4A.4 resistivity

Question 1

What is resistivity?

Answer 1

A property of a material that measures how strongly it resists electric current.

Question 2

What symbol represents resistivity?

Answer 2

ρ (rho).

Question 3

What is the SI unit of resistivity?

Answer 3

Ohm metre (Ω m).

Question 4

What equation relates resistance and resistivity?

Answer 4

R = ρL/A.

Question 5

What do the symbols represent in R = ρL/A?

Answer 5

R = resistance, ρ = resistivity, L = length, A = cross-sectional area.

Question 6

How does resistance change with length of a conductor?

Answer 6

Resistance increases as length increases.

Question 7

How does resistance change with cross-sectional area?

Answer 7

Resistance decreases as cross-sectional area increases.

Question 8

What does resistivity depend on?

Answer 8

The material and temperature.

Question 9

Why do materials have resistance?

Answer 9

Because electrons collide with atoms in the conductor.

Question 10

What happens to resistance when collisions increase?

Answer 10

Resistance increases.

Question 11

What is the resistivity of copper (approx)?

Answer 11

1.7 × 10⁻⁸ Ω m.

Question 12

What is the resistivity of aluminium (approx)?

Answer 12

2.8 × 10⁻⁸ Ω m.

Question 13

What is the resistivity of constantan (approx)?

Answer 13

4.9 × 10⁻⁷ Ω m.

Question 14

What is conductivity?

Answer 14

The reciprocal of resistivity.

Question 15

What is the equation for conductivity?

Answer 15

σ = 1/ρ.

Question 16

What is the unit of conductivity?

Answer 16

Siemens per metre (S m⁻¹).

Question 17

How is resistivity measured experimentally?

Answer 17

By measuring resistance for different lengths of wire and calculating using R = ρL/A.

Question 18

What instrument measures wire diameter in a resistivity experiment?

Answer 18

Micrometer screw gauge.

Question 19

Why measure wire diameter several times?

Answer 19

To reduce uncertainty and calculate a mean value.

Question 20

Why should current be kept low in resistivity experiments?

Answer 20

To prevent heating of the wire.

Question 21

Why does heating affect resistivity measurements?

Answer 21

Because resistivity changes with temperature.

Question 22

What graph can be used to find resistivity experimentally?

Answer 22

Resistance vs length graph.

Question 23

What does the gradient of a resistance vs length graph represent?

Answer 23

ρ/A.

Question 24

How can resistivity be calculated from the graph gradient?

Answer 24

ρ = gradient × A.

Question 25

What is the cross-sectional area of a wire?

Answer 25

A = πr².

Question 26

Why is resistivity a property of the material?

Answer 26

Because all samples of the same material have the same resistivity regardless of shape or size.