The UK site for KS3 and KS4 Physics

KS4 Power

You should already have read Energy changes in systems.

On that page we looked at how we can calculate the amount by which energy changes in systems when temperature was altered.

Now, on this page, we introduce a new term, power, which is all about how quickly energy is transferred or used.

In the everyday world, the word "power" has a whole load of meanings but in Physics it has a single precise meaning which you need to know:

Power is the term or the word that we use to tell us how quickly energy is used or transferred.

For example we can compare the "power" of two electric motors: let's say that Motor 1 lifted a 2Kg load, a height of 4m, in 10seconds, whilst Motor 2 lifted the same 2Kg load, the same height, in 5 seconds. Which has the highest "power"?

The answer is obvious; Motor 2 has the highest power because it lifted the same load, the same height, in less time.

So, that's what power is all about - how quickly something transfers energy.

Another way of saying this is - how quickly something does work.
Motor 2 did the same amount of work as Motor 1 (lifting the same load, the same height) but it did it more quickly, so it has more power.

"Doing work" and "transferring energy" are the same thing, just in different words.

OK, so here is the official definition for Power:

This definition leads on to an equation, first using "energy transferred":

Now, the same again but using "work done" instead of energy transferred.

Before we do a few examples using the equations we should say something about the unit for power. You can see that it is the "watt", W.

Like the unit joule, the unit watt is named after a famous scientist and engineer. This time it is James Watt, who had a lot to do with developing the steam engine and starting the Industrial Revolution which we discussed in our introduction to Energy.
But, what is a "watt"? What does it really mean if something has a power of 1 W, one watt ?

If you look at the first equation; for P to be 1, E and t have to be 1.
So, a Power of 1W is when 1J of energy is transferred in 1s.

One watt is one joule per second (1W = 1 J/s)

So, think of a 60W light bulb!

Now you know what the "60W" means; it means that the bulb is transferring 60 J of energy from the chemical store of the power station into Radiation (light waves) each second.

If you use a 100W bulb instead, then you will expect it to be brighter because it will transfer 100 J of energy per second from the chemical store of the power station into Radiation. But, on the downside, that will cost you more and you will be using up more of the fossil fuel at the power station which is not a good thing!

OK, now let's have a go at a few example calculations using the equations.

Power in electrical circuits

You will study energy in electrical circuits in a lot more detail when you do section 4.2 Electricity, but you need to know a little bit about it now in this section.

What do we mean by "power in an electrical circuit"?

We mean, the rate at which a component transfers energy, being carried by an electrical current, into another store (eg a thermal store, if the current is going through a kettle).

But how do we work out the power in a electrical circuit? We can't see electrons moving as a current in order to time them!

Well, it turns out to be really easy and all you need to know about are two fundamental quantities that you should have learnt at KS3.
These are:
potential difference, V
and, current, I.

Let's say you want to find the power of an unmarked bulb when the bulb is being used in a circuit.
All you do is connect a voltmeter across the bulb to measure potential difference, V, and connect an ammeter in the circuit to measure current, I.

Then you use the following simple equation:

So, all you do is multiply the p.d by the current and you have worked out the power of the bulb!