### Electromagnetism

I. CONCEPTUAL UNDERSTANDING

1. Are the magnetic field lines closed? Explain.  (2 Marks)

A: * Magnetic field lines are closed.

Explanation: The direction of a magnetic line at any point gives the direction of the magnetic force on a unit north pole placed at that point.

* No two field lines can ever intersect each other.

* Each line emerge from the north pole and merge into the south pole.

* Inside the magnet, the direction of field lines is from its south pole to its north pole.

* It is therefore clear that the magnetic field lines are closed.

2. See the figure below, magnetic lines are shown. What is the direction of current flowing through the wire?  (1 Mark)

A: * The magnetic field lines are in anti clock wise direction. So the current flow is vertically upward direction from the page.

3. A bar magnet with north pole facing towards a coil moves as shown in the figure. What happens to the magnetic flux passing through the coil? (2 Marks)

A: * When the bar magnet with its north pole is moved towards the coil, an induced current 'i' is produced in the coil.

* Due to this induced current in the coil it becomes a magnet and a north pole will be formed on the side of the coil facing the bar magnet.

* This north pole of the coil opposes the forward motion of the bar magnet towards it.

4. A coil is kept perpendicular to the page. At P, current flows into the page and at Q it comes out of the page as shown in the figure. What is the direction of magnetic field due to the coil?      (2 Marks)

A: * If current flows into the page in this downward direction, the field lines are in clockwise direction at P according to right hand rule.

* As the current comes out of the page at Q, the magnetic field lines will be in anti - clockwise direction.

* It means that the magnetic lines of force will be on the right side away from the coil.

5. The direction of current flowing in a coil is shown in the figure. What type of magnetic pole is formed at the face that has flow of current as shown in the figure?              (2 Marks)

A: * The flow of current in the coil is in the anti clock wise direction.

* This means a north pole is formed at the face that has flow of current as shown in the figure.

* This way current is induced in the coil when a bar magnet with its N - Pole approaches towards the coil.

* Then the face of the coil towards the approaching magnet behaves as a North pole and it opposes the motion of the magnet towards it.

6. Why does the picture appear distorted when a bar magnet is brought close to the screen of a television? Explain.  (2 Marks)

A: * The formation of a clear picture on the screen of a television is due to the motion of charges inside the screen.

* When a bar magnet is brought close to the screen of the television. The picture on the screen appears to be distorted.

* This is because the motion of the electrons reaching the screen is affected by the magnetic field of the bar magnet.

* It means the magnetic field exerts a force on the moving charges. So the picture gets distorted.

* This force is called magnetic force.

7. Symbol '×' indicates the direction of magnetic field into the page. A straight long wire carrying current along its length is kept perpendicular to the magnetic field. What is the magnitude of force experienced by the wire? In what direction does it act?  (4 Marks)

A: * B is uniform magnetic field directed into the Page.

* A straight long wire of Length L is carrying a current I. This is kept perpendicular to the magnetic field.

* Current means moving electric charges. Let these charges move with a drift velocity 'v'. Let the total charge inside the magnetic field be.

* Magnetic force on the current carrying wire is

F = Q.v.B. ............ (1)

* Time taken to cross the field by the charge (Q) is

* We know  = I (electric current)

...   F = I.LB

Direction of force: By using right hand rule we can find the direction of the force.

Accordingly the direction of the force is perpendicular to the direction of current and perpendicular to the direction of magnetic field.

8. Explain the working of an electric motor with a neat diagram.     (4 marks)

A: Electric motor: An electric motor is a device which converts electrical energy into mechanical energy.

Principle of working: A motor works on the principle that when a rectangular coil is placed in a magnetic field and current passes through it, a couple acts on the coil which rotates it continuously.

Working of an electric motor:

* A rectangular coil ABCD is kept in a uniform magnetic field produced by the horse shoe magnet NS.

* When current passes through the rectangular coil, this current produces a magnetic field around it.

* The magnetic field due to horse shoe magnet interacts with the magnetic field of the coil and as a result the coil rotates continuously.

* If the coil ABCD is in the horizontal position current enters the coil through the brush B, and commutator half ring C1.

* Current flows in the direction ABCD and goes out through the ring C2 and brush B2.

* When the current flows from A to B in the rectangle it experiences a force F from the magnetic field in the downward direction.

* As the current flows from C to D in the rectangle it experiences a force F from the magnetic field in the upward direction.

* These upward and downward forces on the rectangle coil of wire ABCD constitute a couple and rotate it in the anti-clockwise direction.

* When the coil rotates and comes to the vertical position than the brushes B1 and B2 will touch the gap between the commutator rings and so the current supply to the rectangular coil is cutoff.

* Due to the inertia of rotation the coil continues to rotate and the brushes B1 and B2 come in contact with the commutator which supplies current in the reverse direction.

* Now couple is established in reverse direction and the coil continues to rotate.

* The reversal of current in the coil continues for every half rotation. This makes the coil to rotate continuously.

* The rotating shaft of electric motor is connected to machines for their working.

9. Derive Faraday's Law of induction from the law of conservation of energy. (4 Marks)

A: Faraday's Law of induction from the law of conservation of energy.

* Arrange the apparatus as shown in the figure.

* It consists of parallel bare conductors which are spaced 'l' meters apart in uniform magnetic field of B.

* We can hold another bare conductor in such a way that it is in contact with the two parallel wires.

* A galvanometer is connected to the ends of parallel conductors to complete an electric circuit.

* If the cross wire placed across parallel conductors is moved to the left, the galvanometer needle will deflect in one direction.

* If the cross wire is moved to the right its needle deflects in a direction opposite to the previous deflection.

* As the cross wire is moved, the magnetic flux through the enclosed area changes giving rise to induced e.m.f. which in turn gives rise to induced current that can be observed in the galvanometer.

* Let in time Δt the cross - wire be moved through a distance S.

* As per the law of conservation of energy, the work done in moving the cross wire is converted into electrical energy.

* Work done in moving the cross - wire through a distance S in time Δt.

W = F × S

= BIL × S = BLS. I

* Change in magnetic flux

= Δ∅ = BA = B [LS].

[Change in area: LS]

...     W = Δ∅. I dividing by Δt

* But we know power P = εI

[We know power = E.M.F. (ε) × current (I)]

* or induced E.M.F.

* The induced E.M.F. (ε) generated in a closed loop is equal to the rate of change of magnetic flux  passing through it.

* This is Faraday's Law of electromagnetic induction.

10. The value of magnetic field induction which is uniform is 2T. What is the flux passing through a surface of area 1.5 m2 perpendicular to the field? (2 Marks)

A: Given: Magnetic induction of uniform field B = 2T

Surface area A = 1.5 m2

Flux ∅ = ?

Formula: We know B =  or ∅ = BA cos θ A

Here θ = 0, So cos θ cos 0 = 1

...   Flux ∅ = 2 × 1.5 = 3 Weber

11. An 8N force acts on a rectilinear conductor 20 cm long placed perpendicular to a magnetic field. Determine the magnetic field induction if the current in the conductor is 40 A.  (2 Marks)

A: Given: Force on the conductor F = 8 N

Length of the conductor L = 20 cm =  m

Current in the conductor I = 40 A

Magnetic field induction B = ?

Formula: F = BIL or B =

...  Magnetic induction  = 1 Tesla

12. Explain with the help of two activities that current carrying wire produces magnetic field.    (4 Marks)

A: Activity: 1

* Take a thin wooden plank covered with white paper and make two holes on its surface as shown in figure.

* Pass insulated Copper wire (24 gauge) through the holes and wind the wire 4 to 5 times through holes such that it looks like a coil (figure).

* The ends of the wire are connected to terminals of the battery through a switch. Now switch on the circuit.

* Place a compass needle on the plank at the centre of the coil.

* Put dots on either side of the compass. Again place compass at one of the dots, put other dot further.

* Do the same till we reach the edge of the plank. Now repeat this for the other side of the coil from the centre.

