Video Transcript
A 120-watt television has a useful
power output of 30 watts. What is the efficiency of the
television?
Okay, so we have this television,
and the television is rated at 120 watts. That means if we were to monitor
the power coming into this television during its normal operation, that power would
be at 120 watts. So that’s the total power
input. And we’re told that this television
has a useful power output of 30 watts. That is, the power that it uses in
support of creating a moving image is 30 watts. So that leaves 90 watts doing
something other than what we want them to be doing. Most of that wasted power goes into
generating heat in the system. So then our television usefully
outputs some of the power input to it but not all of it. And we want to calculate its
efficiency.
Efficiency can be represented using
the Greek letter 𝜂. It’s equal to the useful output
from a process or a device divided by the total input. These outputs and inputs can be
energies or, like in our case, they can be powers. In this instance, we’re told that
the useful power output from this television is 30 watts and that the total power
input is 120 watts. We can see when we calculate this
fraction that the units, watts, will cancel out. And we’re left with 30 divided by
120. Written as a decimal, this is equal
to 0.25.
Often though, efficiencies are
expressed as percents. To write 0.25 as its equivalent
value as a percent, all we need to do is multiply it by 100 percent. 0.25 times 100 percent is equal to
25 percent. That’s the efficiency of this
television. It means that 25 percent of the
total power input to it is converted to useful power output.
