| You might be asked to calculate the efficiency of a
transformer. What you must do is think about how much energy the
transformer draws from the mains and how much of this energy is used in a
useful way. ie What percentage of the energy is used usefully? It would be
nice to say 100% but it is probably only 80% efficient.
Well nothing is perfectly efficient. If you have a mobile phone, you
will have noticed that the charger gets warm when you use it to recharge
the battery of your mobile. This means that some of the electrical energy
has been wasted ie turned into heat.
You can actually do the calculation for yourself if you already have a
mobile phone and a charger. I hope that it is as cool as my mobile phone.
On the bottom of my charger it says that it uses 21 mAmps at 230 volts.
I can now calculate how much power it draws from the mains. Remember P =
VI.
Power = 230 volts x 21 mAmps = 4.83 watts
To get the answer in watts I had to divide by 1000 because I started
with mAmps instead of Amps.
The charger also says that it has an output of 355 mAmps at 3.7 volts.
So I can calculate how much of the power goes into the battery of my
mobile phone.
Power = 3.7 volts x 355 mAmps = 1.31 watts
Well that is a bit disappointing. It seems that quite a lot of the
energy is being wasted. My mobile is cool, but the charger is not very
efficient and does get quite hot when I use it to recharge the phone. So
how efficient is it?
Power is the rate of doing work. So every second it is charging the
phone, it uses 4.83 joules of energy, but only puts 1.31 joules into the
battery. The other 3.52 joules per second is wasted. Here is the
calculation of % efficiency:
% efficiency = 1.31 x 100 / 4.83 = 27%
That is not quite the end of the story for my mobile phone. The
transformer is only 27% efficient, but there is also a loss of energy in
the mobile when it is charging. I know this because the battery also gets
hot by the time it is fully charged. Some of the electrical energy is
converted into chemical energy in the battery and the rest is wasted as
heat.
You can calculate the % efficiency of any machine provided that you
know how much energy has to be put into it and how much useful energy
comes out. Here is the equation:
% efficiency = useful energy produced x 100 / total
energy used
|