* Then draw a line joining the dots, we will get a field line of the circular coil.

* Do the same for the other points taken in between the holes. Draw corresponding lines. We will get field lines of the circular coil.

* These are magnetic field lines around the coil through which current is passing.

Activity: 2

Aim: To study the pattern of the magnetic field lines (or) pattern produced by a solenoid.

Apparatus: Wooden plank, White paper, Copper wire, Coil, Iron fillings.

Procedure:

* Take a wooden plank covered with white paper.

* Make equidistant holes on its surface as shown in figure.

* Pass Copper wire through the holes as shown in figure (a). This forms a coil.

* Join the ends of the coil to a battery through a switch. Switch on the circuit. Current passes through the coil.

* Now sprinkle iron filings on the surface of the plank around the coil.

* Give small jerk to it. An orderly pattern of iron filings is seen on the paper.

* This long coil is called solenoid. A solenoid is a long wire wound in a close packed helix.

* The field of solenoid is shown in the figure (b). The magnetic field lines set up by solenoid resemble those of bar magnet indicating that a solenoid behaves like a bar magnet.

* The field lines outside the solenoid are continuous with those inside.

* Out side the solenoid the direction of the field lines inform north to south while inside the direction is from south to north.

13. How do you verify experimentally that current carrying conductor experiences a force when it is kept in magnetic field?    (4 Marks)

A: * Take a wooden plank. Fix two long wooden sticks on it. These wooden sticks are split at their top ends.

* A Copper wire is passed through these splits and the ends of the wire are connected to battery of 3 Volt, through a switch.

* Close the switch to make the circuit. Current passes through the wire.

* Now bring a horseshoe magnet near the Copper wire as shown in figure.

* Now a force is experienced on the wire.

* Reverse the polarities of the magnet, then the direction of the force is also reversed.

* Right hand rule explains to find the direction of magnetic force exerted by the magnetic field on current carrying wire.

14. Explain Faraday's Law of induction with the help of activity.

A:

* Connect the terminals of a coil to a sensitive ammeter or a galvanometer as shown in the figure.

* Now if we push a bar magnet towards the coil, with its north pole facing the coil, a remarkable thing happens. While the magnet is moving towards the coil, the needle in galvanometer deflects, showing that a current has been set up in the coil, the galvanometer does not deflect if the magnet is at rest.

* If the magnet is moved away from the coil, the needle in the galvanometer again deflects, but in the opposite direction, which means that a current is set up in the coil in the opposite direction.

* If we use the end of south pole of a magnetic instead of north pole in the above activity, the experiment works just as described but the deflections are exactly reversed.

* Further experimentation enables us to understand that the relative motion of the magnet and coil set up a current in the coil. It makes no difference whether the magnet is moved towards the coil or the coil towards the

magnet.

* "Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil".

* This is one form of Faraday's Law.

15. Explain the working of AC electric generator with a neat diagram. (4 Marks)

A:

* Consider a rectangular coil. Let it be held between the poles of Curve-shaped permanent magnet as shown in figure.

* As the coil rotates, the magnetic flux passing through the coil changes.

* According to the law of electromagnetic induction an induced current is generated in the coil.

Direction of Current

* Consider initially the coil, positioned in such a way that magnetic flux passes through it. When the coil is at rest in vertical position, with side (A) of coil at top position and side (B) at bottom position, no current will be induced in

it. Thus current in the coil is zero at this position.

* When the coil is rotated in clockwise direction, current will be induced in it and it flows from A to B. During the first quarter of rotation, the current increases from zero to a maximum and reaches peak value when the coil is in horizontal position.

* If we continue the rotation of coil, current decreases during the second quarter of the rotation and once again becomes zero when coil comes to vertical position with side B at top (A) at bottom position. During the second part of the rotation, current generated follows the same pattern as that in first half except that the direction of current is reversed. (see figure (b))

Using induced Current:

* As shown in figure (a) the ends of the coil are connected to two slip rings. Two carbon brushes arranged in such a way that they press the slip rings to obtain current from the coil.

* When these brushes are connected to external devices like TV, radio, we can make them work with current supplied from ends of carbon brushes.

* The current obtained by this process changes its direction alternatively for each half cycle as shown in figure (b). This current is called alternating current (AC) in which, the direction of charge flow reverses periodically. So AC

possesses certain frequency. The generator that we discussed here is called AC generator.

16. Explain the working of DC generator with a neat diagram.

A:

* Consider a rectangular coil held in between the poles of curve - shaped permanent magnet as shown in the figure (a).

* As the coil rotates the magnetic flux passing through the coil changes.

* As a result of this changing magnetic flux an induced current is generated in the coil.

Direction of current:

* When the coil is in the vertical position the induced current generated during the first half rotation, rises from zero to maximum and then falls to zero again.

* As the coil moves further from this position, the ends of the coil go to other slip rings.

* Hence during the second half rotation, the current is reversed in the coil itself, the current generated in the second half rotation of the coil is identical with that during the first half of direct current (DC) as shown in figure (b) for one revolution.

II. ASKING QUESTIONS AND MAKING HYPOTHESIS

17. Raj Kumar said to you that the magnetic field lines are open and they start at the north pole of a bar magnet and end at south pole. What questions do you ask Raj Kumar to correct him by saying 'field lines are closed'?   (4

Marks)

A: * How can you say the alignment of lines passing through the magnet?

* If the magnetic lines are open how can you say they start from the north pole?

* Why the magnetic lines are concentric circles around a current carrying wire?

* What does a field line indicate?

* Can you explain why is the magnetic compass needle follow a curved path from one pole to another pole.

* What is the direction of field lines?

18. As shown in the figure both coil and bar magnet move in the same direction. Your friend is arguing that there is no change in flux. Do you agree with his statement? If not what doubts do you have? Frame questions about the doubts you have regarding the change in flux.   (4 Marks)

A: * If both the coil and the bar magnet move in the same direction with the same velocity, there will be no change in flux.

* To disagree with his statement, the following answers are to be elicited by putting questions to him.

a) Are the magnet and coil moving with same velocity?

b) Is there any change in the orientation of the coil?

c) Suppose if both magnet and coil move in opposite directions what happens?

d) If magnet alone moves towards the coil which is stationary what do you observe?

e) Do you observe any change in the induced current of the coil when the magnet is reversed and moved towards the coil.

III. EXPERIMENTATION AND FIELD INVESTIGATION

19. What experiments do you suggest to understand Faraday's Law? What items are required? What suggestions do you give to get good results of the experiment? Give precautions also?    (4 Marks)

A: Materials required: Wooden base, Soft Iron Cylinder, Copper wire, metal ring, AC source, DC sources, switch.

Experiment:

* Take a wooden base as shown in figure (a).

* Fix a soft iron cylinder on the wooden base vertically.

* Wind Copper wire around the soft iron as shown in figure (a).

* Now take a metal ring which is slightly greater in radius than the radius of the soft iron cylinder and insert it through the soft iron cylinder on the wooden base.

* Connect the ends of the coil to an AC source and switch on the current.

* We notice that the metal ring is levitated on the coil.

* Switch off the current, the ring will jump into the air very dramatically.

* Remove the AC supply and connect a DC supply.

* If DC is used, the metal ring lifts up and falls down immediately.

* When DC is supplied at that instant there is a change influx linked with ring. Hence the ring rises up.

* Thereafter, there is no change in flux linked with the coil, hence the ring falls down.

When AC is used

* The free body diagram of metal ring is shown in fig (b). The weight W of the ring acts down to balance it, a force F equal in magnitude and opposite in direction should act.

* In this activity AC is used. AC changes both in magnitude and direction in regular intervals of time.

* The current through the coil produces a magnetic field so that one end of the coil behaves like north pole and other end behaves like south pole for a certain time interval.

* For the next time interval, coil changes its polarities. Hence we can say that coil undergoes changes in its poles in the same interval of time.

