Question

An electric iron is rated at 3kW. Electrical energy costs Rs. 4 per kWh. What is the cost of using the iron for 5 hours?
A) Rs.12
B) Rs.20
C) Rs.150
D) Rs.60

Answer 1

Hint: We can find the cost of using the electrical power using the amount of energy consumed for a given time. We need to analyse the units of energy given and calculate the power consumption for finding the cost of using that power.

Complete answer:
We know that Watt (W) is the unit of power. Every equipment in our house-hold are rated with a power rating in kiloWatts. It is the amount of energy used up by the device in unit time. The electricity boards that supply electricity to our sites collects the cost by monitoring the power consumption.
Usually, the power consumption in an hour has a rate, which is multiplied by the numbers of hours used.
We are given the power consumption rate of an electric iron as 3kW.
The energy used in an hour will be 3kWh by the electric iron. ‘kWh’ is the unit of energy used in power consumption measuring in house-holds.
It is given that the Electricity authority charges Rs.4 per kWh.
Now, we are given that the electric iron works for 5 hours.
\[\begin{align}
& \Rightarrow \text{ Cost of power consumed = Power rating}\times \text{Number of hours}\times \text{cost per hour} \\
 & \Rightarrow \text{ Cost= }3kW\times 5\times Rs.4 \\
 & \therefore \text{ Cost of power consumed }=Rs.60 \\
\end{align}\]

So, the correct answer is “Option D”.

Additional Information:
In commercial terms, ‘unit’ is the unit used for energy consumption. It is equivalent to 1kWh.

Note:
We know that kWh is a unit of energy, but often it is mistreated as power. Also, it is used as the power consumption unit in commercial aspects. As a Physics student, we should be careful in dealing with the units and should not get confused with them.