Question

QUESTION
State and derive joule’s law. An electric iron consumes energy at rate of 420w when heating is at maximum rate and 180 w when heating is at minimum. The voltage is 220V .What is the current and resistant in each case?

Answer 1

(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=40ohms, R2=60ohms (in series), V=220V, t=30sec

We know that

Total resistance, R=40+60=100ohms

By Ohm's law,

V = IR

I = V/R

I = 220/100 = 2.2amp

Putting the values of I, R and t in eq.

H = I2RT

H = 2.22 X 100 X 30

H = 14520 J