* The levitation of ring is possible only when the metal ring behaves like a magnet and should change its polarities in the same time interval but in a sense opposite to that of the solenoid (coil) as shown in figure (c).

* Assume that the current flows in an clockwise direction in the solenoid as viewed from the top. Then the upper end becomes a south pole.

* An upward force is applied on the ring only when the upper side of the ring becomes a north pole (i.e. south pole of the ring faces towards the south pole of solenoid). It is only possible when there exists a anticlockwise current (viewed from the top) in the ring.

* After certain intervals, solenoid changes its polarities, so that the ring also changes its polarities in the same intervals.

* This is the reason why the metal ring is levitated.

* AC is not a constant current, so that, the magnetic induction changes both in magnitude and direction in the solenoid and in the ring.

* Here the area of the metal ring is constant. But the field through the metal ring changes so that flux linked with the metal ring changes.

* Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil. This is one form of Faraday's Law.

Precautions: When AC is switched on necessary care should be taken when the metal ring is levitated from the cylinder.

20. How can you verify that a current carrying wire produces a magnetic field with the help of an experiment?  (4 Marks)

A: * Take a thermocole sheet and fix two thin wooden sticks of height 1 cm which have small slit at the top of their ends.

* Arrange a Copper wire so that it passes through these slits and make a circuit.

* The circuit consists of a 3 (or 9) volt battery, key and Copper wire which are connected in series as shown in figure.

* Now, keep a magnetic compass below the wire. Bring a bar magnet close to the compass.

* We notice the deflection of the needle by the bar magnet.

* This is because we applied magnetic force on the needle.

* Take away the bar magnet from the circuit.

* Now switch on the circuit, we observe a deflection of the needle in the magnetic compass.

* Thus we conclude that the current carrying conductor (Copper wire) produces a magnetic field around it. (or) It behaves like a magnet.

IV. INFORMATION SKILLS AND PROJECTS

21. Collect information about generation of current by using Faraday's Law.   (2 Marks)

A: Faraday's Law is useful in generation of current.

a) Faraday's Law is the base for generator which produces electricity.

b) Transformer also works according to this law. This is helpful in transmission of electricity.

c) Dynamo is a very good example which makes use of Faraday's Law. This produces direct current by using a commutator.

22. Collect information about material required and procedure for making a simple electric motor from internet and make a simple motor on your own.  (4 Marks)

A: Materials required:

1.5 V Battery,

2 safety pins,

magnets,

rubber bands cut from cycle tube,

1.5 m enamelled Copper wire (25 gauge)

Procedure:

* Make a coil of wire with the enamelled Copper wire by winding it on the battery.

* Remove the 15 turns of this coil from the battery and put its ends by scrapping their insulation in the end bits of the safety pins as shown in the figure.

* Put the magnet on the 1.5 V battery and fix it with the help of the rubber bands.

* As the two safety pins are connected to the positive and negative terminals of the battery, the circuit is completed and current flows in the coil.

* The coil begins to rotate. This is how a motor works. Current flowing in the coil makes it rotate when it is in the magnetic field.

23. Collect information of experiments done by Faraday. (2 Marks)

A: * Faraday's first recorded experiment was construction of a voltaic pile with seven half pence pieces, staked together with seven disks of sheet zinc and six pieces of paper moistened with salt water.

* He decomposed sulphate of magnesia with this pile.

* Electromagnetic induction is one of the experiments done by Faraday.

* Faraday's Laws of electrolysis. According to the principles of electrolysis.

a) It became possible to refine metals in the process of extraction of metals.

b) Electroplating is developed.

c) Electrotyping another innovation came in to practice.

V. COMMUNICATION THROUGH DRAWING AND MODEL MAKING

24. Draw a neat diagram of electric motor and name its parts    (4 marks)

A:

25. Draw a neat diagram of an AC generator.

A:

VI. APPRECIATION AND AESTHETIC SENSE, VALUES

26. How do you appreciate Faraday's Law, which is the consequence of conservation of energy?    (2 Marks)

A: * According to the law of conservation of energy 'energy can neither be created nor destroyed'.

* But energy can be converted from one form to another.

* When a bar magnet its north pole is pushed towards a coil of wire, an induced current is generated in the coil.

* If the magnet is moved away from the coil again current is generated in the coil but in the opposite direction.

* When the magnet is at rest no current is generated in the coil.

* The work that is done in moving the magnet appeared in the form of current in the coil. This is in accordance with the law-of conservation of energy.

* I appreciate this very much.

27. How do you appreciate the relation between magnetic field and electricity that changed the life style of man kind?  (2 Marks)

A: * Electricity and magnetism are interlinked.

* Cranes, calling bells, motors, electric trains, generators are some of the electric appliances which are changing the life style of man kind.

* Both electricity and magnetism are used in the working of these appliances.

* With the invention of electromagnetic induction several new devices are developed which are at the service of man kind.

* I appreciate the relation between electricity and magnetism that lead to many inventions which changed our life style.

VII. APPLICATION TO DAILY LIFE AND CONCERN TO BIODIVERSITY

28. Give few applications of Faraday's Law of induction in daily life. (2 Marks)

A: * There are many applications of Faraday's Law of induction in our daily life.

Some of them are

a) Tape recorder                                          b) Transformers

c) Wind mills                                                 d) Induction stoves

e) Use of ATM cards                                    f) Spark plugs in automobiles

g) Brake system in Railway wheels           h) Security checking of metal detectors

i) AC and DC generators                             j) Generation electric current

29. Which of the various methods of current generation protects nature well. Give examples of support your answer. (4 Marks)

A: * Current generation can be made by several methods. Some of them are

a) thermal power                           b) hydal power.

c) wind power                                 d) solar power

e) nuclear power                             f) power from garbage .... etc.

* Generally now a days power is generated from hydal and thermal resources.

* In hydal power production water is stored at a very high place. It is allowed to fall on the generators. This is a non-renewable source. This type of power production is more when enough water is available.

* Thermal power production is done by burning coal. This is a non-renewable source. Further this type of production is associated with pollution.

* Renewable resources are best for production of power. Solar power, wind power, geothermal power etc. But these are always not available. Further at present the cost of production of this power is more. Now solar power is becoming more and more popular. Shortly it is likely to become our major energy resource as so much of research is being done in this direction to make its production much cheaper.

* Nuclear power is another major resource for man kind. But people are afraid of the dangers that may creap in nuclear power plants.

QUESTIONS AND ANSWERS GIVEN IN THE LESSON

1. Why does a needle under a conductor carrying current get deflected? (1 Mark)

A: Due to the establishment of magnetic field as a result of current flowing in the conductor the needle gets deflected.

2. How can we find the strength and direction of the magnetic field?   (1 Mark)

A: The magnetic flux give us the strength of the magnetic field and the tangent drawn to the field line at a point gives the direction of the field.

3. What are the curved lines around a magnet drawn with the help of a compass needle?   (1 Mark)

A: Technically these curves are called 'magnetic field lines'. Field lines are imaginary lines. These lines help us to understand the nature of the field.

4. Can we give certain values to magnitude of the field at every point in the magnetic field?    (1 Mark)

A: * When the magnetic field is uniform the magnitude of the field at every point is the same.

* The magnitude of the field is different at different points in a non uniform magnetic field.

5. What is the flux through unit area perpendicular to the magnetic field?  (2 Marks)

A: * The flux through unit area perpendicular to the magnetic field is magnetic flux density (B).

6. What are the units of magnetic flux density?   (1 Mark)

A: * Units of magnetic flux density is Weber/ (meter)2

* It is also called Tesla.

7. Can we generalise the formula of flux for any orientation of the plane taken in the field?  (1 Mark)

A: * If θ is the angle between the magnetic field (B) and normal to the plane with area (A) then the effective area of the plane perpendicular to the field is A cos θ.

8. What is the flux through the plane taken parallel to the field?   (1 Mark)

A: The magnetic flux through the plane taken parallel to the field is zero.

