Lesson 3, Topic 3
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Electric Power

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Electric Power

Electric power is the time rate of transfer of energy or time rate of doing work

Power = Energy expended time $$\frac {Energy \; expended}{time}$$

= Energy expended time $$\frac {Work \; done}{time}$$

P = $$\frac {IVt}{t}$$

= IV

P = $$\frac {V^2}{R}$$

= I2R

Electrical power is measured in watt (W) or J/s. Larger units are in kilowatt, 1kw = 1000w.

Electrical energy is sold in commercial units of kilowatthour (KWh)

Examples

1. An electric kettle is rated 1000w, 220v, calculate the resistance and current supplied.

Solution: P = 1000w, V = 220V, I = ?

P = IV

I = $$\frac {P}{V} = \frac {1000}{220}$$

= 4.54A

(ii) V = IR

R = $$\frac {V}{I} = \frac {220}{4.54}$$

= 48.5Ω

2. A PHCN supplied power to a household as follows;

40wbulb (6), Freezer (750w) and a TV set, 60w for 8hours. If 1kwh is sold at ₦20.50k, calculate the cost of supplying energy for 8hrs.

Solution

Bulb = 40w x 6 = 240w

Freezer = 750 x 1  = 750w

TV Set       =   60w

1050w

Ikw = $$\frac {P}{V} = \frac {1050}{1000} = \scriptsize 1.050kw$$

For 8hours, 1.050 x 8 = 8.40kwh

Cost of 8hours            = 8.40 x 20.50

= ₦172.20k

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