Edexcel IGCSE Physics 6(d) Electromagnetic Induction — Notes
Written for Pearson Edexcel International GCSE Physics (4PH1) and Science Double Award (4SD0), linear and modular. Electromagnetic induction is not part of Science Single Award (4SS0).
Which parts do I need?
Single Award candidates: nothing on this page is in your specification — sub-topic 6(d) is not in 4SS0. Double Award candidates need induced voltage, its factors and generators, and can skip every section tagged Physics only · 2P: all of the transformer work and the transmission of electricity.
What this page covers
| Spec | Modular no. | You need to be able to… | Route |
|---|---|---|---|
| 6.15 | 7.15 | know that moving a conductor through a field, or changing the field through a coil, induces a voltage; describe what makes that voltage larger | Double · Physics |
| 6.16 | 7.16 | describe how electricity is generated by spinning a magnet in a coil, or a coil in a field, and what affects the voltage | Double · Physics |
| 6.17P | 7.17P (4XPH1 only) | describe how a transformer is built and why different numbers of turns change an alternating voltage | Physics only · 2P |
| 6.18P | 7.18P (4XPH1 only) | explain why step-up and step-down transformers are used in the national transmission of electricity | Physics only · 2P |
| 6.19P | 7.19P (4XPH1 only) | recall and use the link between the voltage ratio and the turns ratio of a transformer | Physics only · 2P |
| 6.20P | 7.20P (4XPH1 only) | recall and use input power = output power for an ideal (100% efficient) transformer | Physics only · 2P |
Modular candidates: electromagnetic induction is in Topic 7 of the modular specifications — statements 7.15–7.20 of 4XPH1 Unit 2 (4WPH2), and 7.15–7.16 of 4XSD1 Physics Unit 6 (4WSD6).
In one minute
- A voltage is induced in a wire or coil when it moves through a magnetic field (cutting field lines) or when the magnetic field through it changes. With a complete circuit, the induced voltage drives an induced current.
- The induced voltage is larger with faster movement, a stronger magnet, and more turns on the coil. Reversing the movement or the field reverses the voltage.
- A generator spins a magnet inside a coil, or a coil inside a field, and produces an alternating voltage.
- A transformer has a primary coil and a secondary coil on a soft iron core and works only with a.c.: \(\dfrac{V_p}{V_s}=\dfrac{N_p}{N_s}\). Physics only · 2P
- For a 100% efficient transformer, \(V_pI_p=V_sI_s\). Transmission lines use a high voltage so the current is low and less energy is wasted heating the cables. Physics only · 2P
How is a voltage induced? Double · Physics
- The voltage is induced only while there is relative movement (or the field is changing). A stationary magnet inside a coil induces nothing, however strong it is.
- It does not matter whether the magnet moves or the coil moves — only the relative motion counts.
- Direction: reversing the motion (pulling out instead of pushing in), or using the other pole, reverses the induced voltage, so the meter deflects the other way.
- The induced voltage drives an induced current only if there is a complete circuit. With the coil disconnected there is still a voltage, but no current.
A field can also change without anything moving. If a second coil is placed next to the first and the current in it is switched on or off, the field it makes through the first coil grows or collapses, and a voltage is induced for that moment. This is the principle of the transformer (below).
When the N pole of a magnet is pushed slowly into a coil, the needle of a centre-zero meter connected to the coil moves 2 divisions to the right. Describe what the meter shows when (a) the magnet is held still inside the coil, (b) the N pole is pulled out slowly, (c) the S pole is pushed in quickly.
(a) Zero — no relative motion, so no voltage is induced.
(b) A deflection to the left, about 2 divisions — the motion is reversed, so the induced voltage is reversed.
(c) A deflection to the left, larger than 2 divisions — the pole is reversed (left) and the movement is faster (larger).
What affects the size of the induced voltage? Double · Physics
| To increase the induced voltage… | Why it works |
|---|---|
| move the magnet or the wire faster | field lines are cut at a greater rate (more per second) |
| use a stronger magnet | more field lines to cut in the same movement |
| use more turns on the coil | a voltage is induced in every turn and they add up |
| wind the coil on an iron core | the iron concentrates the field through the coil |
For a single straight wire, only the first two apply. The size of the induced voltage depends on how quickly the field lines are cut, not on how many have been cut in total.
How do generators produce electricity? Double · Physics
Rotating a magnet within a coil
- As the magnet turns, the magnetic field through the coil keeps changing — in size and direction — so a voltage is induced.
- Each half turn, the pole near the coil changes from N to S, so the voltage reverses: the output is an alternating voltage.
- The coil is fixed, so no sliding contacts are needed. Power-station generators work this way, with an electromagnet turning inside fixed coils.
Rotating a coil within a magnetic field
- As the coil turns, its sides cut the field lines, so a voltage is induced.
- Each side moves across the field lines one way for half a turn, then back the opposite way for the next half, so the voltage reverses every half turn — again a.c.
- The voltage is largest when the coil is parallel to the field (sides cutting the lines at right angles) and zero when the coil is at right angles to the field (sides moving along the lines).
- Slip rings and brushes connect the turning coil to the outside circuit without twisting the wires.
Factors that increase the peak voltage of either type: turning faster, a stronger magnet, more turns on the coil (and, for the rotating coil, a larger coil area or an iron core). Turning faster also increases the frequency of the a.c.
The solid line on the graph shows the output of a small generator. (a) State the peak voltage. (b) Determine the time period and the frequency of the output. (c) The coil is now turned twice as fast. Describe two changes to the graph.