9. What is the use of introducing the ideas of magnetic flux and magnetic flux density?   (2 Marks)

A: * Magnetic flux is useful in understanding the nature of the magnetic field.

* Magnetic flux density is useful in knowing the magnetic field induction.

10. Are there any sources of magnetic field other than magnets?  (1 Mark)

A: * A straight wire carrying current produces magnetic field around it.

* A loop of wire carrying current produces magnetic field.

11. Do you know how old electric calling bells work?   (2 Marks)

A: * When current passes through the circuit a soft iron bar on which conducting wire is wound makes it an electromagnet.

* This electromagnet attracts the shaft infront of it and the circuit breaks there by the current flow is arrested.

* The shaft goes back as there is a spring arrangement. So again circuit is completed and current flows. With the repetition of this process the bell rings.

12. What happens when a current carrying wire is kept in a magnetic field? (2 Marks)

A: * When a current carrying wire is kept in a magnetic field it experiences a force and gets deflected.

* The direction of deflection can be foundout by right hand rule.

13. Do you feel any sensation on your skin when you stand near the TV Screen?  (1 Mark)

A: The hair on the skin rises up when I stand near the TV screen when it is switched on.

14. What could be the reason for this sensation when you stand near the TV screen?    (1 Mark)

A: As there is electron flow in the screen of the TV these induce electric charges on the hair above our skin and as a result the hair rises up due to repulsion of like charges between the neighbouring hairs.

15. Why does a picture get distorted when a magnet is brought nearer to the TV screen.  (1 Mark)

A: The picture formed on the screen due to the motion of the electrons is affected by the magnetic field of the magnet. So the picture gets distorted.

16. Is the motion of electrons reaching the screen affected by the magnetic field of the magnet?    (1 Mark)

A: Yes. The motion of electrons reaching the screen is affected by the magnetic field of the magnet.

17. Can we calculate the force experienced by a charge moving in a magnetic field.  (1 Mark)

A: * We can calculate the force experiences by a charge moving in a magnetic field.

* If F is that force then F = qvB where q is the charge, B is magnetic induction and v is the velocity with which the charge is moving.

18. Can we generalise the equation for magnetic force on charge when there is an angle 'θ' between the direction of the field B and velocity v.   (1 Mark)

A: It is experimently proved that when there is an angle between direction of field and velocity, the magnetic force experienced by the charge is given by F = qvB sin θ.

19. What is the magnetic force on the charge moving parallel to a magnetic field?     (2 Marks)

A: * When the charge moves parallel to the magnetic field the value of θ becomes zero.

* So F = qvB sin θ becomes F = qvB sin 0

...  F = 0 since sin 0 = 0

20. What is the direction of magnetic force acting on a moving charge? (2 Marks)

A: * The direction of magnetic force acting on a charge moving in a magnetic field is found out by right hand rule.

* Keep your right hand fingers along the direction of velocity of moving charge and next curl your fingers towards the direction of the magnetic field then the thumb gives the direction of magnetic force.

21. Can you determine the magnetic force on a current carrying wire which is placed along a magnetic field?    (2 Marks)

A: * We know that each charge experiences no magnetic force because they are moving parallel to the direction of the field along the field. So the force acting on the wire is zero when kept along a magnetic field.

22. What is the force on the wire if its length makes an angle 'θ' with the magnetic field?    (1 Mark)

A: Let 'θ' be the angle between direction of current and magnetic field, then the force acting on the current carrying wire is given by F = ILB sin θ (at any angle).

23. How can you find the direction of the force?    (1 Mark)

A: By using right hand rule we can find the direction.

24. Is the direction of deflection observed experimentally same as that of the theoretically expected one?  (1 Mark)

A: It depends on polarities of the horseshoe magnet.

25. Does the right hand rule give the explanation for the direction of magnetic force exerted by magnetic field on the wire?  (2 Marks)

A: * Right hand rule helps us to find the direction of magnetic force exerted by the magnetic field on current carrying wire.

* It does not help us to explain the reason for the deflection of the wire.

26. Can you give the reason for the deflection of the wire?   (2 Marks)

A: * There exists only magnetic field due to external source (horseshoe magnet).

* When there is current in the wire, it also produces a magnetic field.

* These fields overlap and give non-uniform field. This is the reason for the deflection of the wire.

27. Does this deflection fit with the direction of magnetic force found by right hand rule?   (1 Mark)

A: This deflection fits with the direction of magnetic force found by right hand rule.

28. What happens when a current carrying coil is placed in a uniform magnetic field?  (1 Mark)

A: The current carrying coil gets deflected placed in a uniform magnetic field.

29. Can we use this knowledge to construct an electric motor?  (1 Mark)

A: * We can make use of this knowledge to construct an electric motor.

* Infact this is the working principle of an electric motor.

30. What is the angle made by AB and CD with the magnetic field?        (1 Mark)

A: AB and CD the sides of the rectangular coil of wire are always at right angles to the magnetic field.

31. Can you draw the direction of magnetic force on sides AB and CD of the rectangular coil of wire placed in the uniform magnetic field?   (2 Marks)

A: * By applying right hand rule we get the direction of the magnetic force.

* At AB, the magnetic force acts inwards perpendicular to field of the magnet and on CD it acts outward.

32. What are the directions of forces on BC and DA?   (1 Mark)

A: At BC, magnetic force pulls the coil up and at DA magnetic force pulls it down.

33. Why does the coil rotate?   (2 Marks)

A: The rectangular coil comes into rotation in clockwise direction because of equal and opposite pair of forces acting on the two sides of the coil.

34. What happens to the rotation of the coil if the direction of current is the coil remains unchanged?   (2 Marks)

A: * If the direction of current in the coil is unchanged, it rotates up to a vertical position than due to inertia it rotates further in clockwise direction.

* But now the sides of the coil experience forces which are in the opposite direction to the previous case.

* Hence these forces try to rotate in the anti clockwise direction.

* As a result, the coil comes to halt and rotates in anti clock wise direction. This will go on if the direction of current remains unchanged.

35. How could you make the coil rotate continuously?   (2 Marks)

A: * If the direction of current in the coil, after the first halfrotation, is reversed, the coil will continue to rotate in the same direction.

* Thus if the direction of current through the coil is reversed every half rotation, the coil will rotate continuously in one and the same direction.

36. How can we achieve the continuous rotation of the coil in electric motors? (4 Marks)

A: * To achieve this, brushes B1 and B2 are used as is observed in the figure of electric motor.

* These brushes are connected to the battery. The ends of the coil are connected to slip rings C1 and C2 which rotate along with the coil.

* Initially C1 is in contact with B1 and C2 is in contact with B2. After half rotation, the brushes come into contact with the other slip rings in such a way that the direction of the current through the coil is reversed.

* This happens every half rotation. Thus the direction of rotation of coil remains the same.

37. What happens when a coil without current is made to rotate in magnetic field?   (1 Mark)

A: Due to the change in magnetic flux that is passing through the coil, current is induced in the coil.

38. How is current produced?    (1 Mark)

A: When the magnetic flux passing through a coil changes current is induced in the coil.

39. What do you notice?     (1 Mark)

A: We notice that the metal ring is levitated on the coil.

40. Why is there a difference in behaviour in these two cases? (1 Mark)

A: * When AC is used the direction of current supply changes a number of times in one second.

* DC is unidirection current, As such there is difference in behaviour of the metal ring in these two cases.

41. What force supports the ring against gravity when it is being levitated?  (1 Mark)

A: The magnetic force developed in the coil of Copper wire supports the ring against gravity when it is being levitated.

42. Could the ring be levitated if DC is used?   (1 Marks)

A: The metal ring lifts up and falls down immediately if DC is used.

43. What is the unknown force that acts on the ring that is being levitated?   (1 Mark)

A: The change in polarities at certain intervals at the ends of the solenoid causes the unknown force acting on the metal ring.

44. What is responsible for the current in the metal ring?   (1 Mark)

A: * AC is not a constant current. So that the magnetic induction changes in both magnitude and direction in the solenoid and in the ring.

