4 exam-style question sets on Electrodynamics (Paper 1), each with a hint and a fully worked answer. The app holds 20 questions on this section in total, including variants of every set below, and lets you mark yourself part by part.
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Question 1
A generator in a school laboratory drives a current through a device of resistance 12 Ω. The graph shows how the output current of the generator varies with time. The current reaches a maximum value of 3 A, and one complete cycle takes 0,04 s.
- aIs this an AC generator or a DC generator? Give a reason for the answer by referring to the graph. (2)
- bState the energy conversion that takes place in the generator while it is in operation. (2)
- cName the phenomenon on which the operation of the generator is based. (1)
- dCalculate the frequency at which the coil of the generator rotates. (2)
- eDefine the term root mean square (rms) current. (2)
- fCalculate the rms current delivered by the generator. (3)
- gCalculate the average power dissipated in the 12 Ω device. (3)
This question has a diagram, shown in the app.
Hint
One full cycle on the graph gives the period; use Irms = Imax/√2 and Pave = I²rms R.
Worked answer
a) AC generator ✓ — the graph shows that the current periodically reverses direction (it alternates between positive and negative values) ✓.
b) Mechanical (kinetic) energy is converted to electrical energy. ✓✓
c) Electromagnetic induction. ✓
d) ✓ Hz ✓
e) The rms current is the equivalent direct (DC) current that would dissipate the same amount of energy in a resistor as the alternating current. ✓✓
f) ✓✓ A ✓
g) ✓✓ W ✓
Modelled on Nov 2024 Paper 1, Question 9 — same skills, new scenario.
Question 2
In a direct-current (DC) motor, the component that reverses the direction of the current in the coil every half rotation is the …
Hint
Which component swaps the coil's connections to the circuit every half turn?
Worked answer
B. The split-ring commutator swaps the coil's connections to the external circuit every half rotation, reversing the current in the coil so that the turning force stays in the same rotational direction. A belongs to AC machines — slip rings maintain continuous contact and never reverse the current. C only conducts current between the stationary circuit and the rotating rings. D provides the magnetic field but does not switch the current.
Modelled on Nov 2024 Paper 1, Question 1.9 — same skill, new scenario.
Question 3
An AC generator at a hiking lodge supplies a heater of resistance 44 Ω. The maximum (peak) potential difference produced by the generator is 339,41 V. The graph of the generator's output voltage versus time is a sine curve with a period of 0,02 s, as shown.
- aDefine the term rms potential difference. (2)
- bWrite down the function of the slip rings in this generator. (2)
- cCalculate the cost of operating the heater for 2,5 hours if electricity costs R2,85 per kWh. (5)
- dRedraw the given voltage-time graph in the ANSWER BOOK and label it A. On the same set of axes, draw the graph for ONE rotation of the coil when the rotation speed is DOUBLED. Label this graph B. (3)
- eState ONE change that must be made to this generator to convert it into a DC generator. (1)
This question has a diagram, shown in the app.
Hint
Convert the peak voltage to rms first, then work in kW and hours so the energy comes out in kWh.
Worked answer
a) The rms potential difference is the value of the constant (DC) potential difference that would dissipate the same amount of energy in a resistor as the alternating potential difference does. ✓✓
b) The slip rings maintain electrical contact between the rotating coil and the external circuit (via the brushes) while allowing the coil to rotate freely. ✓✓
c) V ✓
✓ W ✓
Energy kWh ✓
Cost R9,33 ✓
d) Graph A: sine curve, amplitude 339,41 V, period 0,02 s. ✓
Graph B: amplitude DOUBLED to 678,82 V ✓ and period HALVED to 0,01 s (one complete cycle between 0 and 0,01 s) ✓.
e) Replace the two slip rings with a split-ring commutator. ✓
Modelled on Nov 2025 Paper 1, Question 9 — same skills, new scenario.
Question 4
In a DC generator, the function of the split-ring commutator is to ...
Hint
What is the difference between the outputs of AC and DC generators?
Worked answer
B. The emf induced in the rotating coil alternates, but the split-ring commutator swaps the coil's connections to the brushes every half rotation, so the external current always flows the same way — that is what makes the generator DC. A is wrong: the commutator does not change the size of the emf. C is wrong: the mechanical driving system, not the commutator, sets the rotation speed. D is wrong: the field magnets are fixed. Modelled on Nov 2025 Paper 1, Question 1.9 — same skills, new scenario.