(a) What is meant by “electric power”? Write the formula for electric power in terms of potential difference and current.
(b) The diagram below shows a circuit containing a lamp L, a voltmeter and an ammeter. The voltmeter reading is 3 V and the ammeter reading is 0.5 A
(ii) What is the power of the lamp?
(a) The rate at which electrical work is done or the rate at which electrical energy is consumed, is known as electric power.
It is given by
P = VI = watt
(b) Given: V = 3V, I = 0.5 amp
(i) R = ?
We know that V = IR
3 = 0.5 R
R = 6 ohms
(ii) Power of lamp = VI
= 3 x 0.5
= 1.5 watt
(c) One kilowatt hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour.
1kWh = 3.6 x 106J
(d) Given P = 500W = 0.5kW, t = 20 hr
We know that
Energy consumed = P x t
= 0.5 x 20
= 10kwh
Total cost = 10 x cost per unit
Cost per unit = Rs. 3.9 per unit
Therefore, total cost = 10 x 3.9
= Rs. 39