P2bL 3 Resistance 2

Key Words

Accelerated - made faster

Deflections - bouncing off to move in another direction

Kinetic energy - energy in moving objects.

Repulsive - pushing apart

Resistance - the amount by which a material opposes the flow of electricity.

Transfer of energy - energy passed from one object to another.

Vibrate - to move forwards and backwards (or from side to side, or up and down).

Grade E

Resistance of a wire depends on:

• Its length; the longer it is, the more the resistance.

• Its thickness; the thicker it is, the lower the resistance.

• The material it's made from; some materials are better conductors than others.

You can think of it as a school corridor between lessons.  A long, narrow corridor tends to get congested and pupils cannot pass easily.  A wide corridor allows freer movement of pupils.

Grade C

We have seen that as a current flows in a wire, the electrons bump into other electrons and are deflected.  The collisions transfer energy to the metal ions which vibrate more, increasing the chance of more collisions.  So the resistance goes up.

Some materials have more widely spaced lattices, so the chances of collisions are less.  They have lower resistance.  For high-voltage transmission cables used in the national grid, we want materials with low resistance, and light weight.  The copper wires used for overhead electricity transmission to railway locomotive weighed 8 kg per metre.  The lighter modern cables made of aluminium and copper weigh about 2 kg per metre.

The tungsten filament in a light bulb has quite a high resistance.  It gets very hot and glows brightly.  Tungsten also has a very high melting point.

Grade A

When an electron hits an ion, there is transfer of energy which makes the metal ion vibrate more.  The electron is slowed down, but the electric field from the voltage accelerates it until the next collision

The more the vibration, the more the chances are of a collision.  The wire gets hotter.