Question

Kaveri conducted an experiment to study the energy efficiency of different bulbs. She connected a bulb A having a resistance of 100 ohms to a 240 V power supply in a laboratory.

(a) How much energy will be consumed by the bulb, if it is kept ON for 4 hours each day for a week? Express your answer in kJ.

(b) Kaveri connects another similar bulb B in series with bulb A and connects the combination to a 240 V supply. Will there be any change in the brightness with which bulb A glows now? Explain mathematically.

Answer 1

(a) Given V = 240 V and R = 100 ohms 

Therefore, 

Power (P) = V2/R = (240)²/100 

= 576 W  

Energy consumed by bulb A = P × t 

E = 576 x 4 x 7 × 60 × 60 

E = 58,060.8 kJ 

(b) When bulbs A and B are connected in series: 

R_net = R1 + R2 

= 100 + 100 

R_net = 200 ohms 

Total power consumed by bulb A when connected in series with bulb B 

P_tot = V²/R_net = (240)²/200 = 288 W 

P_A' = Ptot/2 = 144 W 

Power consumed by bulb A when connected without bulb B to 240 V 

P_A= V²/R 

= (240)²/100 

= 576 W 

As PA' < PA, the brightness of the bulb A decreases when connected in series with bulb B.