3.5.1.3 Resistivity

Question 1

What is resistivity?

Answer 1

The resistance of 1m length with a 1m^2 cross sectional area
rho=RA/L
(resistivity = (Resistance*cross sectional area)/length of component

• it is a property of a material, so not affected by shape

Question 2

What are the factors which affect resistance of an ohmic conductor?

Answer 2

Length of the conductor
Cross-sectional area of the conductor
Material of the conductor

Question 3

How does length of wire affect resistance?

Answer 3

Longer wire = more resistance

Question 4

How does cross sectional area of wire affect resistance?

Answer 4

More area = current flows more easily so less resistance

Question 5

How does the material of wire affect resistance?

Answer 5

Different materials have different resistivities

Question 6

What happens in NTC thermistors?

Answer 6

Resistance decreases with temperature. An increase in temperature releases electrons. Resistance is lower then the charge carrier density is higher. This effect outweighs the effect of increasing resistance due to increasing lattice vibrations

Question 7

What are thermistors used for?

Answer 7

Sensor circuits
Thermostats

Question 8

What happens in LDRs?

Answer 8

Resistance decreases with light intensity because electrons are released when light is incident therefore more charge carriers and lower resistivity.

Question 9

What is superconductivity?

Answer 9

The phenomenon of zero electrical resistance below a critical temperature

Question 10

What is a critical temperature?

Answer 10

The temperature below which a material is superconducting. This temperature depends on the material

Question 11

What are superconductors used for?

Answer 11

To make high-power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators..
Improved power transmission efficiency - no resistance means no resistive power losses