Efficiency

Efficiency is a very easy concept. The calculations you will be given in your GCSE exam should not give you any problems at all. You just need to be able to calculate percentages. 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 useful and how much is wasted? 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