Question

$1Wh$ (Watt hour) is equal to:
a. $36 \times {10^5}J$
b. $36 \times {10^4}J$
c. $3600J$
d. $3600J{s^{ - 1}}$

Answer 1

Hint: Watt-hour is the unit of electrical energy. Remember, $1Watt = 1J{s^{ - 1}}$ and $1hour = 60 \times 60\operatorname{s} = 3600s$

Step by Step Solution:
This problem requires unit conversions to approach the solution.
Firstly, we will need to find which quantity is being represented in the given problem.

Watt hour is a unit of energy that is equal to one watt of power consumed for one hour. Mostly, it represents electrical energy. This unit is used widely in electrical appliances.
Therefore, the quantity being represented in the problem is energy.

The SI unit of energy is Joules$(J)$.
One joule is the energy transferred to a body when a force of $1$ Newton is exerted to move a body by a distance of $1$ metre.
$1J = 1 Newton - metre$
Energy is also equal to the product of the power consumed and the total time taken.
$1J = 1Watt - second$
We also know that,
$1Watt = 1J{s^{ - 1}}$ and $1hour = 60 \times 60\operatorname{s} = 3600s$

Now to approach our solution,
$1Watt - hour$ should be converted into $Joules$.
The units will be considered into observation and will be converted into the SI forms to finally get converted into $Joules$.
$1Watt - hour = 1 \times 3600Watt - seconds = 1 \times 3600J$
$1Watt - hour = 3600J$

$\therefore $ Option (C) is correct.

Additional Data:
The Other units of energy are Electron-volts $(eV)$, kilo-watt hours$(kWh)$, therms, Calories etc.

Note: The students may commit mistakes if they don’t have a proper knowledge of the unit systems and the quantities that are represented by these units.
The quantities like Energy, power, force and time hold a special role in this problem. Their units can be derived by knowing their brief definitions and further can be interrelated to reach conclusions.
But, the main point to remember is that in what context these units are being used. For example, $Watt - hour$ and $Electron - Volts$ are used to represent electrical energy and $Calorie$ is used to represent heat energy.