What you'll learn
This topic explains how movement between a conductor and a magnetic field produces an electric current—the principle behind power stations, wind turbines and bicycle dynamos. You'll learn how to predict the direction and size of induced currents, understand alternating and direct current, and explain how transformers work in the National Grid.
Key terms and definitions
Electromagnetic induction — the process of inducing a potential difference (and current if there's a complete circuit) in a conductor when it experiences a change in magnetic field.
Generator effect — the production of a potential difference across a conductor when it moves relative to a magnetic field or when the magnetic field around it changes.
Magnetic flux — a measure of the quantity of magnetism, taking into account the strength and extent of a magnetic field (represented by the number of magnetic field lines).
Induced current — the electric current that flows in a conductor as a result of electromagnetic induction when the circuit is complete.
Alternating current (a.c.) — electric current that continuously changes direction, typically in a sinusoidal pattern with a specific frequency.
Direct current (d.c.) — electric current that flows in one direction only, maintaining constant polarity.
Transformer — a device that uses electromagnetic induction to change the potential difference of an alternating current supply.
National Grid — the network of cables and transformers linking power stations to consumers across the UK.
Core concepts
The generator effect and electromagnetic induction
When a conductor (such as a wire) cuts through magnetic field lines, or when the magnetic field through a conductor changes, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, this causes an induced current to flow.
The three essential requirements for electromagnetic induction:
- A magnetic field
- A conductor (usually a wire or coil)
- Relative movement between the field and conductor OR a changing magnetic field strength
Common examples you'll encounter:
- Moving a wire through a magnetic field between two poles
- Moving a magnet into or out of a coil of wire
- Rotating a coil in a magnetic field (generators)
- Changing the current in one coil near another coil (transformers)
The size of the induced potential difference (and therefore induced current) can be increased by:
- Moving the wire or magnet faster
- Using a stronger magnet
- Increasing the number of turns on the coil
- Increasing the area of the coil
Direction of the induced current
The direction of the induced current depends on the direction of movement and the direction of the magnetic field. Fleming's right-hand rule helps you predict this direction (note: this is different from Fleming's left-hand rule for motors).
Fleming's right-hand rule:
- thuMb — direction of Motion (of the conductor relative to the field)
- First finger — direction of the magnetic Field (north to south)
- seCond finger — direction of induced Current
Hold your right hand with thumb, first finger and second finger at right angles to each other. Point your thumb in the direction of motion and your first finger in the direction of the magnetic field; your second finger then shows the direction of the induced conventional current.
Reversing either the direction of motion or the direction of the magnetic field will reverse the direction of the induced current. Reversing both will keep the current in the same direction.
Alternating current generators
An a.c. generator (alternator) converts kinetic energy into electrical energy using electromagnetic induction. The basic structure includes:
- A rectangular coil of wire
- A magnetic field (between north and south poles)
- Slip rings and brushes to connect the rotating coil to an external circuit
As the coil rotates in the magnetic field:
- The sides of the coil cut through magnetic field lines
- A potential difference is induced across the coil
- If connected in a circuit, an induced current flows
The induced potential difference and current continuously change direction because:
- Each side of the coil moves up through the field for half a rotation, then down for the other half
- When moving up, current is induced in one direction
- When moving down, current is induced in the opposite direction
- This creates alternating current (a.c.)
The output can be displayed on an oscilloscope showing a sinusoidal (sine wave) pattern. The frequency of the a.c. depends on how fast the coil rotates.
Direct current generators and dynamos
A d.c. generator has the same basic structure as an a.c. generator but uses a split-ring commutator instead of slip rings. This is a single ring split into two halves, with each half connected to one end of the coil.
The split-ring commutator swaps the connections every half turn, so although the current in the coil itself reverses, the current in the external circuit always flows in the same direction — producing direct current (d.c.).
A bicycle dynamo is a practical example of a small generator. As the wheel rotates, it turns a magnet inside a coil (or a coil inside a magnetic field), inducing a current that powers the bicycle lights.
Microphones and loudspeakers
Microphones use electromagnetic induction to convert sound energy into electrical energy:
- Sound waves cause a diaphragm to vibrate
- The diaphragm is attached to a coil of wire within a magnetic field
- As the coil moves, it cuts through magnetic field lines
- A varying potential difference is induced, creating an electrical signal that matches the sound wave pattern
Loudspeakers work in reverse, using the motor effect:
- A varying current (from an amplifier) passes through a coil in a magnetic field
- The coil experiences a force that varies with the current (motor effect)
- The coil is attached to a paper or plastic cone that vibrates
- These vibrations create sound waves
Together, microphones and loudspeakers demonstrate the reversibility of energy conversion between electrical and sound energy.
Transformers and the National Grid
A transformer consists of two coils of wire (the primary and secondary coils) wound around the same iron core. Transformers only work with alternating current.
