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

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

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

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

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

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 

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

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