NDA 2 2024 GAT, question 93

Topic: Physics

Q93

An incandescent electric bulb converts $20\%$ of its power consumption into light, and the remaining power is dissipated as heat. The bulb’s filament has a resistance of $200\ Ω$ and $2\text{ A}$ current flows through it. If the bulb remains ON for $10\text{ h}$ and the rate of electricity charge is ₹ $5/\text{unit}$ , then which among the following is the correct amount for the money spent on producing light ?

(a) ₹ $5$
(b) ₹ $6$
(c) ₹ $7$
(d) ₹ $8$ Correct answer

Solution.

Answer: (d) ₹ $8$

Electrical power P=I²R=2²×200=800 W.
In 10 hours the bulb uses 0.8×10=8 kWh, costing ₹40 at ₹5 per kWh.
The 20% allocated to useful light energy costs 0.20×40=₹8.
The total electricity bill remains ₹40.