Question

An electric lamp is marked $60W$, $220V$. The cost of kilowatt hour of electricity is $\text{Rs}\text{. 1}\text{.25}$ the cost of using thing lamp on $220V$ for $8\text{ hours}$ is:
$\begin{align}
  & \text{A}\text{. Rs}\text{. 0}\text{.25} \\
 & \text{B}\text{. Rs}\text{. 0}\text{.60} \\
 & \text{C}\text{. Rs}\text{. 1}\text{.20} \\
 & \text{D}\text{. Rs}\text{. 4}\text{.00} \\
\end{align}$

Answer 1

Hint: First convert the power in watts to kilo-watt. Now electrical energy consumed by the lamp in 8 hours can be calculated by the product of power with the time. As the power is in kilo-watt and the time is in hours you will get energy consumed by the lamp and will be in a kilo-watt-hour unit. Cost of one kilo-watt-hour of electricity is given. You just multiply this cost with the total energy consumed by the lamp to get the cost of using this lamp.

Formulas used: Electrical power is given by
$P=Vi={{i}^{2}}R=\dfrac{{{V}^{2}}}{R}$
Electrical energy consumed is given by
$E=Pt=Vit={{i}^{2}}Rt=\dfrac{{{V}^{2}}}{R}t$

Complete answer:
Given in the question is that the value of the power of the lamp is
$P=60W=\dfrac{60}{1000}kW=0.06kW$
But we know that the because $1Watt=\dfrac{1}{1000}Watts$
So the electrical energy consumed by the lamp in $8\text{ hours}$ is
$E=Pt=0.06kW\times 8hour=0.48kWh$ .
We are also given that the price per kilowatt hour of electricity is $\text{Rs}\text{. 1}\text{.25}$.
So for $0.48kWh$ of energy the cost will be $0.48\times 1.25=\text{Rs}\text{. }0.60$

So the correct option is B.

Note:
In this question the power is given. But in some problems power is not given so use the formula for electrical power to calculate the power of the instrument. The power calculated will be in watts. Convert the power to kilo-watt. The energy can be calculated in kilo-watt-hour. If the cost of one kilowatt hour is given the total cost can be calculated. Make sure all the variables are in the same unit system to avoid mistakes.
Also note that one kilowatt hour is called Board Of Trade unit.