Question

How much energy in joules and in KWh is consumed by a 120W light bulb if it is left on for 15 minutes.

Answer 1

Consumption of energy depends on time and as well as the power of the device. Energy consumption is actually the energy lost during the process. Energy consumed is basically a multiplication of power of the device and the time. If a device of 1000W is kept on for 1 hour then it will consume 1KW of energy. These are very basic terms that we use in our everyday life. You can check your electricity bill and all these terms will be written there.

Complete answer:
In this question we are given with some of the quantities
Time = 15 minutes or $\dfrac{15}{60}$ hours
Time = 0.25 hours
We know that
1KWh = 1000 Watts
Therefore, 120W = 0.12KW
Now, energy can be given by
Energy (E) = $0.12\times 0.25$
Energy (E) = 0.03 KWh
Now, we need to convert this into joules
We know that 1KWh = $3.6\times {{10}^{6}}Joules$
Therefore, 0.03KWh in terms of Joules can be written as
$0.03(3.6\times {{10}^{6}})=1.08\times {{10}^{5}}Joules$
Hence, we can say that the bulb will consume 0.03 KWh or $1.08\times {{10}^{5}}$ Joules of energy if it is kept on for 15 minutes.

Note:
The energy consumption is actually the energy lost during the process. As we all know that there is nothing ideal in the practical world and this is the reason that when we turn on a bulb for a few minutes it gets heated. This heat is actually the loss of energy in the form of heat. So, if in future we will be able to reduce the energy loss in our resistors or capacitors or any electrical instrument then the consumption of energy will also be reduced.