On passing electric current through the filament of an electric bulb, it gets heated and glows brightly. Similarly, on passing current through an electric heater, the coil of the heater becomes red hot. Why? It is because in an electric circuit, electrical energy is converted into heat energy. This effect is known as thermal effect of electric current or Joules' heating.
Consider a conductor XY of resistance R. Let current 'i' is passed for t seconds through the conductor on applying a potential difference V across the ends X and Y. If the charge Q is to be transferred from point X to Y, the work is done in moving the charge Q across the ends of the conductor. Work done in transferring the charge Q,
W = Potential difference (V) × Charge (Q)
Since, Q = it
W = Vit
According to Ohm's law, V = iR
W = (iR)it
W = i2Rt
Here, the work done in moving the electric charge across a resistance appears in the form of heat. Therefore, the heat produced in the conductor is
H = i2Rt
The amount of heat produced in a conductor on passing the current i is directly proportional to the square of the current (i2), the resistance of the conductor (R) and the time (t) for which the current flows through the conductor. This is known as Joule’s law of heating. SI unit of heat is joule (J) (4.18 J = 1 cal).
There are many household appliances based on thermal effect of electric current. For example, electric iron, electric kettle, electric immersion rod, electric geyser, cooking range, electric oven, electric toaster, electric stove, room heater.
Beside appliances heating effect of electric current is also used in electric fuse, electric welding and electric arc. In all these appliances potential difference is applied across a conductor, the free electrons inside the conductor get accelerated and during the course of their motion electrons collide with other electrons and atoms or ions of the material of the conductor on their way and transfer their energy to them.
The electrons move with constant drift velocity and do not gain kinetic energy. But due to collision with free electrons, the atoms or ions begin to vibrate with increased amplitude. In other words, the average kinetic energy of vibrations of the atoms of conductor increases which results in increase in temperature of the conductor i.e., the heat is produced in the conductor.
Thus, on applying potential difference, loss in potential energy of the electrons appears in the form of increase of average kinetic energy of the atoms of the conductor which finally appears as heat energy in the conductor.