Electricity 3

Length of wire in a circuit

Electricity comes into our homes through underground and overhead wires which stretch all the way from a power station which could be many miles away. Above the ground we can see the long wires as they loop from pylon to pylon.


So we know that electricity can travel a long way but does it make any difference how far it has to travel? Does it take it a lot longer to get there if it travels a long way? Is it just as ‘powerful’ when it gets to the end of a long journey?

This is an investigation:

To find out if the length of wire in a circuit makes any difference to the way the circuit works


To find out if the length of a wire has an effect on the electricity flowing in the wire.

What do we know about electricity so far?

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1. It can make a bulb light up.
2. It can make a buzzer ‘buzz’ or make a sound.
3. It needs metal wires to allow it to travel or ‘flow’.
4. A circuit must be ‘complete’ if electricity is to flow around it.
5. Materials which allow electricity to flow in them are called ‘conductors’ eg. metals.
6. Materials which do not allow electricity to flow in the m are called ‘insulators’ eg.        plastics.
7. A switch can control the electricity by making a ‘break’ in the circuit. It uses a             combination of conductors and insulators to do this.
8. A battery or cell is the source of the electricity in a simple circuit
9. Care must be taken with the PLUS
+ and MINUS - connections to a battery in            some circuits or it may not work properly eg. with buzzers.

Length of wire investigation

We will need a simple circuit and a way to connect various lengths of wire.

Which components will we need in our circuit and why?

How will we join the wire to the circuit?

How will we know if the wire is having any effect?

What will we change?

What will we keep the same.

What will we record during our investigation?

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We could add one extra length of wire at a time by simply clipping one crocodile clip wire to the next and so on then switch on and see if the bulb lights up.

This is rather messy and you may run out of wires.

Are any other types of wire available?

Left: plastic coated or ‘insulated’ wire.

Middle: varnished wire.

Right: Thin bare wire.

The thin bare wire is best because we can clip a croc. wire to it at any point along its length so we don’t need separate pieces of wire of different lengths.

We need a 1 metre length of the wire but it can tangle and be awkward to handle!

This problem can be solved by wrapping the wire around a piece of card and fastening the ends using staples or paper fasteners.

The loops of wire must not be allowed to touch as it is wrapped around the card!

Now the long piece of wire is much easier to handle and can be joined to the circuit using a croc. clip at one end.

Another croc. clip can be used to touch the bare wire at any point but a good idea is to clip a paper fastener to the croc. clip and use this a a ‘probe’ to gradually move along the card from loop to loop gently pressing to make a good contact with the wire.

Make sure that the circuit is switched on then start at one end of the card and move from loop to loop observing the bulb each time.

What do you see?

Now try it the other way so that the shortest length of wire is tested first then gradually move so as to make the wire longer.

One problem with this investigation is that the bulb may not be very bright and the room lights make it even more difficult to see.

This problem can be solved using a simple cylinder of card cut from a tube eg. kitchen roll. The tube is placed over the bulb and this shields the outside light making it easier to see the bulb alone. Cupping your hands around helps too!

What have we found out?

With the wire at its longest the bulb is off! Or is it?????

With the wire at its shortest the bulb is brightest.

As the length of wire increases the bulb gets dimmer.

As the length of wire decreases the bulb gets brighter.

There may be a point where the wire is so long that the bulb is too dim to see! But, if we make the room darker we may still be able to see a glow from the bulb. This teaches us that ‘how’ we observe things is very important.

We can conclude that:

The longer the wire the smaller the flow of electricity and the smaller the flow of electricity the dimmer the bulb glows.

When the wire is very long there is so small a flow of electricity that we can’t see the bulb light up at all but there is still electricity flowing!

We can prove this in an interesting way by connecting a buzzer into our circuit when the wire is at its longest. The buzzer still buzzes even though the bulb is not lit! This is because a buzzer only needs a very tiny flow of electricity to make it work.

Bulbs need a minimum amount of electricity in order to make them light up and different bulbs have different needs. A car headlight requires lots of electricity and would not light up at all using one of our small batteries. It needs a very powerful, and large, car battery to provide the large flow of electricity.

A flow of electricity is called a ‘current’ so we can use the terms low current and high current to describe the amount of electricity flowing through a wire.

We have used the brightness of a bulb to investigate the amount of electricity flowing in a wire. Can you think of how other components might be affected by changing the flow of current?

What about the pylons and the miles of wire between them?

The length of a wire does affect the current flowing. This means that a lot of ‘energy’ is lost as the electricity makes it’s way along the wires from the power station to our homes.

The wires ‘resist’ the flow of electricity and the longer the wires the greater the resistance.

The power stations push the electricity through the wires by using a special electrical force called the VOLTAGE.

A high Voltage can make a large current flow. The Voltage used by power stations can be up to 400,000Volts.

A simple cell or battery used in our circuits has a Voltage of only 1.5 Volts.

High Voltages are very dangerous so that is why we are always warned to keep away from pylons - you musn’t even fly a kite near one in case the string touches the wires because such a high Voltage can even make electricity travel through string!!!!


How fast is electricity?

When the circuit is complete the bulb seems to light up immediately. This means that electricity must be traveling very quickly. In fact electricity travels at about the same speed as light! There is one problem though! A bulb is made from a tiny coil of fine wire called a filament. The filament needs to get hot before it glows and makes light. Heat takes time to travel in a metal so there is a small delay as the bulb warms up. Similarly there is a delay when the bulb is switched off. This is easily noticed with a car headlamp bulb which seems to glow for a short while after the lights have been switched off!

Another possible area for investigation!

We have used a bulb to give us an idea of the amount of electricity flowing but we haven’t actually got a measurement in the form of a number!

It is possible to measure the brightness of a bulb using layers of tracing paper to gradually block the light. The number of sheets of tracing paper is then a measure of brightness.

The same number is also a measure of the electric current flowing in the bulb!

How could the brightness experiment be performed accurately?



One final thing about wires....

A wire is a long thin strand of metal (or it may be lots of finer strands twisted together so that it is more flexible).

When electricity flows in a wire it gives up some of its ‘energy’ as heat and it actually makes the wire warm!

Most of the time we don’t notice it because there is only a very small amount of heat but this is a really important effect.

If a wire gets hot enough it glows red and if it gets really hot it glows white! Some types of wire are so good at glowing white hot that they are used in light bulbs!

Many years ago an English Scientist and inventor called Joseph Swan made use of this special ‘property’ of electricity and invented the ‘Incandescent lamp’ which we now call the light bulb. An American inventor called Thomas Edison was working on the same invention and they both worked together to set up the world’s first electric light bulb company in about 1880.

The top picture shows a glowing wire ‘filament’ in an old style electric light bulb.

Left is the original light bulb invented by Joseph Swan in 1878. It has a filament made from Carbon which is not really a true metal but it is like a metal in some ways.


Here is a question about light bulbs.

Both of the light bulbs pictured above have a glass bulb so that the filament is protected and still allows the light to come out but there is another very important reason for the glass bulb!

The glass bulb is ‘evacuated’. In other words there is no air inside the bulb!

Why is it essential that the air is kept outside the glass bulb and not allowed in?

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