Question

A bulb of $40W$ is used for $12.5h$ each day for a total of thirty days. Calculate the electrical energy consumed.

Answer 1

Hint: The total energy consumed can be calculated as the product of power consumed and the time of consumption. Time of consumption is the total time the bulb is switched on over the given time period. The total energy consumed will be in terms of Wh.
Formulas used:
$E=P\times t$

Complete answer:
Let us first calculate the total time the bulb is switched on in the total time given, that is thirty days.
$\begin{align}
  & t=12.5h\times 30 \\
 & t=375h \\
\end{align}$
The energy consumed by the bulb is equal to the product of power of the bulb and the total time the bulb was switched on,
$\begin{align}
  & E=P\times T \\
 & \Rightarrow E=40W\times 375h \\
 & \Rightarrow E=15000Wh \\
\end{align}$
Hence, the total energy consumed by the bulb for a period of thirty days is $15000Wh$.

Additional information:
Power is the time rate at which work is done or energy is transformed. Simply, power is the derivative of work with respect to time. Power is measured in energy per time, as joules per second. One of the most common units of power is horsepower. Horsepower can be converted to other units of power as energy per time. One watt is also used to measure the power. One watt is equal to one joule per second. One horse power is $756$ watts. As power is in watts, energy can be defined as watt-hour. We can easily say a hundred-watt bulb uses four times the energy used by a twenty-five-watt bulb in a given period of time. In daily use, we are charged for the amount of energy we use and we usually measure it in kilowatt-hours(kWh).

Note:
Energy is defined as the product of power and time. Time consumed is the total time a switch has been on in a given period of time. When some bulbs are connected in series, the current flowing through them is the same. When some bulbs are connected in parallel, they will have the same voltage across each bulb but not the same current flowing through each of them.