Question

Kilowatt is the unit of electrical ____________ but kilowatt-hour is the unit of electrical ______.
(A). Power, energy
(B). Power, electric potential
(C). Power, current
(D). Energy, power

Answer 1

Hint: Try to understand the concept of electric power. Learn the formulas or equations involved and try to express the unit in different forms. In some questions you can try to convert the derived quantities in terms of the fundamental quantities and then you can find the unit. Then try to find the unit for electric energy and compare them with the energy.

Complete step by step solution:
Electric power of an electric circuit can be defined as the rate of work done or the work done per unit time of the electric circuit. The unit of electrical power is watt.
Electric power can be given by the equation,
$\text{Electrical power = }\dfrac{\text{work done}}{\text{time}}$
Electrical work done (or energy) can be defined as the amount of energy required to move an electric charge Q through a potential difference V from one point to another point.
 $E=QV$
Where e is the electrical energy or work done, Q is the charge and V is the potential difference or voltage
Putting this in the above equation,
$\text{electrical power, P = }\dfrac{QV}{t}$
Again, current can be defined as the flow of charge per unit time,
$I=\dfrac{Q}{t}$
So, we can write,
$P=V\dfrac{Q}{T}=VI$
$P=VI$
Where P is the electrical power, V is the voltage and I is the current
The unit of power is watt.
1kilowatt is equal to 1000 watts.
The kilowatt-hour is a unit of energy. kilowatt-hour is a unit of energy representing 1000 Watt of power expended for 1 hour of time.
Power in kilowatt(kW) can be converted to energy by multiplying power in kilowatt by time in hour(s)
So,
$\text{Energy}=\text{(Powe}{{\text{r)}}_{\text{W}}}\times \text{(Tim}{{\text{e)}}_{H}}$ ------- (1)
$E={{P}_{W}}\times {{T}_{H}}$
Converting into units we can write the equation as,
$\text{energy}=\text{kilowatt}\times \text{hour}$
So, the correct option is (A).

Note: From ohm’s law we have V=IR, where R is the resistance.
So, we can also write the formula for electric power as,
$\begin{align}
  & P=VI \\
 & P=IR\times I \\
 & P={{I}^{2}}R \\
\end{align}$
Power can be defined as work done per unit time (or energy per unit time). So, we can write energy as a product of power and time.