How transformers work:
- Alternating current flows through the primary coil
- This creates a changing magnetic field in the iron core
- The changing magnetic field passes through the secondary coil
- This induces an alternating potential difference across the secondary coil
- If connected in a circuit, an induced current flows
The iron core is essential because it:
- Concentrates the magnetic field
- Ensures almost all the magnetic field from the primary coil passes through the secondary coil
- Is made of iron (a soft magnetic material that can be easily magnetised and demagnetised)
Step-up and step-down transformers:
- Step-up transformer — has more turns on the secondary coil than the primary; increases potential difference
- Step-down transformer — has more turns on the primary coil than the secondary; decreases potential difference
The transformer equation (for 100% efficient transformers):
$$\frac{V_p}{V_s} = \frac{n_p}{n_s}$$
Where:
- $V_p$ = potential difference across primary coil (V)
- $V_s$ = potential difference across secondary coil (V)
- $n_p$ = number of turns on primary coil
- $n_s$ = number of turns on secondary coil
For calculations involving power (assuming 100% efficiency):
$$V_p \times I_p = V_s \times I_s$$
Where $I_p$ and $I_s$ are the currents in the primary and secondary coils respectively.
The National Grid
The National Grid distributes electricity from power stations to consumers across the UK. Transformers play a crucial role:
Step-up transformers at power stations:
- Increase the potential difference to 400,000 V or 275,000 V
- This reduces the current needed to transmit the same power
- Lower current means less energy wasted as heat in the cables (since energy lost = $I^2 R t$)
- Makes long-distance transmission efficient
Step-down transformers:
- First reduction at local substations (to around 33,000 V)
- Final reduction near homes (to 230 V for domestic use)
- Ensures safe potential difference for household appliances
The efficiency gain from high-voltage transmission is significant. For the same power transmitted, doubling the voltage halves the current, which quarters the energy lost as heat (since power loss depends on $I^2$).
Worked examples
Example 1: Transformer calculation
Question: A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input potential difference is 240 V. Calculate the output potential difference. [3 marks]
Solution:
Write down the transformer equation: $$\frac{V_p}{V_s} = \frac{n_p}{n_s}$$
Substitute values: $n_p = 200$, $n_s = 50$, $V_p = 240$ V
$$\frac{240}{V_s} = \frac{200}{50}$$
Rearrange: $V_s = \frac{240 \times 50}{200}$
$V_s = 60$ V ✓
Mark scheme: 1 mark for correct equation, 1 mark for correct substitution, 1 mark for correct answer with unit.
Example 2: Power and transformers
Question: A step-up transformer increases the potential difference from 25,000 V to 400,000 V. The current in the primary coil is 800 A. Assuming the transformer is 100% efficient, calculate the current in the secondary coil. [3 marks]
Solution:
For a 100% efficient transformer: $V_p \times I_p = V_s \times I_s$
Substitute: $V_p = 25,000$ V, $I_p = 800$ A, $V_s = 400,000$ V
$25,000 \times 800 = 400,000 \times I_s$
Rearrange: $I_s = \frac{25,000 \times 800}{400,000}$
$I_s = 50$ A ✓
Mark scheme: 1 mark for correct equation or method, 1 mark for correct substitution, 1 mark for correct answer with unit.
Example 3: Explaining electromagnetic induction
Question: Explain why a potential difference is induced when a magnet is pushed into a coil of wire. [3 marks]
Solution:
When the magnet moves into the coil, the magnetic field through the coil changes ✓
The coil (conductor) experiences a changing magnetic field, which induces a potential difference across it ✓
This is electromagnetic induction / the generator effect ✓
Mark scheme: Marks awarded for: identifying the changing magnetic field; linking this to induced p.d.; naming the process.
Common mistakes and how to avoid them
Confusing Fleming's left-hand and right-hand rules. Left-hand rule is for motors (force on a current-carrying wire); right-hand rule is for generators (induced current from motion). Remember: Right for geneRators.
Stating that transformers work with d.c. Transformers require alternating current because they need a changing magnetic field. A steady d.c. creates a constant magnetic field that won't induce a potential difference in the secondary coil.
Forgetting units in transformer calculations. Always include V for voltage/potential difference and A for current. Show your working clearly to gain method marks even if your final answer is incorrect.
Thinking induced current requires movement in any direction. The conductor must cut through magnetic field lines (move across them), not along them. Moving parallel to field lines produces no induced current.
Reversing the transformer equation. Check whether it's step-up (secondary > primary) or step-down (secondary < primary). The ratio $\frac{V_p}{V_s} = \frac{n_p}{n_s}$ always holds.
Assuming all transformers are 100% efficient. Real transformers lose energy as heat in the coils and core. Only use $V_p I_p = V_s I_s$ when the question states or implies 100% efficiency.
Exam technique for "Electromagnetic induction and the generator effect"
Command word "Explain" requires you to give reasons why something happens, not just describe what happens. For induction questions, state that the field is changing and link this to the induced p.d. or current. Typically worth 2–3 marks.
Drawing and interpreting graphs of induced current or p.d. against time is common. For a.c. generators, show a smooth sine wave; for d.c. with a commutator, show the direction staying the same but the magnitude still varying.
Transformer calculations often appear as multi-step problems. Write the equation first, substitute values clearly, then solve. Show every step to maximise method marks if you make an arithmetic error.
When explaining the National Grid, focus on the relationship between high voltage, low current, and reduced energy loss ($P = I^2 R$). Link step-up transformers at power stations to efficiency and step-down transformers to safety for homes.
Quick revision summary
Electromagnetic induction occurs when a conductor experiences a changing magnetic field, inducing a potential difference. The generator effect is used in a.c. and d.c. generators, with alternators using slip rings and dynamos using split-ring commutators. Transformers change potential difference using two coils around an iron core and only work with a.c. The National Grid uses step-up transformers for efficient transmission at high voltage (low current, less energy wasted) and step-down transformers for safe domestic supply.