Electricity

Question 1

Resistivity

Answer 1

Resistivity is a measure of how much resistance a material provides against the current, regardless of other factors e.g. length. However, it does change with temperature.

ρ = RA/L

Question 2

How does temperature affect a thermistor?

Answer 2

As temperature increases, the resistance of the thermistor decreases.

Question 3

Superconductors

Answer 3

Superconductors are materials with a resistivity of zero below a critical temperature. This means they can reduce energy loss in the transmission of electrical power.

The critical temperature is around a few degrees above absolute zero for most superconductors, which is very expensive to get to and so limits their use. Superconductors are used in magnets needing high currents, such as MRI scanners.

Question 4

Circuit conservation laws

Answer 4

Energy and charge are always conserved in a circuit.

Question 5

Potential divider

Answer 5

Voltage is divided between components in a circuit depending on their resistance. This means that if we have an input voltage, a resistor and another component, we can control the voltage of the component by changing the resistance of the resistor.

For example, you could have a potential divider circuit with a thermistor and the compressor of a fridge. The higher the temperature, the lower the resistance of the thermistor, so the compressor gets more voltage, and thus, it cools down the fridge more, keeping the temperature of the fridge constant.

Question 6

Electromotive force

Answer 6

EMF is the voltage of a cell when the current is 0.

ℰ = E/Q
ℰ = IR + Ir

Question 7

Cells in parallel

Answer 7

Cells with the same EMF and internal resistance in parallel get their voltage cancelled out so that the total output voltage equals the EMF of one of them.