* The field through the metal ring changes so that flux linked with the metal ring changes and this is responsible for the current in the metal ring.

45. If DC is used, the metal ring lifts up and falls down immediately. Why?  (4 Marks)

A: * The flux linked with metal ring is zero when no current flows through the solenoid.

* When the current is allowed to flow through the solenoid, it behaves like a bar magnet.

* So the flux is linked to the metal ring when the switch is on. At that instant there is a change in flux linked with ring. Hence the ring rises up.

* Thereafter, there is no change in flux linked with coil, hence it falls down.

* If the switch is off, the metal ring again lifts up and falls down. In this case also, there is a change in flux linked with ring when the switch is off.

46. What could you conclude from the above analysis?  (1 Mark)

A: * Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil.

*The relative motion of the magnet and coil sets up a current in the coil.

47. What is the direction of induced current?  (1 Mark)

A: "The induced current will appear in such a direction that it opposes the changes in the flux in the coil".

48. Can you apply conservation of energy for electromagnetic induction.   (1 Mark)

A: * We can apply the law of conservation of energy for electromagnetic induction.

* Here the mechanical energy is converted into electrical energy.

49. Can you guess what could be the direction of induced current in the coil in such case? (1 Mark)

A: In the context, the direction of induced current in the coil must be in anticlock wise direction.

50. Could we get Faraday's Law of induction from conservation of energy.  (1 Mark)

A: We could get Faraday's Law of induction from conservation of energy.

51. Can you derive an expression for the force applied on cross wire by the field B.  (1 Mark)

A: * We can derive an expression for the force (F) applied on cross wire by the field B. Its value is F = BIL.

52. How could we use the principle of electromagnetic induction in case of using ATM card when its magnetic strip is swiped through a scanner?  (2 Marks)

A: * When the ATM card is moved through the card reader, change in magnetic flux is produced in one direction.

* This change in magnetic flux induces current in the pickup coil.

* This current goes through signal amplification and is translated into binary code.

* This binary code is read by the computer.

53. What happens when a coil is continuously rotated in a uniform magnetic field.  (1 Mark)

A: Due to change in magnetic flux continuously induced current is produced in the coil continuously.

54. Does it help us to generate electric current?    (1 Mark)

A: It helps to generate electric current.

55. Is the direction of current induced in the coil constant? Does it change? (1 Mark)

A: The direction of current induced in the coil is not constant. It changes.

56. Can you guess the reason for variation of current from zero to maximum and vice versa during the rotation of the coil.   (2 Marks)

A: * As the coil rotates from horizontal to vertical position variation of induced current from zero to maximum is observed.

* Again when the coil rotates from vertical to horizontal in the clock-wise direction variation of induced current from maximum to zero is observed because of change of flux from maximum to zero passing through the coil.

57. Can we make use of this current? If so how?     (2 Marks)

A: * The current induced in the coil is drawn out with the arrangement of two carbon brushes and slip rings.

* This current is then made use of for the working of the electric appliances like TV fans etc.

58. How can we get DC using a generator?   (1 Mark)

A: By connecting two half-slip rings instead of a slip ring commutator on either side to the ends of the coil we get DC Current.

59. What changes do we need to make in an AC generator to be converted into a DC generator.    (1 Mark)

A: If two half slip rings are connected to ends of the coil, then AC generator works as DC generator to produce DC current.

ACTIVITIES

Activity: 1

1. Explain with the help of an activity that current carrying wire produces magnetic field.   (2 Marks)

A: * Take a thermocole sheet and fix two thin wooden sticks of height 1 cm which have small slit at the top of their ends.

* Arrange a Copper wire so that it passes through these slits and make a circuit.

* The circuit consists of a 3 (or 9) volt battery, key and Copper wire which are connected in series as shown in figure.

* Now, keep a magnetic compass below the wire. Bring a bar magnet close to the compass.

* We notice the deflection of the needle by the bar magnet.

* This is because we applied magnetic force on the needle.

* Take away the bar magnet from the circuit.

* Now switch on the circuit, we observe a deflection of the needle in the magnetic compass.

* Thus we conclude that the current carrying conductor (Copper wire) produces a magnetic field around it. (or) It behaves like a magnet.

* Draw a small line segment connecting the two dots. Draw an arrow on it from South Pole of the needle to North Pole of the needle.

* Repeat the same by placing the compass needle at various positions on the paper. The compass needle settles in different directions at different positions.

* From these observations we conclude that the strength of the field varies with distance from the bar magnet.

* Now hold the compass a little above the table and at the top of the bar magnet. We can observe that field exists in all directions around the bar magnet.

* Hence we can say that the magnetic field is three dimensional i.e., magnetic field surrounds its source such as bar magnet. From the above discussion we can generalise that.

* A magnetic field exists in the region surrounding a bar magnet and is characterised by strength and direction.

Activity: 2

2. By doing an activity establish the fact that the magnetic field around a bar magnet is three dimensional and its strength and direction varies from place to place.  (4 Marks)

A: * Take a sheet of white paper and place it on the horizontal table. Place a bar magnet in the middle of the sheet.

* Place a magnetic compass near the magnet. It settles to a certain direction.

* Use a pencil and put dots on the sheet on either side of the needle. Remove the compass.

* Draw a small line segment connecting the two dots. Draw an arrow on it from South Pole of the needle to North Pole of the needle.

* Repeat the same by placing the compass needle at various positions on the paper. The compass needle settles in different directions at different positions.

* From these observations we conclude that the strength of the field varies with distance from the bar magnet.

* Now hold the compass a little above the table and at the top of the bar magnet. We can observe that field exists in all directions around the bar magnet.

* Hence we can say that the magnetic field is three dimensional i.e., magnetic field surrounds its source such as bar magnet. From the above discussion we can generalise that.

* A magnetic field exists in the region surrounding a bar magnet and is characterised by strength and direction.

Activity: 3

3. How do you trace the magnetic field lines Describe your activity in doing this process.  (4 Marks)

A:

* Place a white sheet of paper on a horizontal table. Place a compass in the middle of it. Put two dots on either side of the compass needle.

* Take it out. Draw a line connecting the dots which shows the North and South of the earth.

* Now place the bar magnet on the line drawn in such a way that its north pole points towards geographic north.

* Now place the compass at the north pole of the bar magnet. Put a dot at the north pole of the compass needle. Now remove the compass and place it at the dot. It will point in other direction.

* Again put a dot at the north pole of the compass needle. Repeat the process till you reach the south pole of the bar magnet. Connect the dots from 'N' of the bar magnet to 'S' of the bar magnet.

* We will get a curved line. Now select another point from the north pole of the bar magnet.

* Repeat the process for many points taken near the north pole. We will get different curves as shown in figure.

* Technically those curves are called 'magnetic field lines'. Field lines are imaginary lines.

Activity: 4

4. Describe an activity to find the magnetic field due to a straight wire carrying current. How can you find the direction of the magnetic field? (4 Marks)

A:

* Take a wooden plank and make a hole as shown in figure (a). Place this plank on the table. Now place a retort stand on the plank as shown in figure (a).

* Pass 24 gauge Copper wire through hole of the plank and rubber knob of the retort stand in such a way that the wire be arranged in a vertical position and not touch the stand.

* Connect the two ends of the wire to a battery via switch. Place 6 to 10 compass needles in a circular path around the hole so that its centre coincides with the hole.

* Use 3 (or 9) volt battery in the circuit. Switch on. Current flows through the wire.

* We notice that they are directed as tangents to the circle.

* The shape of the magnetic field line around the wire must be a circular line. So we conclude that magnetic field lines are closed lines.

* The magnetic field lines due to straight wire. Carrying the current is shown in fig (b) and fig (c).

