## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

In the world of Physics, the words "Energy" and "Power" are related BUT they do not have the same meaning and they are not interchangeable!

You need to learn what they do mean.

#### Energy and Power

Energy is a measure of the amount of Work that is done in a certain situation.

Power is a measure of how quickly that Work is done, or a measure of how quickly Energy is transferred.

We will come back to these descriptions later, but for now our interest is in how we quantify Power in Electrical systems, and luckily for us, it is really easy to do this.

Power in Electrical Systems

To find the "Power" of a component in an electrical circuit all we need to know is the pd across the component, V, and the current flowing through it, I. Then we make use of the following simple equation:

But what do we mean when we say that this lamp has a "power" of 7.5 watts?

Well, this is where we need to think back to what we read at the start of this section;
Power is a measure of how quickly Energy is transferred.

A power of 1 watt is when 1 joule of energy is transferred in 1 second,
so, a power of 7 watts is when 7 joules of energy is transferred in 1 second.

So our lamp, with a power of 7W, is transferring 7 joules of energy from the power source into light (and heat) every second.

For electrical components it is better to refer to their "power rating" rather than to their "power"; the former correctly gives the impression that the components use up energy, whereas the latter gives the incorrect impression that they posses some kind of inner energy or power, which they don't.

Resistors are good examples of components that very definitely use up energy or "dissipate power" whenever a current flows through them.

Power dissipated in a resistor

It is very easy to calculate the amount of power dissipated (used up) in a resistor when a current flows through it. We use:

Now its time for you to have a go at using these 2 new equations, P = V I and P = I2 R.

#### Energy transfers in everyday appliances

Everyday electrical appliances such as kettles and washing machines are designed to use electricity in order to bring about energy transfers.

They bring about energy transfers in order to do something useful.

eg. The kettle makes use of electricity in order to heat up water, to bring it to boiling point;
the washing machine uses electricity in order to both heat up water and to rotate a drum full of clothes, often at very high speed.

In each of these 2 examples, and in every other example of how electrical appliances use electricity, an electric current, coming from a mains or battery power source, brings about an energy transfer.

Now at this point in the AQA syllabus you should already have done AQA Section 4.1 Energy, so you should already know about Energy Stores and Energy Pathways.
If you need to, you can read the relevant section by clicking here: 4.1.1.1 Energy stores and systems. (But remember to click the Back Arrow at the top of your browser in order to return to this page.)

Here is a reminder of energy stores and energy pathways with a relevant example:

When an electric current travels from a power station (energy store 1) to a bulb (energy store 2) in your house, or from a battery (energy store 1) to a buzzer (energy store 2), as shown below, it travels along an Electrical Work Energy Pathway.

The amount of energy transferred from the power source to the appliance will depend on two things, the power rating of the appliance, eg the buzzer, and the length of time it is on.
Then we simply multiply these two numbers together to get a value for the amount of energy transferred.

Here is the relevant equation:

The amount of energy transferred when electrical appliances are used is directly related to the amount of money people pay to use them! I hope you can see that this is obvious. And you should be able to see from the equation that two factors determine the cost of using an appliance:
1. Its power rating, P, and
2. The length of time it is on, t.

A low power rating appliance such as an LED light bulb can be "on" for quite a long time before it uses the same amount of energy as an electric shower that is "on" for only 5 min (300s).

Let's do an example where we make use of this equation to calculate the amount of energy transferred from a power source to a typical electrical appliance.

The above equation is simple to use when we have just one electrical appliance and we know its power rating, P. But what if we are presented with a circuit with a few electrical components and we don't know any of their power ratings, such as in the circuit below.

Can we still work out the amount of energy transferred from the power source to these two components? As you might guess, the answer is Yes!

We do so by combining the above equation with our first equation for power rating, P = I x V:

Let's make use the above circuit to do an example of how to use our new equation.

(NB. If you want, you can rewrite this equation as E = I t V which might make it easier to remember!)

Now its time for you to have a go at using these 2 equations, E = P x t and E = I x t x V.

Before we finish this section on energy transfer to electrical appliances, we need to add, believe it or not, a third energy transfer equation.

This is a very useful energy transfer equation especially if it is rearranged as:

Q = E/V

because with this version we can work out the Quantity of charge (Q) that flows in a circuit when we know the amount of energy that has transferred.

You can have a go at similar questions.

#### The Electricity National Grid

Electrical power is transferred from power stations to consumers using the National Grid - a system of cables and transformers linking all the power stations spread across the UK to consumers in cities, towns and villages.

The cables of the Grid

The National Grid ensures that electricity generated anywhere in England, Scotland and Wales can be used to satisfy demand elsewhere. So, at any moment the electricity in your house or school could be coming from a power station on the other side of the UK, then later it could be coming from somewhere else; but more likely, it is coming from a variety of power stations at any one time.

Can you see the advantage of this National Grid system?
It means that a shutdown due to a failure, or for maintenance, of a local power station will not interrupt your electricity supply.

The way to visualise the National Grid system is as a set of criss-cross lines that span the country as shown below. The red grid lines represent the power cables.

Now, let's show the approximate location of a number of power stations. Some of these will be big gas, coal, oil or nuclear power stations, but others will be much smaller wind farms.

Notice that all of the Power stations are "connected to" the Grid. Each of them feeds electrical power "into" the Grid.

Now let's imagine that you live near Birmingham.
We'll show your location on the map below.

Birmingham is also "connected to" the Grid, meaning that it can draw electrical power from the Grid. It does not rely on its nearest power station which could go offline for any reason and yet the supply of electricity to Birmingham would not be interrupted.

So long as cities, towns and villages are "connected to" the Grid, then they too will have an uninterrupted supply of electricity.

Some power stations, such as the one shown in Wales are connected to the Grid, but are only operating and delivering power into the Grid during peak demand periods such as around 6pm when lots of homes are cooking and boiling water to make tea; it supports the whole UK Grid during this period even though it is located on a mountain top in the west of Wales!

So, wherever you are in the UK, you too will be connected to the National Grid.

The Transformers of the National Grid

We have already learnt that the National Grid consists of very long cables.
When electrical power is transmitted through a long cable the cable heats up and so energy is wasted.
The higher the current of the electricity passing through any length of cable, the greater the amount of energy wasted!

So, in order to reduce the current being used and yet still transmit the same electrical power, the electricity power stations increase or step up their output voltage.

Transformers are used to "step up" the output voltage of the power station (automatically reducing their output current by the equivalent amount) making the energy transmission through the long cables more efficient.

The following diagram shows a transforer "stepping up the voltage" at the power station end of the network.

But what happens at the consumer end of this connection? A "very high voltage" would be both very dangerous for consumers and it would not be suitable for electrical appliances which are designed to work with a much lower voltage of about 230V.

The simple solution is to use another transformer near to where consumers live which will also "transform" the voltage, but this time the transformer is turned round so that it steps the voltage back down.

Putting these two diagrams together we can see the whole picture of a Grid connection from a power station to a consumer.
(You might have to zoom in to this picture to read the labels clearly)

If we did not have Transformers, we would not have the ability to transmit electricity over the verylong cables that we currently use; we would not have a National Grid system with all its advantages.

For this section of the AQA specification you do NOT need to know how the Transformer does its amazing job, but you would be wise to find out because you DO need to know the answer for section 4.7.3.4.