Cause of Resistance

Aim

The resistance of a conductor is well known to be dependent upon the Material it is made from, its length, diameter, and temperature. I am going to be studying the resistance of wire to see how a length of Nichrome wire affects the dependent variable, resistance.

Prediction

I predict that, as the length of the wire doubles, the resistance will also double. This means that the length will affect the resistance.

Hypothesis

I predict that as the length of wire is increased a greater amount of resistance will be created. This is because electrons colliding into ions cause resistance, so if the length of the wire is doubled, the electrons bump into twice as many ions so there will be twice as much resistance.

A current is formed by moving charges. These charges loose energy as they move through the materials forming the circuit. Some materials, like metals, make it very easy for the charges to move, providing millions of mobile electrons that flow easily. Others like polythene provide no mobile electrons and current cannot flow through them.

In between these two extremes are many materials, which do allow charges to move but not as easily as metals do. As charges flow through these materials they transform lots of their energy into heat as they bump their way through the microscopic structure. These materials are known as poor conductors or resistors (they resist the flow of charges). A resistor transforms the electrical energy of the charges into heat.

The unit used to measure resistance is called the Ohm ( ) it is a measure of how many volts are needed to push 1 amp through the component.

Resistance ( ) = Potential difference (V)

Current (A)

What causes RESISTANCE in a wire?

• The charges are pushed around the circuit by the power supply.

• The wire is made from ATOMS.

• If a charge hits an atom on its journey through the wire it loses ENERGY.

• This causes the charge to slow down, and reduces the CURRENT.

• If the current IS LESS then the RESISTANCE must be LARGE (If the POTENTIAL DIFFERENCE doesn’t change).

What is the effect of length on resistance?

• The more atoms there are in a wire, the bigger the chance that the charges will hit the atoms.

• A LONG wire has more atoms than a SHORT wire, so a long wire has a HIGHER resistance than a short one.

• This means that the resistance of a wire INCREASES as the length of the wire INCREASES.

What is the effect of wire diameter on resistance?

• If the charges can get through the gaps between the atoms, the charges can’t collide with the atoms, so they can’t lose energy, so the resistance will be LESS.

• There are more gaps between atoms in a THICK wire than a THIN one, so a thick wire has LESS resistance than a thin one.

• This means that the resistance of a wire DECREASES as the thickness of the wire DECREASES.

Why do different metals have different resistances?

The atoms in different metals are different SIZES, so different metals have different chances of having collisions between the charges and the atoms. This means that different metals have different Resistance’s. Some metals are good conductors, and have LOW resistance, but others are poor conductors, and have HIGH resistance. Gold, Silver and Copper are good conductors; they have low resistances.

Potential difference provides energy for the coulombs of charges to move through electrical resistance. The amount of charge which the pd can 'push' around