* This can be verified by sprinkling iron ring around the wire when current flows in the wire Direction of magnetic field induction at any point.

a) If the current flow is vertically upwards (out of the page), the field lines are in anticlockwise direction, as shown in figure (b).

b) If current flows into the page, in downward direction, the field lines are in clockwise direction as shown in figure (c).

c) How do we determine the direction of field lines? It can be easily determined with right hand thumb rule.

d) If you grab the current carrying wire with your right hand in such way that thumb is in the direction of current, then the curled fingers show the direction of the magnetic field as shown in figure (d).

Activity: 5

5. Explain with the help of an activity how you could trace the magnetic field due to a circular coil.  (4 Marks)

A:

* Take a thin wooden plank covered with white paper and make two holes on its surface as shown in figure.

* Pass insulated Copper wire (24 gauge) through the holes and wind the wire 4 to 5 times through holes such that it looks like a coil (figure).

* The ends of the wire are connected to terminals of the battery through a switch. Now switch on the circuit.

* Place a compass needle on the plank at the centre of the coil.

* Put dots on either side of the compass. Again place compass at one of the dots, put other dot further.

* Do the same till we reach the edge of the plank. Now repeat this for the other side of the coil from the centre.

* Then draw a line joining the dots, we will get a field line of the circular coil.

* Do the same for the other points taken in between the holes. Draw corresponding lines. We will get field lines of the circular coil.

* These are magnetic field lines around the coil through which current is passing.

Activity: 6

6. How do you find the magnetic field due to a solenoid? Explain it through an activity.

A: Aim: To study the pattern of the magnetic field lines (or) pattern produced by a solenoid.

Apparatus: Wooden plank, White paper, Copper wire, Coil, Iron fillings.

* Take a wooden plank covered with white paper.

* Make equidistant holes on its surface as shown in figure.

* Pass Copper wire through the holes as shown in figure (a). This forms a coil.

* Join the ends of the coil to a battery through a switch. Switch on the circuit. Current passes through the coil.

* Now sprinkle iron filings on the surface of the plank around the coil.

* Give small jerk to it. An orderly pattern of iron filings is seen on the paper.

* This long coil is called solenoid. A solenoid is a long wire wound in a close packed helix.

* The field of solenoid is shown in the figure (b). The magnetic field lines set up by solenoid resemble those of a bar magnet indicating that a solenoid behaves like a bar magnet.

* The field lines outside the solenoid are continuous with those inside.

* Out side the solenoid the direction of the field lines is from north to south while inside the direction is from south to north.

Activity: 7

7. Describe the activity of finding the magnetic force on a moving charge and current carrying wire. How is the direction of magnetic force acting on a moving charge is found out.   (4 Marks)

A:

* Let a charge 'q' move with a velocity 'v' perpendicular to the magnetic field 'B' as shown in figure.

* The value of magnetic force on the moving charge can be found experimentally and it is given by, F = qvB

* Magnetic force on the charge is the product of three quantities charge, speed and magnetic flux density.

* The equation for magnetic force acting on a charge 'q' is F = qvB and holds well only when the direction of velocity of charged particle 'v' is perpendicular to the direction of the magnetic field 'B'.

* It is experimentally proved that when there is an angle between the direction of field and velocity the magnetic force experienced by the charge is

F = qvB sin θ

Direction of magnetic force acting on a moving charge:

We have an easy method to find out the direction of magnetic force acting on a charge moving in a magnetic field.

* Keep your right hand fingers along the direction of velocity of moving charge and next curl your fingers towards the direction of magnetic field then the thumb gives the direction of magnetic force as shown in figure (a). This rule is applied for any case of angle between directions of velocity and field. The direction of magnetic force is always perpendicular to the direction of both velocity and magnetic field.

* Generally right hand rule is used when velocity and field are perpendicular to each other. This law states that, "If the fore - fingers points towards the direction of velocity of charge or current, middle finger points to the direction of field (B) then thumb gives direction of force when the three fingers are stretched in such a way that they are perpendicular to each other as shown in figure b.

This rule is applicable to positive charge.

Activity: 8

8. Describe an activity to explain the result of magnetic force applied on a current carrying wire. Draw the figures showing the field lines of horse shoe magnet, and magnetic field lines when current passes through the wire.   (4 Marks)

A:

* Take a wooden plank. Fix two long wooden sticks on it. These wooden sticks are split at their top ends.

* A copper wire is passed through these splits and the ends of the wire are connected to battery of 3 volt, through a switch.

* Close the switch to make the circuit. Current passes through the wire.

* Now bring a horseshoe magnet near the Copper wire as shown in figure (a).

* Change polarities of the horse shoe magnet. Again observe the deflection. Repeat this by changing the direction of current in the circuit.

* When there is current in the wire, it also produces a magnetic field. These fields overlap and give non - uniform field.

* The field in between north and south pole of horse-shoe magnet is shown in fig (b).

* Let us imagine a wire passing perpendicular to the paper. Let the current pass through it into the page. It produces a magnetic field as shown in figure (c).

* Now let us try to sketch the resultant field by observing the field lines. We can see that the direction of the field lines due to the wire in upper part (of circular lines) coincides with the direction of the field lines of horse shoe magnet.

* The direction of field lines by wire in lower part (of circular lines) is opposite to the direction of the field lines of horse shoe magnet.

* So that the net field in upper part is strong and in lower part it is weak. Hence a non-uniform field is created around the wire. This non uniform field is shown in figure (d). Therefore the wire tries to move to the weaker field
region.

Activity: 9

9. With the help of an activity explain Faraday's Law of electromagnetic induction. Activity instead of AC what happens if DC is used.

A: Materials required: Wooden base, Soft Iron Cylinder, Copper wire, Metal ring, AC source, DC source, Switch.

Experiment:

* Take a wooden base as shown in figure (a).

* Fix a soft iron cylinder on the wooden base vertically.

* Wind Copper wire around the soft iron as shown in figure (a).

* Now take a metal ring which is slightly greater in radius than the radius of the soft iron cylinder and insert it through the soft iron cylinder on the wooden base.

* Connect the ends of the coil to an AC source and switch on the current.

* We notice that the metal ring is levitated on the coil.

* Switch off the current, the ring will jump into the air very dramatically.

* Remove the AC supply and connect a DC supply.

* If DC is used, the metal ring lifts up and falls down immediately.

* When DC is supplied at that instant there is a change influx linked with ring. Hence the ring rises up.

* Thereafter, there is no change in flux linked with the coil, hence the ring falls down.

When AC is used:

* The free body diagram of metal ring is shown in fig (b). The weight Wof the ring acts down to balance it, a force F equal in magnitude and opposite in direction should act.

* In this activity AC is used. AC changes both in magnitude and direction in regular intervals of time.

* The current through the coil produces a magnetic field so that one end of the coil behaves like north pole and other end behaves like south pole for a certain time interval.

* For the next time interval, coil changes its polarities. Hence we can say that coil undergoes changes in its poles in the same interval of time.

* The levitation of ring is possible only when the metal ring behaves like a magnet and should change its polarities in the same time interval but in a sense opposite to that of the solenoid (coil) as shown in figure (c).

* Assume that the current flows in an clockwise direction in the solenoid as viewed from the top. Then the upper end becomes a south pole.

* An upward force is applied on the ring only when the upper side of the ring becomes a north pole (i.e. south pole of the ring faces towards the south pole of solenoid). It is only possible when there exists a anticlockwise current (viewed from the top) in the ring.

* After certain intervals, solenoid changes its polarities, so that the ring also changes its polarities in the same intervals.

* This is the reason why the metal ring is levitated.

* AC is not a constant current, so that, the magnetic induction changes both in magnitude and direction in the solenoid and in the ring.

* Here the area of the metal ring is constant. But the field through the metal ring changes so that flux linked with the metal ring changes.

* Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil. This is one form of Faraday's Law.

Precautions: When AC is switched on necessary care should be taken when the metal ring is levitated from the cylinder.

I. Conceptual understanding

1. What are the curves drawn with a magnetic compass drawn in a magnetic field.    (2 Marks)

A: * Technically those curves are called magnetic field lines. Field lines are imaginary lines.

