Important Questions

1. How can you verify that a current carrying wire produces a magnetic field with the help of an experiment?    (AS 3) 4 Marks
A: Required Apparatus: Copper wire, 3 volt battery, key, thermocol sheets, wooden sticks, barmagnet, magnetic compass.
Procedure:
                              
‣ Take a thermocol 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 of 24 gauge so that it passes through these slits and make a circuit.
‣ The circuit consists of 3 volt battery, key and Copper wire which are connected in series as shown in the figure.
‣ Now, keep a magnetic compass below the wire. Bring a bar magnet close to the compass.
‣ The needle in the compass deflects. This deflection is due to magnetic field produced by bar magnet.
‣ Take the bar magnet far away from the circuit and switch on the circuit, observe the changes in compass.
‣ The needle in the compass deflects.
‣ This deflection is due to the magnetic field produced by current carrying wire.
 

2. How can you verify the Faraday's law of induction with the help of an experiment.  (AS 3) 4 Marks
A:

‣ Connect the terminals of a coil to sensitive ammeter (galvanometer) as shown in the figure given above.
‣ Push a bar magnet towards a coil, with its north pole facing the coil, the needle in the galvanometer deflects.
‣ It shows 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 has been setup in the coil in the opposite direction.
‣ If we use the end of south pole of a magnet instead of north pole, the experiment works just as described but the deflections are exactly reversed.
‣ Further experimentation enables us to understand that relative motion of the magnet and coil set 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 closed coil, the current is generated in the coil. This is one form of Faraday's law.

3. How can you verify experimentally the formation of magnetic field due to straight wire current? How can you determine the direction of field lines?  (AS 3) 4 Marks

A: Apparatus: Conducting wire, switch, battery, stand, compass needles - 10, wooden plank.

Procedure:
‣ Take a wooden plank and make a hole as shown in the figure. Place this plank on a table and now place a retort stand on the plank as shown in figure.
‣ 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 it should not touch the stand.
‣ Connect the two ends of the wire to a battery of 3 or 9 volts via switch.
‣ Place 6 to 10 compass needles in a circular path around the hole so that its centre coincides with the hole.
‣ Now on the switch and allow to flow the current.
‣ When current is passed through the wire, the magnetic needle deflects and it is directed as the tangent to the circle.
‣  If the current flows vertically in upward direction the field lines are in clock-wise direction. If the current flows vertically in downward direction the field lines are in anti-clock wise direction.
‣  The direction of the current and magnetic line of force can be explained with the help of right hand thumb rule.  

‣ According to the right hand thumb rule if you hold a current carrying conductor with your right hand such that, the direction of the thumb shows the direction of current, then other finger shows the direction of magnetic lines of force.

4. Compare by drawing the diagrams of line of force formed by bar magnet and lines of force formed by solenoid.   (AS1) (AS5) 4 Marks
A:
                      
       Comparisons among lines of force formed by bar magnet and lines of force formed by solenoid.

Magnetic lines of force formed by bar magnet	Magnetic lines force formed by solenoid
* Field lines are imaginary lines.	* Magnetic lines are also imaginary lines.
* These lines directed from North pole to South pole of bar magnet.	* These are directed from North to South of solenoid.
* Direction of these lines is from South to North inside the bar magnet.	* Direction of these lines are towards South to North inside the solenoid.
* These are closed loops.	* These are also closed loops.
* This is not uniform field.	* This is also not uniform field.

5. How can you verify experimentally that the magnetic lines of force are like closed loops?   (AS 3) 4 Marks
A:
                           
                                      Magnetic field lines
Aim: To show that magnetic lines are closed loops.
 

Apparatus: Horizontal table, white sheet of paper, compass needle, bar magnet.
 

Procedure:
* 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. You 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. You will get different curves as shown in the figure.
 

6. As shown in figure a coil is suspended. A bar magnet is moved perpendicular to the plane of the coil with north pole pointing the coil.     (AS1) (AS4) 4 Marks
                                              

a) What is the change in magnetic flux in coil?
b) What is the direction of induced current in coil with respect to bar magnet?
c) Draw the magnetic field lines at the end of bar magnet and induced field in coil.
d) Explain the reason for induced current.
A: a) As magnet moves closer to coil flux linkage with coil increases.
b) In the coil current induces in such a way that it opposes the bar magnet. It flows in anticlockwise direction i.e., North Pole will be created with respect to bar magnet.
                                              
c) Straight line - Field lines due to bar magnet. dotted lines - Filed lines due to induced field current in the coil.
                                              
d) The reason for induced current is Electromagnetic induction.
      Whenever there is a relative motion between a bar magnet and coil current will be induced in the coil.

