(a) Write Joule's law of heating.
(b) The lamps, one rated 100 W, 220 V , and the other 60 W, 220 W V, are connected parallel to electric mains supply. Find the current drawn two bulbs from the line,if the supply voltage is 220 V.

Explanation

(a) Joule's law of heating . When a current (I) flows through a resistor of resistance for time (t), then the amount of heat produced (H) is given by
`H = I^(2) xx R xx t`
This is known as Joule's law of heating.
The heat produced in the wire is directly proportional to :
square of current `(I^(2))`
resistance of wire (R )
time (t) for which current is passed.
Given : Lamp (1) = 100W , 220 V
Lamp (2) `= 60 W, 220 V`
`I_(1) = ?, , I_(2) = ?`
`V_(1) = V_(2) = V = 220 V`
`P_(1) = V_(1) xx I_(1)`
`100 = 200 xx I_(1)`
`I_(1) = 100/220 = 5/11 = 0.45 A`
`P_(2) = V_(2) xx I_(2)`
`60 = 220 xx I_(2)`
`I_(2) = 60/220 = 0.27 A`