3.5.1.3 Resistivity

Question 1

What three things does the resistance of a wire depend on?

Answer 1

Length of the wire.
Cross sectional area of the wire.
Resistivity of the material of which the wire is made from.

Question 2

Describe and explain the relationship between length and resistance.

Answer 2

As length of a wire increases, the resistance of the wire increases. This is because, the longer the wire is, the more difficult it is for current to flow through it.

Question 3

Describe and explain the relationship between cross sectional area and resistance.

Answer 3

A cross-sectional area increases, the resistance of the wire decreases.
The wider a wire, the easier it is for electrons/charge carriers to flow through it, as there are more pathways for them to flow through.

Question 4

What is resistivity?

Answer 4

A measure of how much a particular material resists current flow. It is a property of material. It depends on environmental factors, like temperature and light intensity.

Question 5

Define resistivity.

Answer 5

The resistivity of a material is defined as the resistance of a 1 metre length with a cross sectional area of 1 metre squared.

Question 6

What equation links resistance, resistivity, length and cross sectional area?

Answer 6

Resistance = Resistivity x  Length / Cross sectional area.
R=  ρl/A

Question 7

Give two equations which can be used to calculate the cross sectional area of a wire.

Answer 7

=πr^2 or A= (πd^2)/4

Question 8

What is the SI unit for length?

Answer 8

Metres, m

Question 9

What is the SI unit for resistivity?

Answer 9

Ohm-metres, Ωm

Question 10

What is the SI unit for cross sectional area?

Answer 10

Metres squared.

Question 11

Explain, in terms of structure, the relationship between length and resistance.

Answer 11

When the length increases, there are more positive ion cores in the way of the gas of electrons moving through the conductor, so it is harder for current to flow. Resistance therefore increases.

Question 12

Explain, in terms of structure, the relationship between area and resistance.

Answer 12

When the cross sectional area of the conductor increases, there are more conducting pathways through the conductor for electrons to move through. Therefore it is easier for current to flow, so resistance decreases.

Question 13

What is an intrinsic property of a material?

Answer 13

An intrinsic property of a material is a property of itself, and is independent of other factors.

Question 14

Which intrinsic properties does the resistivity depend on?

Answer 14

It directly relates to the number of free, conducting electrons that can flow through the structure.
The mobility of these electrons to flow through the structure.
The arrangement of the atoms in the conductor, and any distribution of impurities will affect the mobility of the electrons.

Question 15

How is resistivity dependant on temperature, in a conductor, semiconductor and superconductor?

Answer 15

As the temperature of a metal increases, the resistivity increases.
As the temperature of a semiconductor increases, the resistivity decreases.
The resistivity of a superconducting material decreases with decreasing temperature above its critical temperature, but will drop below to 0 ohm meters below the critical temperature.

Question 16

What are semiconductors?

Answer 16

Semiconductors are a group of materials that are not as good at conducting electricity as metals, because they have far fewer charge carriers available.

Question 17

What happens to a semiconductor when energy is supplied to it?

Answer 17

If energy supplied to a semiconductor, more charge carriers can be released and the resistivity of the material will decrease.

Question 18

What are semiconductors commonly used for?

Answer 18

Semiconductors are used for sensors for detecting environmental changes.

Question 19

What are three common semiconducting componentd?

Answer 19

Thermistors
Diodes, including LEDs
Light dependant resistors/LDRs

Question 20

What is a thermistor?

Answer 20

A thermistor is a component with a resistance that depends on its temperature. It is made with an intrinsic semiconductor.

Question 21

What is an NTC?

Answer 21

An NTC is a negative temperature coefficient. This means that the temperatures goes up, meaning resistance will decrease.

Question 22

What happens in an NTC Thermistor, in terms of resistance per kelvin temperature change?

Answer 22

The percentage change of resistance per kelvin change of temperature is much greater than for a metal. For this reason, thermistors are often used as the temperature-sensor in a temperature sensitive component.

Question 23

What does the resistance against temperature graph for an NTC Thermistor show?

Answer 23

The graph shows the non-linear relationship between resistance and temperature.

Question 24

What happens when a thermistor is warmed?

Answer 24

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

Question 25

What is a superconductor?

Answer 25

A superconductor is a wire or device made of a material that has zero resistivity at, and below, a critical temperature that depends on the material.

Question 26

How can you lower the resistivity of a material?

Answer 26

You can lower the resistivity of a material by cooling the material. By cooling it enough to get the temperature below the critical level, the resistance/resistivity disappears.

Question 27

What happens when a current passes through a superconductor?

Answer 27

When a current passes through a superconductor, there is no potential difference across it, because its resistance is zero. Therefore the current has no heating effect.

Question 28

Give three used of superconductors?

Answer 28

Superconductors are used to make high-power electromagnets.
They are also used in transmission power cables.
They can also be used in electronic circuits.

Question 29

Why are superconductors used to make high-power electromagnets?

Answer 29

They can generate very strong magnetic fields, which can be used in devices such as MRI scanners and particle accelerators.

Question 30

Why are superconductors used in power cables?

Answer 30

They can transmit electricity without any loss of power. They have no resistance, so no electrical energy will be transferred as wasted heat as current passes through the wire.

Question 31

Why are superconductors used in electronic circuits?

Answer 31

They can work very fast, with minimal energy loss. There is no resistance to slow the current down, so the circuit will be very fast.