7. Which device converts electrical energy into mechanical energy? Explain the working of the device. (AS1) 4 Marks
A: Electric motor is a device which converts electrical energy into mechanical energy.
Principle: Electric motor works on the principle that when a current carrying coil is placed in a magnetic field, a force acts upon it. When two equal parallel and opposite force acts on a body, it tends to rotate.
Working:
* Let coil ABCD be in horizontal position, when the key is switched on the current flows in the direction DCBA and then leaves through the brushes B2 via ring C2.
Applying Fleming's left hand rule.
                     

   (a) No force acts on arm CB and BA because as they are parallel to the magnetic field.
   (b) Arm AB will experiences a force in upward direction.
   (c) Arm CD will experience an equal force in downward direction.
   (d) A torque, thus acts on the coil and it rotates in anti-clockwise direction.
* While rotating, when the coil reaches the vertical position, brushes B1 and B2 lose contact with rings and the current stops flowing. But the coil does not stop rotating due to inertia of motion.
* When the coil passes the vertical position, the ring automatically change their position and came in contact with opposite brushes.
* This reverses the direction of the current through the coil, but the direction of current on right hand side will be the same.
* So, the force on right hand side is always upward and force on left hand side of the coil is always in downward direction.
* So, the coil continues to rotate anti-clockwise direction.
* The split rings in an electric motor acts as commutator, it reverses the direction of current flowing through the armature coil.

8. Draw a neat diagram of electric motor. Name the parts.   (AS5) 4 Marks
A:
            

9. Draw a neat diagram of an AC Generator. Name of parts (AS5) 4 Marks
A:
                

10. How can you verify experimentally that the formation of Magnetic field is due to circular coil. (AS 3) 4 Marks
A: Activity:
Aim: To study the pattern of the magnetic field lines produced by a current carrying circular coil.
Apparatus: Thin wooden plank, white paper, Copper wire, battery and compass.
Procedure:
             
* Take a thin wooden plank covered with white paper and make two holes on its surface as shown in figure.

* Pass insulated Copper wire through the holes and wind the wire 4 to 5 times through holes such that it looks like a coil as shown in the figure.
* The ends of the wire are connected to terminals of the battery through a switch and switch on.
* Now place a compass 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 you reach the edge of the plank.
* Now do the other side of the coil from the centre
* Then draw a line joining the dots, you 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 circular coil.
 

Observation:
We observed that a pattern of the magnetic lines are formed around the wire.

11. Let us assume that we made a small magnet (M) to fall through the metallic ring.
(i) Explain the direction of flow of current induced in the metallic ring as a result of movement of the magnet.
(ii) We know that acceleration due to gravity is g. How will the magnetic acceleration changes when the bar magnet comes near or moves away from the ring?
(iii) What changes you observe when the magnetic poles are inverted? (AS1) (AS4) 4 Marks
A: (i) When a bar magnet is pushed towards a coil with its north pole facing the coil and induced current is set up in the coil. The direction of induced current is anti clockwise direction with respect to north pole of bar magnet.
(ii) When the bar magnet comes near to the ring then the magnetic acceleration is away from the ring. When the bar magnetic moves away from the ring then the magnetic acceleration is into the ring.
(iii) When magnetic poles are inverted then the direction of induced current also inverted (reversed) that means the direction is in clockwise.

12. As shown in figure of a bar magnet is moved towards a coil, current is induced then (a) What is the direction of induced current? (b) Draw the magnetic field lines created by bar magnets pole at coil and also lines formed by the coil.   (AS1) (AS4) 2 Marks
A: (a) In the coil North pole will be created to indicate North pole and backside South pole. The direction of induced current is anti-clockwise.
(b)
                                    

13. Mention the method of preparing a solenoid with Copper wire on a wooden plank to find out the electric field formed by a solenoid.  (AS 3) 2 Marks
A:
                    

* Take wooden plank covered with white paper. Make equidistant holes on its surface as shown in the above figure.
* Pass Copper wire through the holes as shown in the figure. 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 a small jerk to it. An orderly pattern of iron fillings is seen on the paper.

