alleryA 99 W bulb is kept switched on for 10 hours. How many units (kWh) of electrical energy does it consume?
A9.9 kWh
B990 kWh
C0.99 kWh
D0.099 kWh
Answer & Solution
Correct answer: C. 0.99 kWh
1. Energy in watt-hours: $E = P \times t = 99\ \text{W} \times 10\ \text{h} = 990$ Wh.
2. Convert to kilowatt-hours: $990\ \text{Wh} = \dfrac{990}{1000}$ kWh.
3. $E = 0.99$ kWh, i.e. 0.99 units.
4. The option 9.9 kWh wrongly keeps the result in Wh-scale; dividing 990 by 1000 gives 0.99, not 9.9.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 4 "Effects of Electric Current", p.60_
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