Resistivity and Superconductors

Resistance in a component

How difficult it is to get current to flow through it

Ohm’s Law

In an ohmic conductor at constant temperature:

V α I

What is resistance caused by?

Repeated collisions between charge carriers in the material and the fixed positive ions of the material

I-V characteristics graphs: resistor

I-V characteristics graphs: diode

Forward bias

The current flows in the forward direction due to the voltage applied in the forward direction

What is the threshold voltage for a diode?

0.6V

I-V characteristics graphs: filament lamp

Resistivity

The resistance of length 1m with a cross-sectional area of 1m²

What does resistivity tell you?

How difficult it is for current to flow through the material

What does resistance depend on?

• Length

• Area

• Resistivity

Thermistor

A component with a resistance that depends on its temperature

What is a ‘Negative Temperature Coefficient’ (NTC) thermistor?

The resistance decreases as the temperature increases

Thermistor graph

Explain the thermistor graph

Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available, so resistance is lower.

One use of thermistors

Temperature sensor

Superconductor

Carries electricity with 0 resistance

What is so good about superconductors?

No resistance, so none of the electrical energy is converted into heat, so none of it is wasted

How do you make something a supercondutor?

By cooling it down below its critical temperature

Uses of superconductors

• Power cables that transmit electricity without any loss of power

• Really strong electromagnets

• Electronic circuits that work really fast with minimal energy loss

Discuss one advantage and one difficulty when using superconductors in electrical transmission over long distances

Advantage - no power will be lost

Difficulty - keeping the superconducting material below its critical temperature