Induction: voltage first, current needs a circuit
A voltage (e.m.f.) is induced whenever a conductor cuts magnetic field lines, or when the field through a coil changes. Keep the sequence in mind: cut field lines → a voltage is induced → and only if the circuit is complete does an induced current flow. Two situations produce it — a wire or coil moves through a field, or the field through a stationary coil changes. No movement and no field change means no induced voltage; reversing the motion reverses it too.
Factors that increase the induced voltage
Four factors increase the induced voltage: move or rotate faster; use a stronger magnetic field; use a coil with more turns (each turn adds to the voltage); and use a larger coil area. In a generator they combine — spinning faster raises both the voltage and the a.c. frequency. There is no induced-voltage formula at this level, so marks come from stating these factors plus the mechanism: faster → more field lines cut per second → larger induced voltage → larger current.
The a.c. generator: rotate a coil or a magnet
Electricity is generated by continuously changing the field through a coil, two ways: rotate a magnet inside a fixed coil, or rotate a coil between the poles of a fixed magnet. The output is alternating (a.c.) because in each half-turn the coil sides cut field lines in the opposite direction, so the induced voltage reverses. A rotating coil connects to the circuit through slip rings and brushes; a split-ring commutator would instead give d.c.
Drawn from real examiner reports.
a.c. reverses direction continuously
Most know d.c. flows one way only; the harder mark is stating that a.c. continuously changes (reverses) direction, its size varying too. To change a generator output you must change the induction itself — spin faster/slower, use a stronger/weaker field, or more/fewer turns. Transformers or reducing the current are not credited: they do not change the induction.
Jun 2023 1P Q12(b)(i): the mark for a.c. continuously changing direction was harder to earn than the d.c. one-direction mark.
Induced current vs induced voltage
The most-penalised error here is writing that a current is induced when the mark scheme wants an induced voltage. A moving conductor always has a voltage induced across it, but a current flows only in a complete circuit. Safe wording: a voltage is induced, and because the circuit is complete it drives an induced current. Name the voltage first, current second.
Jun 2024 1PR Q10a; Jun 2023 1P Q12(b)(ii): induced current stated instead of induced voltage was a main reason marks were missed.
Generator is not a motor
A motor takes electrical energy IN: a supplied current in a coil sitting in a field makes the fields interact, and the coil turns. A generator does the reverse: an external force turns the coil, which induces a voltage (electrical energy OUT). Quick test — if your answer supplies a current to make something spin, you are describing a motor, not a generator.
Cut field lines, not interact
For induction you must say the coil cuts magnetic field lines, and state that the field being cut is due to the magnets. Interacting fields describes the motor effect and does not score for a generator. Examiners saw interacting written often, but the creditworthy word cutting was rarely seen.
Jun 2023 1P Q12(b)(ii): the creditworthy idea of cutting field lines was rarely seen; interacting (which does not score) was seen more often.
Slip rings vs split-ring commutator
Slip rings — two full rings — keep the coil connection to the circuit the same, so a rotating-coil generator outputs a.c. A split-ring commutator swaps the connection every half-turn, which cancels the reversal and gives d.c.; that is the motor arrangement. Drawing a commutator on an a.c. generator loses the mark.
Write induction as a sequence
Approach induction with a sequence of events in mind: the coil or magnet moves (or the field changes) → the conductor cuts field lines → a voltage is induced → because the circuit is complete, this drives an induced current. Order matters — name the voltage before the current.
Bigger voltage: name a factor and effect
For "how could the voltage be bigger?" name a specific factor and its effect: spin faster (cuts more field lines per second), add more turns, or use a stronger magnet. A vague "stronger" does not score. Examiners want higher speed → higher voltage → higher current.
Say a.c.; use the mark tariff
For "what type of current?" always answer a.c., and add that it continuously changes direction. Use the marks available as a guide to how many distinct points to give, and never describe a motor when the question asks about a generator.
There is no induced-voltage formula to learn at this level (Faraday's law is not required on 4SD0/4PH1). Electromagnetic induction is assessed by describing the mechanism and the factors that change the induced voltage. The numeric marks come from applying the electricity relationships to the induced voltage and current:
| Relationship | Formula | Used for |
|---|---|---|
| Ohm's law | the induced voltage drives an induced current through resistance | |
| Electrical power | the electrical power delivered by a generator | |
| Charge | charge moved by an induced current in time | |
| a.c. period | the time for one complete cycle of the a.c. output |
Where = voltage/e.m.f. (V), = current (A), = resistance (), = power (W), = charge (C), = time (s), = frequency (Hz), = period (s). Here e.m.f. means electromotive force, i.e. the induced voltage.
The factors that increase the induced voltage: move/rotate faster, use a stronger magnetic field, use more turns on the coil, use a larger coil area.
Definitions (mark-scheme form):
Define electromagnetic induction.
A bicycle dynamo (a small generator) induces a voltage of 6.0 V. It is connected to a lamp so that there is a complete circuit of total resistance 3.0 .
Calculate the induced current in the circuit.