* These lines help us to understand the nature of the field. So these curved lines represent the field lines.

* If we place a compass at any point on the line, the needle comes to rest along the tangent to the line.

* So we conclude that the tangent drawn to the field line at a point gives the direction of the field.

2. When do you call the magnetic field non uniform and uniform.     (2 Marks)

A: * We may define the nature of the field with its characteristics such as its strength and direction.

* The field is said to be non uniform when any one of the characteristics of field i.e., strength or direction changes from point to point.

* Similarly the field is said to be uniform if both strength and direction are constant throughout the field.

3. Explain the terms magnetic flux and flux density.  (2 Marks)

A:

* Consider a uniform magnetic field in space. Imagine a plane of certain area 'A' placed perpendicular to the field at a certain point in the magnetic field as shown in figure. We notice that a few field lines pass through this plane. This number gives an estimation of strength of the field at the point.

* The number of lines passing through the plane of area 'A' perpendicular to the field is called magnetic flux. It is denoted by Φ.

* Magnetic flux represents the number of lines passing through the imagined plane in the field. Of course, flux depends on the orientation of the plane in the field.

* But here we are concerned only with the perpendicular case. The S.I. unit of magnetic flux is Weber.

* Now strength to the field is easily defined using the idea of flux. If the imagined plane is perpendicular to the field and has unit area, then the flux through this plane of unit area gives the strength of the field.

* This strength of the field is technically called magnetic flux density (B).

* So, magnetic flux density is defined as the magnetic flux passing through unit area taken perpendicular to the field. B is also known as magnetic field induction.

Let the flux through the area 'A' be Φ.

4. What is the flux through unit area perpendicular to the field. What is its unit.    (2 Marks)

A: * It is equal to  . The ratio of magnetic flux passing through a plane perpendicular to the field and the area of the plane is called the magnetic flux density.

* So magnetic flux density =

B =

⇒  Φ = BA

* Units of magnetic flux density Weber/ (meter)2. It is also called Tesla.

5. Can we generalise the formula of magnetic flux for any orientation of the plane taken in the field?    (2 Marks)

A:

* Let θ be the angle between magnetic field (B) and normal to the plane with area (A) as shown in figure.

* The effective area of the plane perpendicular to the field is A cos θ.

* Then magnetic flux density is given by,

B = magnetic flux/ effective area. (This formula is used when plane makes an angle with the field)

Then

* The flux through the plane, is given by

Φ = BA cos θ

6. How can you tell the direction of magnetic field of the coil carrying current?   (1 Mark)

A: * This could be answered from the orientation of the compass needle.

* We can observe this when the compass needle is kept at the centre of the coil. The direction in which the compass needle comes to rest indicates the direction of the field due to the coil.

* Thus the direction of the field is perpendicular to the plane of the coil.

7. Why does the compass needle point in the direction of field?  (2 Marks)

A: * Place the compass in front of one of the faces of the coil and observe the orientation of the compass needle.

* Note the pole of the needle that faces the coil. We know that south pole is attracted to the north pole.

* The needle is oriented in such a way that its south pole points towards the north pole of the coil.

* So we can say that the direction of magnetic filed, due to coil, points towards you when the current in the coil is in anti clockwise direction.

* We can verify this in our experiment we should not touch the wires of the coil. When the current in the coil is in clock-wise direction, the direction of magnetic field due to the coil points away from us. The direction of the field due to coil or solenoid is determined by using right hand rule, which states that, "when you curl your right hand fingers in the direction of current, thumb gives the direction of magnetic field." We can observe the direction of magnetic field in the figure.

8. What is the direction of force acting on a negative charge moving in a field?  (1 Mark)

A: First find the direction of force acting one a positive charge. Next reverse its direction. This new direction is the direction of force acting on the negative charge.

9. Can we generalise the equation for magnetic force on charge when there is an angle 'θ' between the directions of field B and velocity v?  (1 Mark)

A: It is experimentally proved that when there is an angle between direction of field velocity, the magnetic force experienced by the charge is given by

F = qvB sin θ

10. What is the magnetic force on the charge moving parallel to a magnetic field?  (1 Mark)

A: * When charge moves parallel to the magnetic field (along the magnetic field or against the field) the value of θ becomes zero. In the above equation is θ is 0° so that sin θ = 0

* Thus the charge experiences no force when it is moving parallel to the magnetic field (along field direction or against field direction).

11. A charged particle 'q' is moving with a speed 'v' perpendicular to the magnetic field of induction B. Find the radius of the path and time period of the particle.

A: Solution: Let us assume that the field is directed into the page as shown in figures.

* Then the force experienced by the particle is F = qvB.

* We know that this force is always directed perpendicular to velocity.

* Hence the particle moves along a circular path and the magnetic force on a charged particle acts like a centripetal force.

* Let r be the radius of the circular path. We know that centripetal force =

qvB =

Solving this equation, we get;

* Time period of the particle;

Substituting r in above equation,

we get

12. Can you determine the magnetic force on a current carrying wire which is placed along a magnetic field?   (1 Mark)

A: We know that each charge experiences no magnetic force because they are moving parallel to the direction of field along the field. So the force acting on wire is zero when it is kept along a magnetic field.

13. Find the magnetic force on a straight wire carrying current which is kept perpendicular to a uniform magnetic field B.  (4 Marks)

A: * This 'B' is directed into the page. It is represented by '.' as shown in the figure.

* Let the field be confined to the length L.

* So only the part of the wire of length 'L' is inside the magnetic field. Remaining wire is outside the magnetic field.

* We know that electric current means charges in motion hence they move with a certain velocity called drift velocity v.

* The magnetic force on a single charge is given by F0 = qvB

* Let total charge inside the magnetic field be = Q. So the magnetic force on the current carrying wire is given by

F = QvB ............... (1)

* The time taken by the charge (Q) to cross the field be

* The time taken by the charge (Q) to cross the field be

* Substituting this in equation 1, We get,

* We know that  is equal to the electric current in the wire.

I =

* Substituting 'I' in the equation 3, we get

F = ILB .......... (4)

Note: This equation holds well only when direction of electric current is perpendicular to magnetic field. In figure,we can observe the bending in the wire due to the force applied on it.

14. What is the force on the wire if its length makes an angle 'θ' with the magnetic field?  (1 Mark)

A: Let 'θ' be the angle between direction of current and magnetic field, then the force acting on the current carrying wire is given by. F = ILB sin θ (at any angle).

15. How could you find the direction of the force on the wire carrying current placed in a magnetic field which makes an angle 'θ' with the field?  (1 Mark)

A: By using right hand rule we can find the direction of the force.

16. Explain the behaviour of a current carrying coil kept in a uniform magnetic field? (or) Explain the principle of working of an electric motor. (4 Marks)

A: Principle of working of an electric motor:

* Consider a rectangular coil kept in a uniform magnetic field as shown in figure. Switch on the circuit so that the current flows through the rectangular coil.

* The direction of current in the coil is shown in figure (a).

* AB and CD are always at right angles to the magnetic field.

* At AB, the magnetic force acts inwards perpendicular to the field of the magnet and on CD, it acts outward.

* fig (b) is drawn showing top view. The force on the sides BC and DA varies because they make different angles at different positions of the coil in the field.

* At BC, magnetic force pulls the coil up and at DA magnetic force pulls it down.

* The force on AB is equal and opposite to the force on CD due to an external magnetic field because they carry equal currents in the opposite direction. Sum of these forces is zero.

* Similarly the sum of the forces on sides BC and DA is also zero for the same reason so net force on the coil is zero.

* Let us understand how the coil rotates.

* Let us consider opening a cap of the bottle as an example where two equal and opposite forces act on the cap. Two forces equal in magnitude but opposite in direction must act on the either side of cap of the bottle as shown in figure (c).

* These forces bring the cap into rotation. Similarly the rectangular coil comes into rotation in clockwise direction because of equal and opposite pair of forces acting on the two sides of the coil.

