Heating effect of current: the phenomenon of the production of heat in a resistor by the flow of electric current through it is called heating effect of current or joule heating.

Cause of heating effect of current:

When a potential difference is applied across the ends of a conductor, its free electrons get accelerated in the opposite direction of applied field. But the speed of the electros does not increase beyond a constant drift speed. This is because during the cause of their motion, the electrons collide frequently with the positive metal ions. The kinetic energy gained by the electrons during the intervals of free acceleration between collisions is transferred to the metal at the time of collision. The metal ion begin to vibrate about their mean positions more and more violently. The average K.E. of ions increases. This increases the temp. of the conductor. Thus the conductor gets heated due to the flow of current.

Expressions: when a potential difference V is applied across a conductor of resistance R and a current I is flowing through it as shown in fig.

The amount of charge flowing from a to Bin time t is

q = It………………… (1)

work done in flowing q from A to B = V*q

or W = VIt

as V = IR

w = I²RT

this work done appears as heat. So the amount of heat produced is

H = I²RT joules

Electric power: it is the rate at which an electric appliance coverts electric energy in to other form of energy.

It is the rate at which work is done by a source of emf in maintaining an electric current through a circuit.

Electric power, P = w/T = VI = I²R = V²/R

S.I unit of power is watt:

The power of an appliance is one watt if one ampere of current flows through it on applying a potential difference of 1 volt across it.

1w = 1 J s^-1= 1VA

1kW = 1000 W

Electric energy: it is the total work done in maintaining an electric current in an electric circuit for a given time.

Electric energy = Electric power * time

W = P * T = VIt joules or I2Rt joules

Unit of electric energy:

The commercial unit of electric energy is kilo-watt hr or board of trade (B.O.T.). it is the electric energy consumed by an appliance of power 1000 watt in one hr.

1 unit = 1 Kwhr = 1000w hr = 1000 W * 3600 sec

= 3.6 * 10⁶J