Oliver H asked me in class where the energy goes when mains electricity is stepped up or down inside a transformer. He correctly cited the first law of thermodynamics (or the law of conservation of energy) pointing out that when 240 volts from the mains is stepped down to 9 volts to run an electronic balance in the lab, a lot of energy seems to have disappeared. He knew it could not have vanished and asked me where it had gone. I said that I thought that this could be accounted for by a change in the current and subsequent research suggests that I just about got my explanation right. Phew!
How does a transformer work?
If you have ever taken any electrical equipment apart (after unplugging it from the mains please!) in science club or at home, you will probably have met something like the picture to the left. This is a transformer although there are other common designs too.
They all do the same job of changing one voltage up or down to another – and they all require that the electrical supply is alternating not direct. (That’s why I put up the post about AC/DC the other day – I was planning ahead!)
Transformers consist of two coils of insulated copper wire wrapped around a soft iron core. The alternating current flowing through the first coil (the input coil) turns the iron into a pulsing electromagnet which induces an alternating current through the second set of coils (the output coil). The voltage changes in proportion to the number of coils on either side of the iron core.
I hope my diagram above helps to explain the process better. It is a step down transformer that halves the input voltage. If it were wired in reverse it would step the voltage up, and double it. So this is where Oliver’s question becomes relevant. Where does the energy associated with those other 110 volts go?
Power, in watts, is a measure of how quickly a machine is transferring energy from one form to another. It is equivalent to joules per second. In electrical systems the power can be calculated by multiplying the voltage and the current. Oliver was right that the power going into the transformer must be the same as the power coming out, so to make this the case the current increases if the voltage drops and decreases if the voltage is being stepped up. This means that voltage x current either side of the transformer will be the same.
No electrical component is perfect, of course. Energy is lost as heat because of the electrical resistance of the wire in the coils. Wikipedia lists some other ways in which energy can be lost from a transformer.
Questions…
- How can you calculate the resistance in a circuit if you know the voltage and the current?
- What is the Law of Conservation of Energy?
- What device turns mechanical (kinetic) energy into DC electrical energy?
- What is ‘insulated copper wire’ and what might be used to insulate a copper wire?
- What frequency is UK mains alternating current?
- Which of the following is the correct circuit symbol for a transformer?
(a) 
(b) 
(c) 
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