Question

URGENTLY...........

14. State and derive joulc'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

Joules Law states that the heat produced in the conductor is always directly proportional to:

1) the square of the current flowing through the conductor.

2) resistance of the conductor.

3) time for which the current flows through the conductor.

Deriving Joules Law

Potential difference V = Work done W / Charge moved Q

V = W / Q

W = V * Q.......1

Now

P = W / t.

P = V * Q / t ..... from 1

P = V * I ......since Current I = Q / t

.........2

Now,

P = W * t

W = P * t

W = VI * t

By ohm's law

V = I * R

therefore

W = I*R*I * t

W = I^2 *R *t

But as all the work done to move unit charge offered resistance gets converted into heat energy

H = I^2 *R*t

Now

1) In the first case

Energy rating means Power.

Now Power = 420 W

Potential difference= 220 V

Now

P = V*I

I = P / V

= 420/220

= 21/11

I = 21 / 11 Amperes

Now,

V = I * R

220 = 22/21 * R

R = 220 * 21 / 22

R = 210 ohms

2) In the second case

Power = 180 W

potential difference= 220 V

P = V*I

I = P / V

180 = 220/I

I = 180/220

I = 9/11 Amperes

Now,

V = I * R

220 = 9/11 * R

R = 220*11/9 ohms.