Question

How much energy does a $100W$ electric bulb consume in two hours?

Answer 1

We know, whereas most energies are measured in the units of Joules or some higher multiplier of it, electrical energy is calculated in kilowatt-hour. So, convert the power of the electric bulb in kilowatt, first and multiply it with the time of consumption to obtain the electrical energy consumed by this bulb.

Complete step by step answer:
In the problem, it is given, that the power of the electric bulb is $100W$
$\Rightarrow \dfrac{100}{1000}KW$
$\Rightarrow 0.1KW$
And we are to calculate the energy consumed by this bulb in two $(2)$ hours.
We know that electrical energy is usually measured in units of kilowatt-hours, which is equal to about $3.6$ megajoules.
Since, consumed electrical energy = power $\times$ time of consumption,
Electrical energy consumed by an electric bulb of power $100W$ in two hours, $E=0.1KW\times 2$ hours
$=0.2$ kilowatt-hours
Therefore, the correct answer is $0.2$ kilowatt-hours.

Additional information:
If the potential difference at the two terminals of an electrical appliance is $1$ volt and the current passing through it is $1$ ampere, then the power of the appliance is $1$ watt.
Work done or electrical energy spent in one hour by an electrical appliance having power $1KW$ is called $1$ kilowatt-hour or $1$ BOT unit.

Obviously, this is a bigger unit of electrical energy. The electricity supplied to us for use is measured by this unit. Many times, kilowatt-hour or BOT units are simply called “units”.
Electrical energy can also be measured in the unit watt-hour. Work done or electrical energy spent in one hour by an electrical appliance having power $1W$ is called $1$ watt-hour.

Note: On writing $220V-100W$ on the body of an electric lamp or $220V-2000W$ on the body of an electric heater, both voltage rating and watt rating are referred.
To minimize heat loss, electrical power is to be transmitted from one place to another at high voltage and low current.