P2bL 2 Resistance 1

Key Words

Collide - bump into.

Electrons - tiny negatively charged particles.

Force of repulsion - force that pushes things apart.

Nucleus - where the protons and neutrons in the atom are.

Ohm's Law - describes the relationship between potential difference and current.

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

Voltage - potential difference, electrical "pressure".

Grade E

When an electric current flows through a wire, the electrons are moving and collide with atoms in the wire.  The more collisions there are, the more difficult it is for the current to get through the wire.  Therefore the resistance gets bigger.

Grade C

When a voltage is applied across a conductor, a force field called an electric field is set up.  The positive side of the field pulls the electrons along, while the negative side of the field pushes them.  It's a bit like a train with an engine at each end.

The battery in effect acts as an electron pump and voltage is the "electrical pressure" from the pump.

A metal can be thought of as a lattice of ions in a sea of free electrons.  The electrons move randomly and collide with the metal ions as in the picture below.

The metal ions are vibrating.  All materials vibrate with temperature; the hotter the temperature, the more they vibrate. 

The ions still have electron shells that surround the nucleus.  The nucleus is very small, about 1/10 000 the size of the atom.  Although most of the atom is empty space, the electrons cannot penetrate the electron shells.  Since the free electrons have the same negative charge as the electrons in the cloud, the two like charges repel.  The electrons bounce off.

As they bounce off, the electrons put a tiny little bit of energy into the ions, which makes them vibrate even more.  Therefore the temperature goes up.  Therefore there is more chance of a collision.  So the overall flow of electrons is reduced.

In insulators, almost all the electrons are bound, so there are very few electrons to carry the charge.  Insulators have a very high resistance.

In semi-conductors, the electrons are bound, but can be released into what's called the conduction band.  We won't go into detail here.

In a super-conductor, the materials have a very low resistance at very low temperatures.  So big currents can be carried, although a lot of energy is needed to keep the wires cold.

Grade A

Ohm's law links the voltage and current in a conductor by stating:

The current through a conductor is directly proportional to the potential difference, provided the temperature remains the same

This means that if the voltage is doubled, the current is doubled too.  The temperature remaining the same is critical.  If the temperature rises, the resistance goes up too.

Potential difference (V) = Current (A) × Resistance (W)

In Physics Code:

V = IR

In triangle form:

  Many textbooks say that the electrons are slowed down through resisting materials.  This is quite adequate at GCSE level.  However in reality, the electrons move faster in a material with high resistance.

 To say that the electrons are slowed is bad physics.