1) Electric current is defined as the rate of flow of electric charges through a surface area. The unit of current is the ampere. 2) Electric potential difference between two points is the work required to move a unit charge between those points. It is measured in volts. 3) Ohm's law states that the current through a conductor is directly proportional to the potential difference across its ends, provided the temperature remains constant. The relationship is defined by the formula I=V/R, where R is the resistance of the material.

1. Electric Current
ElectricCurrent is defined as the rateof flow of electric charges. Electriccurrent is thequantity of
chargeflowing per unit area.
t
Q
I 
Unit of current is Ampere
Electric Potentialand Electric PotentialDifference
Electric Potential is defined as potential per unit charge.
ElectricPotential difference betweentwo points A and B on a conductor throughwhich a current is
flowing is defined as the amount of work done to move a unit charge from A to B.
Q
W
V 
Unit of potential is volt(V).
Ohms Law
Ohm found out the relationship between the current I, flowing in a metallic wire and the potential
difference across its terminals. The potential difference, V, across the ends of a given metallic wire in
an electric circuit is directly proportional to the current flowing through it, provided its temperature
remains the same. This is called Ohm’s law. In other words –
IV 
RIV 
Resistance
The resistance of a conductor is the ratio of the potential difference across its ends to the current
flowing through it.
I
V
R 
Unit of Resistance is ohm (Ω)
Factors on which resistance of a conductor depends-
we observe that the resistance of the conductor depends (i) on its length, (ii) on its area of cross-
section, and (iii) on the nature of its material.

2. Precise measurements have shown that resistance of a uniform metallic conductor is directly
proportional to its length (l) and inversely proportional to the area of cross-section (A). That is,
A
R
lR
1


A
l
R 
A
l
R


 = resistivity of material
Why cord of electric heater does not glow while heating filament or element does?
Filament of heater is made of tungsten to have high melting point. Melting point of tungsten is 3380
degree Centigrade while melting point of copper is 1084 degree centigrade. Tungsten emits light at
2126 deg centi. As we have studied that high resistance produces more heat so we would like to
make tungsten wire of high resistance. According to
A
l
R

 , to have high resistance area of wire
should be made thin. So tungsten alloy along with high value of resistance is used as filament while
wire connecting heater from switch(cord) is made of metal which has low value of resistance that’s
why heating element is able to emit light while cord does not.
Resistance of System of Combination
1) Resistance in series combination
.........321  RRRRs

3. 2) Resistance in parallel combination
Resistors in parallel
321
1111
RRRReq

Heating Effect of Electric Current
Whenever a current is passed through a conductor, it becomes hot after sometimes. This means that
electric energy is being converted into heat energy. Electric bulb, electric iron, electric heater etc.
are some of appliances which are based on this effect called the heating effect of current or joule
heating.
tRIH 2

Where H = amount of heat produced(H)
where t=time in which current I flows
Electric Power
The rate of doing work by electric current is called Electric Power.
Consider a current I flowing through a resistor of resistance R. Let the potential difference across it
be V. Let t be the time during which a charge Q flows across. The work done in moving the charge Q
through a potential difference V is VQ. Therefore, the source must supply energy
IV
t
Q
VP 

4. Or the energy supplied to the circuit by the source in time t is P × t, that is, VIt. This energy gets
dissipated in the resistor as heat. Thus for a steady current I, the amount of heat H produced in time
t is H = VIt
Electric Energy
The total work done by a electric current in an electric circuit is called electric energy.