1. Example
Example 1: An electric lamp is rated 110 W, 200 V. When the lamp is operated at its rated power and voltage, calculate
a) the current flowing through the lamp
Apply I = P V =
110 W
220 V
= 0.5 A

2. Example b) the resistance of the lamp V2 Apply R = P (220 V)2 110 W =
= 440 

3. Example
Example 2: The rating of a lamp of 2 V, 5 W. It has to be operated from a 9 V supply and a resistor R is connected in series so that the lamp can functions at its rated voltage and power. R
2 V, 5 W
9 V

4. Example a) (i) Find the resistance of the lamp. Resistance of the lamp= V2 P =
(2 V)2
5 W
= 0.8 

5. Example (ii) Find the current flowing in the circuit.
Current drawn by the bulb = P V =
5 W
2 V
= 2.5 

6. Example b) Find the value of R. Voltage across R = 9 V - 2 V = 7 V
Resistance of R = V I =
7 V
2.5 A
= 2.8 

7. Example c) Find the power lost in the resistor R. P = V I
= 7 V  2.5 A
= 17.5 W
The power lost in the resistor is 17.5 W

8. Example End d) Find the efficiency of this circuit. Efficiency=
output power of lamp
input power
 100 % =
5 W
9 V  2.5 A
 100 %
= 22.2 %
End