Resistivity and superconductivity

What causes resistance of a wire?

Delocalised (free) electrons colliding with positive ions

Describe the positive ions?

They make up the structure of the metal

What does resistance depend on?

Length, area, temperature, material

What is the trend for length?

If the length increases the resistance increases

What is the trend for area?

If the area increases the resistance decreases

What is the trend for the temperature?

If the temperature increases the resistance increases

Describe the trend for length?

The longer the piece of wire the more collisions the electrons will have

Describe the trend for area?

The wider the piece of wire the more gaps there are between the ions

Describe the trend for temperature?

As temperature increases the ions are given more energy and vibrate more

What happens when the ions are given more energy and vibrate more?

The electrons are more likely to collide with the ions

Describe the material?

The structure of any two metals is similar but not the same

Describe how materials are similar but not the same?

Some metal ions are closer together and others have bigger ions

What is rho?

The resistivity of the material

What is the importance of resistivity?

It accounts for the structure of the metal and the temperature

Describe the resistivity of a metal?

Each metal has its own value of resistivity at each temperature

What is resistivity measured in?

Ohm metres - Ωm

How do you measure the cross-sectional area

Area = π r squared

How do you measure the resistance?

Measure values of current and potential difference

What is the trend of temperature and resistivity?

The resistivity of a metal increases with its temperature

What can be done to certain metals?

They can be cooled below a critical temperature

What happens when certain metals are cooled below a critical temperature?

Their resistivity drops to zero - they have zero resistance

What happens when the metal has zero resistance?

It allows massive currents to flow without losing any energy as heat

What are metals that can have a zero resistance?

They are called superconductors

What happens when a superconductor is heated above its critical temperature?

It loses its superconductivity and behaves like other metals

What is the problem with making superconductors reach their critical temperature?

Large amounts of energy are needed to cool the metal below their critical temperature

What are uses of superconductors?

High-power electromagnets, power cables, MRI scanners

What does MRI stand for?

Magnetic Resonance Imaging