Question

Watt-hour meter measures _________.
A) electric energy
B) current
C) voltage
D) power

Answer 1

A Watt-hour meter must measure that physical quantity which has the unit of Watt-hour. The S.I unit of electrical energy is Joules, the S.I unit of current is Ampere, the S.I unit of voltage is Volts and the S.I unit of power is Watt. The definition of power which gives power as the work done over time might help us to choose the correct option.

Formula used:
-The power of a body is given by, $P = \dfrac{W}{t}$ where $W$ is the work done by the body and $t$ is the time taken.

Complete step by step solution:
Step 1: Express the relation for the power of a body.
Power is defined as the amount of work done by a body in a given interval of time. So we can express the power of a body as $P = \dfrac{W}{t}$ --------- (1) where $W$ is the work done by the body and $t$ is the time taken.
Now the S.I unit of power is Watt while that of work is Joules and time is seconds.
So if we were to express equation (1) in terms of the S.I units of power, work and time then we have ${\text{W}} = \dfrac{{\text{J}}}{{\text{s}}}$
$\Rightarrow {\text{Ws}} = {\text{J}}$ -------- (2)
or in terms of hours we have $\dfrac{{{\text{Wh}}}}{{60 \times 60}} = {\text{J}}$
$\Rightarrow {\text{Wh}} = {\text{3600J}}$
Thus Watt-hour corresponds to the unit of work. Since the work done by a body is essentially converted into a form of energy, here we can say that Watt-hour is the unit of electrical energy and a Watt-hour meter will be used to measure electrical energy.

So, the correct option is A.

Note: To express equation (2) in terms of hours we used to conversion relation, $1{\text{s}} = \dfrac{{1{\text{h}}}}{{60 \times 60}}$. An ammeter is used to measure the current through some resistance while a voltmeter is used to measure the voltage in a circuit. The S.I unit of power is Watt. So we can conclude that a Watt-hour meter must measure the only other possibility which is the electrical energy.