Resistors control how much electric charge flows through a circuit each second – the size of the current in the circuit. They do this by controlling how difficult it is for electric charges to flow in the circuit – that is, the resistance in the circuit.
The amount of resistance depends on what material the resistor is made of, the length and the cross-sectional area (or thickness) of the wire used. Thicker wires allow more electric charges to flow than thinner wires.
Resistors can be made of different materials, such as alloys and carbon. Some resistors are coded with bands of colour to indicate the amount of resistance they have.
A low resistance circuit will allow electric charges to flow easily and may have:
- thick wires
- short lengths of resistance wire
- good conductors
- parallel arrangement of resistors.
A high resistance circuit will reduce the flow of electric charges and may have:
- thin wires
- long lengths of resistance wire
- conductors with higher resistivity
- series arrangement of resistors.
How are resistors used in circuits?
Some components that are used in circuits are very sensitive to too much electricity. They may get damaged if they get too high a current through them – a bit like when a light bulb blows because the wire inside it breaks and stops the flow of electric charges.
Resistors are often placed in circuits with other components to protect the components from getting too much electric current through them. Most electrical components have a limit to the amount of power that they can use, which is usually specified by the manufacturer.
Thin nichrome metal wires have high resistance and are often used to protect components in circuits from too much current. They can act to protect circuits as fuses. When the electric current is too great, the piece of nichrome wire heats up, melts and breaks the circuit. This prevents electricity from flowing through the circuit. We say the fuse (piece of wire) has ‘blown’ but it really means it has melted and broken the circuit.
How can we change the amount of resistance?
We can change the resistance in the circuit by replacing a resistor with a different one that has more or less resistance or we can use a variable resistor if we want the current to be varied over a range of values.
Variable resistors are also used for changing the outputs of a circuit (such as the volume on a sound system or speed at which a robot moves).
Combining resistors in series or in parallel arrangements also helps to vary the amount of resistance in the circuit and controls current.
Nature of science
Science investigations often involve changing a variable quantity and seeing the effect on other quantities in a particular situation. We can study the effect on the resistance of a wire by changing the variables of thickness, length or type of metal that it is made of, one variable at a time.