* Lightening is an electric discharge between two cloud or between cloud and the earth. This electric charge from clouds to the earth through air appears to us as a electric spark or lightening in atmosphere. Lightening is a live example which provides evidence for the motion of charge in the atmosphere.
* The substance which transfers energy from battery (source) to the bulb is called a conductor and the substance which cannot transfer energy from battery (source) to the bulb is called a non-conductor.
e.g.: Copper wires are conductors while Nylon wires are non-conductors.
* Drude and Lorentz, scientists of the 19th century proposed that conductors like metals contain a large number of free electrons while the positive ions are fixed in their locations. The arrangement of the positive ions is called lattice.
* The net charge flowing through a cross section of a conductor in unit time is called electric current.
The SI units of electric current is ampere denoted by A.
Ampere is the electric current through a conductor. When a charge of one coulomb passes through it in one second.
Electrons in the conductor move with a constant average speed. This speed is called drift speed (or) drift velocity (Vd) .
n is the number of electrons in unit volume (electron density)
q is charge of 1 electron
A is the cross-sectional area of the conductor
I is current.
* For the flow of current through a conductor, one end of the conductor should be higher
potential than that of other end.
* Electric potential and potential difference
Electric potential at a point in space is defined as the work done in moving a single unit
positive charge from infinity without any acceleration from infinity to that point.
Work done by the electric force on unit positive charge to move it through a distance 'l' from A to B is called potential difference between those points. potential difference is denoted by a symbol 'V'.
* The SI unit of potential difference is Volt and it is denoted by V.
* Volt is the potential difference between two points, which one joule of work is to be done to move one coloumb of positive charge.
* e.m.f.: Electromotive force is defined as the work done by the chemical force to move unit positive charge from negative terminal to positive terminal of the battery.
d = distance between the terminals
* Electric current is measured using ammeter.
* Generally a Volt meter is used to measure potential difference or emf across an electric device like battery. It must be connected in parallel to the electric device to measure the potential difference, across the ends of the electric device.
Ohm's Law: The potential difference between the ends of a conductor is directly proportional to the electric current passing through it at constant temperature.
V ∝ I (temparature is constant)
* Resistance is the opposition that a substance offers to the motion of electrons.
* The SI unit of resistance is Ohm.
The symbol of Ohm is Ω
* Those which obey Ohm's Law are called Ohmic materials. For example, metals are Ohmic materials.
* Those which do not obey Ohm's law are called non Ohmic materials. For
example, LEDS are non ohmic materials.
(i) Human body is a resistor.
(ii) The resistance of the human body generally varies from 100 Ω (if body is wet with salt water) to 5,00,000 Ω (if the skin is very dry).
(iii) The current passing through our body when we took a line wire of 240 V is 0.00024 A. When this quantity of current flows through the body the functioning of organs inside the body gets disturbed. This disturbance inside the body is felt as electric shock.
(iv) Effects of the electric current on human body.
Current in Ampere
Can be felt
Causes involuntary muscle contraction (spasms)
Causes loss of muscle control
If through the heart causes serious disruption, probably total if current lasts for more than one second.
(v) Electric shock is a combined effect of potential difference, electric current and resistance of the human body.
* A multimeter is an electronic measuring instrument that combines several measurement functions in one unit.
Factors on which the resistance of a conductor depends:
* Resistance of a conductor increases with the increase of its temperature.
* Resistance of a conductor depends on the material with which it is made.
* Resistance of a conductor is directly proportional to its length.
* Resistance of a conductor is inversely proportional to its area of cross-section. or
ρ is the resistivity or specific resistance of the material.
* The resistance of a conductor of unit length and unit area of cross-section is the specific resistance of the material with which the conductor is made.
* The Unit of specific resistance of Ohm-metre (Ω - m)
* Specific resistance depends on the temperature and nature of the material.
* Reciprocal of resistivity is called conductivity.
* Unit of conductivity is mho/metre.
* The filament of an electric bulb is usually made of tungsten, because of its higher resistivity values and melting point (3422°C).
* The values of resistivity of insulators are very high of the order of 1014 to 1016 Ω - m.
* Alloys like Nichrome (Nickel, Chromium and Iron) and Manganin (86% Copper, 12% Manganese, 2% Nickel) have 30 - 100 times larger values of resistivity than those of metals. This makes them suitable for use in the heating elements of electric irons, toasters etc.
* Semi conductor (Si, Ge) are used to make diodes, transistors and integrated circuits (ICs).
* A closed path created by the connecting wires through a battery along which electrons can flow is called a circuit.
* Same electrical current passes through all the resistors connected in series.
* The voltage across a group of resistors connected in series is equal to the sum of the voltage across the individual resistors.
V = V1 + V2 + V3 .............
* The equivalent resistance of a series combination of resistors in equal to the sum of individual resistances.
Req = R1 + R2 + R3 ............
* Total current through the parallel combination of resistors is equal to the sum of currents through individual resistors.
I = I1 + I2 + I3 ...........
* When resistors are connected in parallel the reciprocal of equivalent resistance is equal to the sum of the reciprocals of the individual resistance.
* Resistance of a circuit increases when individual resistances are connected in series.
* Resistance of a circuit decreases when individual resistance are connected in parallel.
KIRCHHOFF'S CIRCUIT RULES:
Junction law: At any junction point in a circuit where the current can divide, the sum of the currents into the junction must equal the sum of the currents leaving the junction.
The loop law: The algebraic sum of the increases and decreases in potential difference across various components of a closed circuit loop must be zero.
* Rate of electric work done is electric power.
* Unit of electric power is 'Watt' (Joule/Sec)
* Electric power = voltage × current.
* Total electrical energy consumed.
W = Power × time
* Wattage of an electrical appliance is the rate at which electrical energy is consumed by it.
Wattage P = VI
* The unit of electrical energy consumed is Kilo-Watt-Hour. (KWH)
* 1 KW = 1000 Watts.
* 1 MW = 106 Watts.
* Watt-Hour is the electrical energy consumed by an appliance of 1 watt power for
* 1 KWH = 3.6 × 106 J
* All the electrical devices of our home are connected in parallel connection, potential drop across each device in 240 V.
* For 240 V, the maximum current that we can draw from the mains is 5-20 A
* When the current drawn from the mains is more than 20 A. Over heating occurs and may cause a fire. This is called over loading.
* To prevent damage due to over loading we cannot an electric fuse to the house hold circuit.
* The fuse consists of a thin wire of low melting point.
Conceptual flow chart: