What you'll learn
Electromagnetic induction is the process by which a voltage is generated when a conductor moves through a magnetic field or when the magnetic field through a conductor changes. This principle underpins the operation of generators, transformers and much of our electrical infrastructure. Understanding electromagnetic induction is essential for GCSE Physics and forms approximately 4-6% of your Paper 2 examination.
Key terms and definitions
Electromagnetic induction — the generation of a potential difference (voltage) across a conductor when it experiences a change in magnetic field
Generator — a device that converts kinetic energy into electrical energy using electromagnetic induction
Alternator — a type of generator that produces alternating current (a.c.)
Dynamo — a type of generator that produces direct current (d.c.)
Transformer — a device that uses electromagnetic induction to change the voltage of an alternating current
Magnetic flux — a measure of the quantity of magnetism, considering the strength and extent of a magnetic field
Step-up transformer — a transformer that increases voltage (has more turns on the secondary coil than the primary coil)
Step-down transformer — a transformer that decreases voltage (has fewer turns on the secondary coil than the primary coil)
Core concepts
The principle of electromagnetic induction
When a conductor (such as a wire) cuts through magnetic field lines, or when the magnetic field through a coil changes, a potential difference is induced across the ends of the conductor. This effect was discovered by Michael Faraday in 1831.
For a voltage to be induced, there must be relative movement between the conductor and the magnetic field. Either the conductor moves through a stationary field, or the field changes around a stationary conductor.
The size of the induced potential difference can be increased by:
- Moving the conductor faster through the magnetic field
- Using a stronger magnetic field
- Increasing the number of turns on a coil
- Increasing the area of the coil
The direction of the induced potential difference (and hence the induced current if the circuit is complete) can be reversed by:
- Reversing the direction of movement
- Reversing the direction (polarity) of the magnetic field
The generator effect
The generator effect occurs when a conductor moves relative to a magnetic field, inducing a potential difference. If the conductor forms part of a complete circuit, an induced current will flow.
When a coil of wire rotates in a magnetic field (or when a magnet rotates inside a coil), the coil continuously cuts through magnetic field lines. As the coil rotates:
- The potential difference induced varies from zero to a maximum value
- The potential difference reverses direction each half turn
- This produces an alternating current (a.c.)
Slip rings in an alternator maintain continuous electrical contact while allowing the coil to rotate, producing a.c. output. The alternating nature means the current repeatedly changes direction.
Split-ring commutators in a dynamo reverse the connections every half turn, ensuring the current always flows in the same direction through the external circuit, producing d.c. output. However, this d.c. is not smooth—it varies in magnitude.
Practical applications of generators
Bicycle dynamos convert the kinetic energy of the rotating wheel into electrical energy to power lights. The wheel turns a magnet inside a coil, inducing a current. The faster the wheel rotates, the brighter the lights (higher induced voltage and current).
Power station generators operate on the same principle but on a much larger scale. Steam or water turbines rotate enormous electromagnets inside extensive coil systems. The UK National Grid relies on generators producing a.c. at 25,000 V, which is then transformed for transmission and distribution.
Wind turbines use the kinetic energy of moving air to rotate blades connected to a generator. The rotation induces a current in the generator coils, converting wind energy into electrical energy.
Transformers and their operation
A transformer consists of two coils of wire (the primary and secondary coils) wound around an iron core. When an alternating current flows through the primary coil:
- It creates a changing magnetic field in the iron core
- This changing magnetic field passes through the secondary coil
- The changing field induces an alternating potential difference in the secondary coil
- If the secondary circuit is complete, an alternating current flows
Transformers only work with alternating current because a changing magnetic field is required to induce a voltage in the secondary coil. Direct current produces a steady magnetic field, which does not induce a voltage.
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
- Increases efficiency by reducing energy losses
The transformer equation
The ratio of potential differences across the primary and secondary coils relates to the ratio of the number of turns on each coil:
Vₚ/Vₛ = nₚ/nₛ
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 a step-up transformer: n_s > n_p, therefore V_s > V_p (voltage increases)
For a step-down transformer: n_s < n_p, therefore V_s < V_p (voltage decreases)
Important principle: Transformers do not create energy. For 100% efficient transformers, the power input equals power output:
Vₚ × Iₚ = Vₛ × Iₛ
Where I_p and I_s are the currents in the primary and secondary coils respectively.
When voltage is stepped up, current must step down (and vice versa) to conserve energy. This relationship is crucial for understanding the National Grid.
Transformers and the National Grid
The UK National Grid transmits electrical energy at very high voltages (up to 400,000 V) to reduce energy losses during transmission.
Why high voltages reduce energy loss:
Power loss in transmission cables occurs due to the resistance of the wires, calculated using:
Power loss = I² × R
Where I is current and R is resistance of cables.
To transmit the same power (P = V × I) at higher voltage requires lower current. Since power loss depends on current squared, reducing current dramatically reduces wasted energy as heat in the cables.
Step-up transformers at power stations increase voltage from 25,000 V to 400,000 V for transmission.
Step-down transformers reduce voltage in stages:
- From 400,000 V to 132,000 V at grid substations
- To 33,000 V for heavy industry
- To 230 V for homes and businesses
This system allows efficient transmission over long distances while providing safe, usable voltages to consumers.
Worked examples
Example 1: Calculating induced voltage
Question: A student moves a bar magnet quickly into a coil of 50 turns. This induces a potential difference of 2.0 V. The student then uses a coil with 200 turns and moves the magnet at the same speed. Calculate the induced potential difference. [2 marks]
Solution:
The induced voltage is directly proportional to the number of turns (all other factors constant).
Ratio of turns = 200/50 = 4
Induced voltage = 2.0 V × 4 = 8.0 V ✓✓
(1 mark for correct method/ratio; 1 mark for correct answer with unit)
Example 2: Step-up transformer calculation
Question: A transformer has 200 turns on the primary coil and 800 turns on the secondary coil. The input voltage is 230 V.
(a) Calculate the output voltage. [2 marks] (b) State what type of transformer this is. [1 mark]
Solution:
(a) Using the transformer equation:
Vₚ/Vₛ = nₚ/nₛ
230/Vₛ = 200/800
Vₛ = (230 × 800)/200 ✓
Vₛ = 920 V ✓
(1 mark for correct rearrangement/substitution; 1 mark for correct answer with unit)
(b) Step-up transformer ✓
(Output voltage is greater than input voltage, or secondary turns greater than primary turns)
Example 3: Current and power in transformers
Question: An ideal (100% efficient) transformer steps down voltage from 11,000 V to 230 V. The secondary current is 5.0 A. Calculate the primary current. [3 marks]
Solution:
For 100% efficient transformer: power in = power out ✓
V_p × I_p = V_s × I_s
11,000 × I_p = 230 × 5.0 ✓
I_p = (230 × 5.0)/11,000
I_p = 0.10 A (or 0.1 A) ✓
(1 mark for recognising power conservation; 1 mark for correct substitution; 1 mark for correct answer)
Common mistakes and how to avoid them
Confusing which transformer is which: Remember step-UP increases voltage (more turns on secondary), step-DOWN decreases voltage (fewer turns on secondary). Link it to lifts: stepping up means going up, stepping down means going down.
Thinking transformers work with d.c.: Transformers require changing magnetic fields. Only a.c. (alternating current) provides this continuous change. D.c. produces a steady field that induces voltage only when switched on/off, not continuously.
Mixing up the transformer equation: Always write it as V_p/V_s = n_p/n_s. Don't cross the letters—primary with primary, secondary with secondary. Check your answer makes sense: more turns should give more voltage.
Forgetting that high voltage means low current: When voltage steps up in the National Grid, current steps down proportionally (for constant power). Students often incorrectly think both increase together.
Not stating all methods to increase induced voltage: In "suggest how to increase..." questions, give multiple methods: move faster, stronger magnet, more turns, larger area. Each valid suggestion earns marks.
Ignoring units in calculations: Always include units (V for voltage, A for current) in final answers. Mark schemes specifically allocate marks for correct units.
Exam technique for "Electromagnetic induction"
"Describe" questions about generators or transformers require clear sequences. Use numbered steps or link words ("This causes...", "Which then...", "Resulting in...") to show the chain of events. Aim for 3-4 linked points for 3-4 marks.
Command word "Explain" requires reasons. Don't just state what happens—say why it happens. For example: "The voltage increases because there are more turns on the secondary coil, meaning each turn cuts field lines."
Calculation questions always show working. Even if your final answer is wrong, you can gain method marks. Write the equation, substitute values, then calculate. For 3-mark calculations, expect: formula (1), substitution (1), answer with unit (1).
National Grid questions commonly ask about efficiency. Remember the key chain: high voltage → low current → low I²R losses → more efficient transmission. Each arrow could be worth a mark.
Quick revision summary
Electromagnetic induction generates voltage when conductors move through magnetic fields or fields change. Generators convert kinetic energy to electrical energy using rotating coils in magnetic fields—alternators produce a.c., dynamos produce d.c. Transformers use electromagnetic induction to change a.c. voltages using the equation V_p/V_s = n_p/n_s. The National Grid uses step-up transformers for high-voltage transmission (reducing current and I²R losses) and step-down transformers to provide safe voltages to consumers.