Topic 5: Electricity and magnetism

Question 1

power = VI

Answer 1

power = power (W)
V = potential difference (V)
I = current (A)

Question 2

R = ρL / A

Answer 2

R = resistance (Ω)
ρ = resistivity (Ωm)
L = length (m)
A = cross-sectional area (m^2)

Question 3

power = I^2 R

Answer 3

power = power (W)
I = current (A)
R = resistance (Ω)

Question 4

Explain, in terms of electrons, what happens to the resistance of the cable as the temperature of the cable increases.

Answer 4

• Increased collisions of electrons
• Drift velocity decreases so current decreases
• Potential difference is constant so if current decreases, resistance has to increase

Question 5

What is drift velocity?

Answer 5

The speed of an electron through a conductor

Question 6

What is often the magnitude of the drift velocity in an everyday situation?

Answer 6

10^–4 m/s

Question 7

What is the relationship between drift velocity and current?

Answer 7

Directly proportional

Question 8

I = nAvq

Answer 8

I = current (A)
n = number of charges per unit volume/charge density (m^-3)
A = cross-sectional area of the conductor (m^2)
v = drift velocity (m/s)
q = charge of charge carriers (C)