Charge, current and potential difference

WPH12Topic 2.64 questions

Current as rate of flow of charge, potential difference as work done per unit charge, and charge carriers in conductors.

Specification points

Understand that electric current is the rate of flow of charged particles and be able to use the equation $I = \frac{\Delta Q}{\Delta t}$

Understand how to use the equation $V = \frac{W}{Q}$

Understand how the distribution of current in a circuit is a consequence of charge conservation

Understand how the distribution of potential differences in a circuit is a consequence of energy conservation

Be able to use $I = nqvA$ to explain the large range of resistivities of different materials

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Original practice questions written in the style of the exam.

Question 13 marks
A current of $0.25\,\text{A}$ flows in a lamp for $3.0$ minutes.

(a) Calculate the charge that flows through the lamp.

(b) Calculate the number of electrons that pass through the lamp in this time.
Mark scheme
- Converts time: $t = 180\,\text{s}$ (1)
- $Q = It = 0.25 \times 180 = 45\,\text{C}$ (1)
- $N = 45/1.60 \times 10^{-19} = 2.8 \times 10^{20}$ (1)
Question 23 marks
A copper wire has a cross-sectional area of $1.0\,\text{mm}^2$ and carries a current of $2.0\,\text{A}$ . Copper has $8.5 \times 10^{28}$ free electrons per cubic metre.

28

Calculate the drift velocity of the electrons.

Mark scheme

Converts area: $A = 1.0 \times 10^{-6}\,\text{m}^2$ (1)
Uses $I = nqvA$ : $v = \dfrac{2.0}{8.5 \times 10^{28} \times 1.60 \times 10^{-19} \times 1.0 \times 10^{-6}}$ (1)
$v = 1.5 \times 10^{-4}\,\text{m s}^{-1}$ (1)

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