* If the direction of current in the coil is unchanged, it rotates upto a vertical position then due to its inertia it rotates further in clockwise direction. But now the sides of the coil experience forces which are in the opposite direction to the previous case. Hence these forces try to rotate it in anti clockwise direction.

* As a result, this coil comes to halt and rotates in anti clockwise direction, this will go on if the direction of current remains unchanged.

* If the direction of current in the coil, after the first half rotation, is reversed, the coil will continue to rotate in the same direction.

* Thus if the direction of current through the coil is reversed every half rotation, the coil will rotate continuously in one and the same direction.

17. Can you apply conservation of energy for electromagnetic induction? (4 Marks)

A: * When we push the bar magnet towards the coils, current is generated, in other words electromagnetic induction takes place and mechanical energy is converted into electrical energy.

* We know that when a bar magnet is pushed towards a coil with its north pole facing the coil an induced current is set up in the coil.

* Let the direction of current in the coil be an clockwise direction with respect to north pole of the bar magnet.

* Then this current carrying a loop behaves like a magnet, with its south pole facing the north pole of bar magnet. In such a case, the bar magnet attracts the coil.

* Then it gains kinetic energy. This is contradictory to conservation of energy. So our assumed clockwise direction of induced current is wrong. Hence the correct direction of induced current has to be in anticlockwise direction

with respect to north pole of the bar magnet.

* In such a case the north pole of the coil faces the north pole of the bar magnet as shown in figure.

* Then north pole of bar magnet is repelled by the north pole of the coil. Hence we need to do work to overcome this force.

* This work done on the magnet is converted into electrical energy in the coil. In this way conservation of energy takes place in electromagnetic induction.

* Let us see a case where the bar magnet is pulled away from the coil, with north pole facing the coin.

* In such case, the coil opposes the motion of bar magnet to balance the conversion of mechanical energy into electric energy. This happens only when the north pole of the magnet faces the south pole of the coil.

18. Derive an expression for the force applied on cross wire by field B. (4 Marks)

A: * The force F applied on cross wire by the field is F = BIL

* This force must oppose the applied force. The direction of applied force determines the direction of current through the cross wire. Here we are doing positive work. The work done by us in moving the cross wire converts into electric energy. So the work done is given by,

W = Fs = BIls ........... (1)

* When we put the cross wire across parallel conductors it makes a complete electrical circuit which encloses a certain amount of magnetic flux.

* Now as we move the cross wire to the left, the area of the loop (formed by the parallel conductors and cross wire) decreases and the flux through the loop also decreases. The decrease in flux is given by,

ΔΦ = Bls ............ (2)

Here B is perpendicular to the area (ls). From equations (1) and (2)

W = (ΔΦ)I

Let us divide both sides of this equation by Δt

* We know that electric power is the product of current and emf or voltage.

is obviously equal to induced EMF.

Electric power, P = εI ........ (4)

* Thus the electric power generated in the circuit is equal to product of induced EMF and the current. Thus the mechanical energy utilised to move the cross wire in one second is converted into electric power  I. This is nothing but conservation of energy.

* Dividing equation (1) by Δt, we have

Here  gives the speed of the cross wire, let it be taken as v. Then we get

Power is also given as force times velocity. From equations 4, 6 We get

This is called motional e.m.f.

19. The magnetic flux inside a coil of 400 turns changes for each single turn with time as shown in figure.

Determine the maximum induced emf generated in the coil. Is there any change in induced EMF from t = 0.1 second to 0.3 second?   (2 Marks)

A: * From the given graph, the increase in magnetic flux through one turn of coil in 0.1 second is 0.001 Wb. According to Faraday's Law, the maximum induced emf generated in the coil is given by.

* Substituting the values, we get

* From graph, there is no change in magnetic flux through coil from t = 0.1 s to 0.3 s hence no emf is generated.

20. Find the length of the conductor which is moving with speed of 10 m/s in the direction perpendicular to the direction of magnetic field of induction 0.8 T, if it induces an emf of 8 V between the ends of the conductor.  (2

Marks)

A: Given that B = 0.8 T, v = 10 m/s and ε = 8 V.

∴  8 = 0.8 (l) (10)

l (length of the conductor) = 1 m

21. Give one application of Faraday's Law of Electromagnetic Induction in our daily life. (2 Marks)

A: * Electromagnetic induction is all around us.

* We know that, during security check, people are made to walk through a large upright coil of wire which produces a weak AC (alternating) magnetic field.

* If we are carrying any significant quantities of iron, the magnetic flux linked with the large coil changes and the induced current generated in coil triggers an alarm.

22. How is electromagnetic induction is made use of in a tape recorder?   (2 Marks)

A: * The tape recorder which we use to listen to songs (or) record voices works on the principle of electromagnetic induction.

* It consists of a piece of plastic tape coated with iron oxide and is magnetised more in some parts than in others.

* When the tape is moved past as a small coil of wire (head of the tape recorder), the magnetic field produced by the tape changes, which leads to generation of current in the small coil of wire.

23. Explain the working of Induction Stove?    (2 Marks)

A: * An induction stove works on the principle of electromagnetic induction. A metal coil is kept just beneath of cooling surface.

* It carries alternating current (AC) so that AC produces an alternating magnetic field. When you keep a metal pan with water on it, the varying magnetic field beneath it crosses the bottom surface of the pan and an EMF is induced in it.

* Because the pan is metal the induced EMF generates an induced current in it.

* Since the pan has a finite resistance, the flow induced current in it produces heat in it and this heat is conducted to the water. That's why we call this stove as induction stove.

24. What is an electric motor?   (1 Mark)

A: An electric motor is a device which converts electrical energy into mechanical energy.

25. How much force is exerted by a magnetic field on a stationary charged particle?   (1 Mark)

A: Zero Force is exerted by a magnetic field on a stationary charged particle.

26. What is the difference between AC and DC?    (1 Mark)

A: DC is direct current. This flows only in one direction. AC is alternating current. This reverses its direction of flow periodically.

27. What is the principle of an electric motor?    (1 Mark)

A: * Electric motor works on the principle of force acting on the current carrying coil kept in a magnetic field.

* Electric motor converts electrical energy into mechanical energy.

28. What is the working principle of a generator?   (1 Mark)

A: * A generator works on the principle of electromagnetic induction.

* If converts mechanical energy into electrical energy.

29. Mention the difference between AC generator and DC generator.   (1 Mark)

A: * The ends of the coil are connected to two slip rings in an AC generator.

* The ends of the coil are connected to two half slip rings in a DC generator.

30. If current in a coil flows in an anti clockwise direction what would be the nature of the face of the coil?    (1 Mark)

A: The face of the coil behaves as a north pole.

31. If current in a coil flows in clockwise direction what is the nature of the face of the coil?   (1 Mark)

A: The face of the coil works like a south pole.

32. Is AC dangerous or DC?   (1 Mark)

A: AC is more dangerous.

Hans Christian Oersted (1777 - 1851): He was one of the leading scientists of the 19th century, played a crucial role in understanding electromagnetism. He gave lectures which were quite popular among the public and also learnt a lot during the tours. During one such lecture in April 1820, Oersted carried out an experiment that was never performed before. He placed a compass needle underneath a wire and then turned on electric current. The needle of compass showed deflection.

Oersted recognised the significance of what he had just done. Earlier, it was believed that electricity and magnetism were two different unconnected sciences. Oersted had demonstrated that they were interconnected. Through this observation he showed that electricity and magnetism were related phenomena. Some scientists, influenced by this experiment, continued to work in the modern field of "electromagnetism". Their research resulted in several new scientific theories and various vital inventions like the dynamo and the electric motor, with this a new technology prospered, leading to inventions such as radio, television and fiber optics.

The unit of magnetic field strength is named Oersted in his honour.

Oersted was made a foreign member of the Royal Swedish Academy of Sciences in 1822.

Writer: C.V. Sarveswara Sarma

Posted Date : 28-05-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.