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Resistance

The electric wires in your houses are made of copper.
The covering of these wires is made of plastic.
Some connecting leads in integrated circuits are made of gold.
Power cables hang from pylons on ceramic holders.
Some houses have dimmer switches to control the lights.
The volume control on a TV or stereo can change how loud the sound is.
An electric blower may have two different settings to change the speed of the motor.
Two different light bulbs both connected to the mains may give out different amounts of light energy.

All these facts are connected with how difficult it is for the electricity to go through certain things.

The property of a material that resists the flow of electricity through it is called the RESISTANCE of the material.

The more resistance something has the smaller will be the electric current flowing through it for a certain voltage.

STUDENT INVESTIGATION
Design and carry out an experiment to place the following things in order of increasing resistance:
(a) a 10 cm long pencil lead (carbon)
(b) a 10 cm long copper wire (28 SWG)
(c) a 10 cm long constantan wire (28 SWG)
(d) a 10 cm long piece of string
(e) a 10 cm long copper wire (34 SWG)
(f) a 1 m long constantan wire (28 SWG)

You should have found that the resistance of a sample depends on:
(a) what it is - copper has a very low resistance and plastic a very high one. Materials that have a very high resistance are called INSULATORS and those that have a low resistance are called CONDUCTORS.
(b) how long it is - a long wire has a bigger resistance than a short one of the same material and diameter
(c) how thick it is - a thick wire has a lower resistance than a thin one of the same length and material

You can explain resistance by thinking about the free electrons moving through the material. As they move they collide with the atoms of the material and this affects their motion and increases the resistance. Some materials have more free electrons than others and so these materials have a low resistance.

A high resistance means that the free electrons will make lots of collisions and lose lots of energy. You can compare this with a person struggling to get through a large crowd; they will also lose lots of energy as they collide with the people in the crowd.

schoolphysics electric current animation

To see an animation of the motion of free electrons in an electric current click on the animation link.

Calculation of resistance

You can work out the resistance by measuring the current through a specimen and the potential difference across its ends. The resistance (R) is then found by dividing the potential difference (V) by the current (I)

(See: Ohm's law and resistance)

Resistance and heating

When an electric current flows through a wire the wire heats up. This is because the electrons collide with the atoms of the metal as they move and the atoms absorb some of their energy. This makes the atoms vibrate more strongly and so the wire heats up.

The bigger the current in a wire the bigger the heating effect

It is for this reason that the wires running to a house immersion heater need to be of a special type. They have to take large currents and so will get quite hot.

The filament in an electric light bulb is very thin and has a high resistance. The heating effect of the electric current flowing through the filament is so great (the temperature of the filament may reach over 1500^oC) that the wire glows – this is how electrical energy is converted to light energy.

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