Question

A table lamp of power $60W$ consumed 9 (commerical) units of electricity in the month of April. For how many hours per day, on an average, was the lamp in use?

• A. 1 h
• B. 3 h
• C. 5 h
• D. 6 h

Answer 1

The correct option is C 5 h

1 unit is $3.6\times {10}^{6}J$

Let us suppose it has been used for $y$ hours a day.

Energy consumed in $y$ hours = $Power\times Time$

$E=60 \ W \times y\times 60\times 60$ ,

$E=\left(60W\right)\times (y\times 60\times 60seconds)$

$y$ is in hours, so converting it into seconds

So $E=60\times 60\times 60\times y$ Joules

Now it has been used for whole month so energy consumed per month:

$=E\times 30$ Joules

$=60\times 60\times 60\times y\times 30$ Joules

Now this energy is equal to 9 commercial units.

$60\times 60\times 60\times y\times 30$ Joules $=9\times 3.6\times {10}^6$ Joules

$\Rightarrow y=\frac{9\times 3.6\times {10}^6}{36\times 9\times 2\times {10}^4}=\frac{10}{2}$

So $y=5hrs$

Option C is the answer.