A 60-watt incandescent light
bulb is left on for 30 seconds. How much energy is supplied to
it over this time?
We have this light bulb then
that uses up 60 watts of power. If we recall that power is
equal to energy divided by time and that the units of power are watts, the units
of energy are joules, the units of time are seconds, then we can see that the
fact that this is a 60-watt bulb means that it uses up 60 joules of energy every
We can see that this way. Let’s say we rearrange this
power equation so that energy is isolated on one side by itself. When we do that, we see that
energy is equal to power times time. Substituting in for those
values, we see it’s equal to 60 watts multiplied by the time the bulb is on, 30
But look at this: power is
equal to energy divided by time, which means that one watt is equal to a joule
per second. This means we can go to our
expression for power 60 watts and rewrite it as 60 joules per second. That’s because a joule per
second is a watt. Once we’ve done that though, we
see that the units of seconds in the denominator cancel with those in the
numerator. We’re left in this expression
with the units of joules, the units of energy.
So the energy 𝐸 then is equal
to 60 joules multiplied by 30. 60 times 30 is 1800. So 1800 joules is the amount of
energy supplied to the bulb over this time. And we found that result by
multiplying the power used by the bulb times the time it was on.