An incandescent electric bulb converts 20% of its power consumption in

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 A current flows through it. If the bulb remains ON for 10 h and the rate of electricity charge is ₹5/unit, then which among the following is the correct amount for the money spent on producing light ?

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₹5

₹6

₹7

₹8

Answer is Right!

Answer is Wrong!

This question was previously asked in

UPSC NDA-2 – 2024

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The power consumed by the incandescent bulb is calculated using the given resistance and current. The total energy consumed over 10 hours is then calculated. Since only 20% of the power (and thus energy) is converted into light, the energy spent on producing light is 20% of the total energy consumed. The cost is calculated based on the rate per unit (kWh) for this amount of energy.

Power consumed (P) = I²R = (2 A)² * 200 Ω = 4 * 200 = 800 W = 0.8 kW.
Total energy consumed (E_total) = Power * Time = 0.8 kW * 10 h = 8 kWh.
Total cost of electricity = E_total * Rate = 8 kWh * ₹5/kWh = ₹40.
Energy converted into light (E_light) = 20% of E_total = 0.20 * 8 kWh = 1.6 kWh.
Cost spent on producing light = E_light * Rate = 1.6 kWh * ₹5/kWh = ₹8.

Incandescent bulbs are inefficient in converting electrical energy into visible light; a large portion of the energy is dissipated as heat, which is why the question specifies that only 20% is converted to light. The remaining 80% (1.6 kWh * 4 = 6.4 kWh) is dissipated as heat, costing ₹32 (₹40 – ₹8).

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