(a) State and explain Joule’s law of heating.
(b) A resistance of 40 ohms and one of 60 ohms are arranged in series across 220 volt supply. Find the heat in joules produced by this combination of resistances in half a minute.
(a) Joule's law of heating states that heat produced in joules when a current of I amperes flows in a wire of resistance R ohms for time t seconds is given by H = I2Rt.
Thus the heat produced in a wire is directly proportional to:
(i) Square of current
(ii) Resistance of wire
(iii) Time for which current is passed
(b) Given: R1 = 40 ohms, R2 = 60 ohms (in series), V = 220V, t = 30 sec
We know that
Total resistance, R = 40 + 60
= 100 ohms
By Ohm's law,
V = IR
I = \(\frac{V}{R}\)
I = \(\frac{220}{100}\)
= 2.2 amp
Putting the values of I, R and t in eq. H = I2RT
H = 2.22 x 100 x 30
H = 14520J