14. Where do we use Faraday's law in our daily life? (OR) Mention few applications of electromagnetism in the daily life. (AS6) 2 Marks
A: * Faraday's law are more useful in production and transmission of electricity and also we are applying these law in A.C. Generator. Back currents in electric motors, Self induction, Mutual induction, transformers.
* Check gates that are arranged at temples and other important places.
* Recording and reproducing of sounds on tape records.
* To read magnet bar when we swipe our ATM Card.
* An induction stove works on the principle of electromagnetic induction.
 

15. Take a long cylindrical Copper tube. Hold it so that it is perpendicular to the horizontal. A stone and a bar magnet are left freely, so the bar magnet pass through the tube and stone outside the tube. Which one reaches the earth first. Guess why it is so? Give proper reasons.  (AS2) 2 Marks
A: * Stone will reach the ground quickly.
* Because when magnet moves through the Copper cylinder, assume the Copper cylinder is a combination of Copper rings.
* When the magnet moves through the rings, current is induced in the rings.
* It opposes the movement of the magnet according to Lenz's law.

16. A spring is suspended as shown in diagram. A battery and switch are connected between the ends of the spring. What happens when the switch is closed? Guess the answer.    (AS 2) 2 Marks

A: * The magnetic lines set up in the spring.
* This is known as solenoid.
* The solenoid behaves like a bar magnet.
* Hence each ring acts as dipole. So mutual attraction exists between the rings and spring compresses.
 

17. How the tape of a tape recorder reproduces voice? (AS1) (AS6)2 Marks
A: * The tape recorder which we use to listen songs or record voices works on the principle of electromagnetic induction.
* It consists of piece of a 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.

* This small current will be converted into sound by speaker.
* Thus the voice will be reproduced from the tape of a tape recorder.
 

18. Explain the working of induction stove. (AS1) (AS6) 2 Marks
A: * An induction stove works on the principle of electromagnetic induction.
* A metal coil is kept just beneath the cooling surface. It carries alternating current (AC). So that AC produces an alternating 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.
* Since pan has a finite resistance, the flow of 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.
 

19. Mention two uses of solenoid. (AS6) 1 Mark
A: It is used in electric bells, fans and motors.

20. Identify the poles of a bar magnet in the figure. (AS5) 1 Mark
                                           
A:
                                           

21. Identify the poles of a bar magnet in the figure.  (AS5) 1 Mark
           
A:
           

22. What are the modifications that are to be made to change AC generator as DC generator?  (AS1) 1 Mark
A: Two half slip rings are to be connected to the ends of the coil in place of full slip rings. Then AC generator works as DC generator to produce DC current.

SHORT QUESTIONS

1. What are the devices which works on the magnetic effects of electric current? (AS6) 1 Mark
A: Electric motors
 

2. What are the devices which works on the electric effects of moving magnets? (AS6) 1 Mark
A: Electric Generators
 

3. Name the components used in oersted experiment? (AS3) 1 Mark
A: Thermocole sheet, wooden sticks of height 1 cm, copper wire, 3 (9) V battery, key, magnetic compass.
 

4. write your observations from oersted experiment. (AS3) 1 Mark
A: i) Deflection of magnetic needle in magnetic compass.
     ii) Current carrying copper wire behaves as magnet

5. What happens if we replace iron wire instead of copper wire in the oersted experiment?  (AS2) 1 Mark
A: Iron wire also behaves as magnet because the electric current flows in it.
 

6. What happens if we use nylon thread instead of copper wire in the oersted experiment?    (AS2) 1 Mark
A: Nylon thread does not behave as magnet because the electric current does not flow through it.
 

7. When a force gets applied on an object by another object without there being any physical contact what is the field that is induced give an example? (AS1) 1 Mark
A: Magnetic Field
 

8. Demonstrate an experiment to show that magnetic field is three dimensional? (AS3) 2 Marks
A: i) Take a bar magnet.
     ii) Take a magnetic compass and place it in all possible directions like left, right, up and down of the magnet with in its field and observe the deflection of magnetic needle in the compass.
     iii) The magnetic needle of the compass shows the deflection in all directions.
     iv) This ensures that magnetic field is three dimensional.