6.5 Resistivity

Question 1

What is resistivity?

Answer 1

The resistivity of a material is defined as the resistance of a 1 m length with a 1 m^2 cross-sectional area
It is a measure of how much a particular material resists current flow. It depends on the structure of the material as well as environmental factors such as temperature and light intensity

Question 2

What is resistivity measured in?

Answer 2

Ohm-meters

Question 3

How does resistivity of a wire depend on length?

Answer 3

The longer the wire, the more difficult it is to make a current flow through it.
The resistance is proportional to the length of the wire.

Question 4

How does resistivity of a wire depend on area?

Answer 4

The wider the wire, the easier it will be for the electrons to pass along it

Question 5

What are semiconductors?

Answer 5

Semiconductors are a group of materials that aren’t as good at conducting electricity as metals, because they have fewer charge carriers available.
However, if energy is supplied to a semiconductor, more charge carriers can be released and the resistivity of the material decreases.

Question 6

What is a thermistor?

Answer 6

A component with a resistance that depends on its temperature

Question 7

What is a NTC thermistor?

Answer 7

NTC stands for ‘negative temperature coefficient’
This means that the resistance decreases as the temperature goes up
Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available, so the resistance is lower.

Question 8

How would you investigate the resistance of a thermistor?

Answer 8

• use a circuit with a power supply, thermistor and ammeter
• you can control the temperature of the thermistor using a water bath (for a waterproof thermistor)
• measure and record the temperature of the water using a digital thermometer and the current through the circuit (keep potential difference constant)
• continue to record the current and temperature for every 5 degree drop in temperature
• use the recorded values for potential difference and current to calculate resistance of the thermistor at each temperature

Question 9

How is a superconductor formed?

Answer 9

You can lower the resistivity of many materials like metals by cooling them down. If you cool some materials down to below a critical temperature, their resistivity disappears entirely and they become a superconductor.

Question 10

What is a ‘transitional temperature’?

Answer 10

The temperature that a material becomes a superconductor at

Question 11

What are the uses of superconductors?

Answer 11

• power cables that transmit electricity without any loss of power
• really strong electromagnets that have lots of applications
• electronic circuits that work very fast with minimal energy loss, because there is no resistance to slow the current down

Question 12

What is the equation for resistivity?

Answer 12

Resistivity = (resistance x area)/length