Joule’s law

When current flows through a resistor, some of the electrical energy delivered to the resistor is converted into heat energy and it is dissipated. This heating effect of current is known as Joule’s heating effect. Just as current produces thermal energy, thermal energy may also be suitably used to produce an electromotive force. This is known as thermoelectric effect.

Joule’s law

If a current I flows through a conductor kept across a potential difference V for a time t, the work done or the electric potential energy spent is

In the absence of any other external effect, this energy is spent in heating the conductor. The amount of heat(H) produced is

This relation was experimentally verified by Joule and is known as Joule’s law of heating. It states that the heat developed in an electrical circuit due to the flow of current varies directly as

(i) the square of the current

(ii) the resistance of the circuit and

(iii) the time of flow.

EXAMPLE 2.27

Find the heat energy produced in a resistance of 10 Ω when 5 A current flows through it for 5 minutes.

Solution

R = 10 Ω, I = 5 A, t = 5 minutes = 5 × 60 s

H = I2 R t

= 52 Ã— 10 × 5 × 60

=25 × 10 × 300

=25 × 3000

=75000 J (or) 75 kJ