Simple Circuits

[The cell and the battery] [The Bulb] [Joining bulbs] [Changing the brightness of a bulb] [A Switch] [Why a circuit may not work]

 

When we use wires to connect a battery with a bulb (or buzzer etc) and the bulb lights then we have a circuit

The inside of a wire is made of COPPER (which is a good conductor of electricity)
The copper wire is covered with coloured plastic (which is an insulator).

 

Example

 

Here is a circuit where a battery (sometimes called a cell) is connected to a bulb using wires.

Below is a circuit diagram of the same circuit using the correct symbols

 

 

The Cell

Circuit symbol

The cell has a voltage which makes an electric current flow around the circuit.

The higher the voltage the greater the current

 

 


A 'battery' of cells

Symbol for a battery

Several cells joined together form a battery.

 

 

The bulb

Circuit symbol

 

 

The BULB

 A light bulb has a thin metal filament, often made out of tungsten, which glows very hot when an electric current flows through it.

 

If we use a higher voltage more current will flow and the filament gets hotter and glows brighter.

Too much current makes the filament melt (and the bulb ‘blows’).

 

How to join a bulb:

 

Circuit diagram

Two bulbs joined end to end (in series)

This is how christmas tree lights are joined together
If one bulb is unscrewed the other bulb will also go out

 

 

Circuit diagram

Two bulbs joined side by side (in parallel)

This is how the lights in a house are wired together.
If one bulb is unscrewed the other bulb will stay alight

 

 

Changing the brightness of a bulb



Circuit diagram

Two cells makes the bulb brighter
because there is a higher voltage

 

 

Circuit diagram

Two bulbs (in series) makes the bulbs dimmer
because it is more diffcult for the electricity to flow

 

 


Making the wire longer will make the bulb dimmer

 

 


Circuit diagram

Two cells and two bulbs is the same brightness as one cell and one bulb

 


Using a switch

 


Circuit diagram

All the switch does is to break the circuit and switch everything off (or on)

It does not matter where we put a switch in the circuit.

A switch like this is sometimes called a SPST switch
( SPST=Single Pole Single Throw)

 

 

 

 

Sometimes a circuit does not work.

The three main causes for a circuit not working are:

 

There is something wrong with each of the circuits below. can you see what it is?
(Hold your mouse over the diagram to find out)
Wire not connected to the battery Wire joined to the bulb at the wrong place
Wire joined to the bulb at the wrong place Here there is a 'short circuit'. The + and the - of the battery should not be joined together.
One of the batteries is the wrong way around

 

 

 

 

 


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