Question

The unit of energy is
A. J/s
B. \[{\text{watt}} \times {\text{day}}\]
C. kilowatt
D. \[{\text{g cm/se}}{{\text{c}}^{\text{2}}}\]

Answer 1

Hint: In simple words energy means the capacity or ability to do work. Its SI unit is the joule.

Complete solution:
First of all, there are two important terms to understand that is, power and energy. Energy is the ability to do work and power is the rate at which work is done (or the rate at which energy is transferred).
The SI units of energy and power are joule (J) and watt (W) respectively. To convert power units to energy units we need to multiply the power units with the time units.
For example,
\[\begin{array}{*{20}{l}}
  {\left( {1{\text{kilowatt hour }} = {\text{ }}1000{\text{ watts and }}1{\text{ hour }} = {\text{ }}1 \times 60 \times 60{\text{ }} = {\text{ }}3600{\text{ seconds}}} \right)} \\
  {1{\text{ kilowatt hour }} = {\text{ }}1000{\text{ watts}} \times 3600{\text{ seconds }} = {\text{ }}3600000{\text{ joules}}} \\
  {{\text{Similarly,}}} \\
  {\left( {1{\text{ day }} = {\text{ }}24{\text{ hours }} = {\text{ }}24 \times 60 \times 60{\text{ }} = {\text{ }}86400{\text{ seconds}}} \right)} \\
  {1{\text{ watt}} \times {\text{day}}\;\; = {\text{ }}1{\text{ watt}} \times 86400{\text{ seconds }} = {\text{ }}86400{\text{ joules}}}
\end{array}\]
This means \[{\text{watt}} \times {\text{day}}\]is the required answer. Here, watt is the unit of power and day is the unit of time.

So, the correct answer is option B.

Note: There are various forms of energy such as heat energy, sound energy, light energy, potential energy, kinetic energy, nuclear energy, and so on.
According to the law of conservation of energy, it can neither be created nor destroyed but can be transformed from one form to another. For example, when we switch on an electric bulb, the electric energy is converted to heat energy then to light energy.
SI units of both energy and work have done are the joule.