Question

Find the cost of operating an electric toaster for 2 hours, if it draws a current of 8A on a 110V circuit. The cost of electric energy is Rs. 2.50 per kWh.
A. Rs.4.40
B. Rs.6
C. Rs.8.20
D. Rs.2.20

Answer 1

First using current and voltage find the power. Then, find the energy consumed in 2 hours by an electric toaster. Use the obtained energy to find the total unit consumption. This will give the total unit consumed by the toaster. Multiply it with the cost of electric energy per kWh and obtain the total cost of operating an electric toaster for 2 hours.

Formula used:
$Power= Voltage \times Current$
$E= Power \times time$
$Total\quad consumption= \dfrac { E }{ 1 kWh }$

Complete step by step answer:
Given: Voltage V = 110V
Current I = 8A
Cost of electric energy= Rs.2.50 per kWh
Power is given by,
$Power= Voltage \times Current$ …(1)
Substituting the values in above equation we get,
$Power=110 \times 8$
$\therefore Power= 880W$
Energy consumed in 2 hours is given by
$E= Power \times time$
$\therefore E= 880 \times 2 \times 3600$
$\therefore E= 6.34 \times {10}^{6}J$
Total unit consumption is given by,
$Total\quad consumption= \dfrac { E }{ 1 kWh }$ …(2)
But, $1kWh=3.6\times { 10 }^{ 6 }J$
Thus, substituting values in the equation. (2) we get,
$Total\quad consumption= \dfrac { 6.34\times { 10 }^{ 6 } }{ 3.6\times { 10 }^{ 6 } }$
$Total\quad consumption= 1.76kWh$
Cost of per kWh electric energy is Rs.2.50. Thus, cost of 1.76kWh energy is,
$Cost= 1.76 \times 2.50$
$\therefore Cost= 4.40$
Thus, the cost of operating an electric toaster for 2 hours is Rs.4.40.
Hence, the correct answer is option A i.e. Rs.4.40.

Note:
Make sure you convert time from hours to seconds. Students may get confused between kW and kWh. Both the units are different. kW stands for kilowatt. It is a unit of power. While, kWh stands for kilowatt-hour. It is a unit of energy. I kWh can be defined as a unit which is equal to the energy consumed by a device of 1 kW power for 1 hour. I kW is equal to 1000W which is equal to 1000 Joules per second.