(a) Peak voltage = 3.0 V
(b) One complete cycle takes 0.04 s. \(f=\dfrac{1}{T}=\dfrac{1}{0.04}=\) 25 Hz
(c) The peak voltage is larger (6.0 V, because field lines are cut at twice the rate) and the period is shorter (0.02 s), so the frequency rises to 50 Hz.
Answers that mention only the higher peak, or only the higher frequency, score half the marks.
How is a transformer built and how does it work? Physics only · 2P
Physics only (4PH1 Paper 2P). Not required for Double or Single Award.
Structure
- A primary coil (input side) and a secondary coil (output side) of insulated wire.
- Both are wound on a closed soft iron core, which links the magnetic field from the primary to the secondary. Soft iron is used because it magnetises and demagnetises easily as the field keeps reversing.
How it works
- An alternating current in the primary coil produces a magnetic field in the core that is constantly changing in size and direction.
- The changing field passes through the secondary coil and induces an alternating voltage across it.
- The size of the output voltage depends on the numbers of turns: more turns on the secondary than on the primary gives a larger output voltage.
| Type | Turns | Output voltage |
|---|---|---|
| step-up | more turns on the secondary (\(N_s>N_p\)) | larger than the input |
| step-down | fewer turns on the secondary (\(N_s<N_p\)) | smaller than the input |
Why not d.c.? A steady direct current makes a steady field in the core. The field through the secondary does not change, so no voltage is induced (apart from a brief pulse when the current is switched on or off).
How do you use the transformer equation? Physics only · 2P
Physics only (4PH1 Paper 2P).
\[ \frac{\text{input (primary) voltage}}{\text{output (secondary) voltage}}=\frac{\text{primary turns}}{\text{secondary turns}}\qquad \frac{V_p}{V_s}=\frac{N_p}{N_s} \]- The voltage ratio equals the turns ratio. Both voltages must be in the same unit (both V or both kV).
- Rearranged for the output: \(V_s=V_p\times\dfrac{N_s}{N_p}\).
- The equation is on the Equation List for Paper 2P, but you still need to choose and rearrange it.
What does input power = output power mean? Physics only · 2P
Physics only (4PH1 Paper 2P).
\[ \text{input power}=\text{output power}\qquad V_pI_p=V_sI_s\quad\text{(for 100\% efficiency)} \]- A transformer cannot create energy. If it is 100% efficient, the power out equals the power in (\(P=I\times V\) on each side).
- So if a transformer steps the voltage up, it steps the current down by the same factor, and the reverse.
- Real transformers are very efficient, but some energy is wasted heating the coils and core, so the output power is slightly less than the input power.
A transformer in a phone charger has 2300 turns on its primary coil, which is connected to the 230 V mains. Its secondary coil has 60 turns. (a) Calculate the output voltage. (b) The output current is 1.5 A. Calculate the current in the primary coil, assuming the transformer is 100% efficient.
(a) \(\dfrac{V_p}{V_s}=\dfrac{N_p}{N_s}\Rightarrow V_s=230\times\dfrac{60}{2300}=\) 6.0 V (a step-down transformer).
(b) \(V_pI_p=V_sI_s\Rightarrow 230\times I_p=6.0\times1.5=9.0\)
\(I_p=\dfrac{9.0}{230}=0.0391=\) 0.039 A
Check: the voltage is stepped down, so the current in the primary must be smaller than the current in the secondary — it is.
Why are transformers used to transmit electricity? Physics only · 2P
Physics only (4PH1 Paper 2P).
- A step-up transformer at the power station increases the voltage (for example from 25 kV to 400 kV).
- For the same power, \(P=I\times V\), a higher voltage means a lower current in the transmission lines.
- A current heats the cables, which have resistance. A lower current means less energy is wasted as thermal energy, so transmission is more efficient (and thinner, cheaper cables can be used).
- Step-down transformers near towns reduce the voltage to a level that is safe to use in homes (230 V).
A power station generator delivers 200 MW at 25 kV. A step-up transformer with 500 turns on its primary coil raises this to 400 kV. Assume the transformer is 100% efficient. (a) Calculate the number of turns on the secondary coil. (b) Calculate the current from the generator and the current in the transmission lines. (c) Explain why the electricity is not transmitted at 25 kV.
(a) \(N_s=N_p\times\dfrac{V_s}{V_p}=500\times\dfrac{400}{25}=\) 8000 turns
(b) \(I=\dfrac{P}{V}\): from the generator \(I=\dfrac{200\,000\,000}{25\,000}=\) 8000 A; in the lines \(I=\dfrac{200\,000\,000}{400\,000}=\) 500 A.
(c) At 25 kV the current in the lines would be 16 times larger. A larger current causes much more heating of the cables, so far more energy would be wasted as thermal energy before reaching users.
Common error: saying the high voltage “reduces resistance”. The resistance of the cables is the same; it is the current that is reduced.
Exam tips for 6(d)
Electromagnetic induction produces a voltage. A current flows only if there is a complete circuit. In explanations, write the sequence: movement / changing field → field lines cut → voltage induced → current (if the circuit is complete).
Credit is given for the idea that the conductor cuts field lines, or that the field through the coil is changing. “The magnet is near the coil” or “the coil is in a field” is not enough.
When a generator turns faster, say both: the peak voltage increases and the frequency increases (period decreases). On a graph, draw the new curve taller and with more cycles in the same time.
A full answer links four ideas: step-up raises the voltage → current is lower (same power) → less heating in the cables → less energy wasted. Then step-down for safe use. Drawing or naming only one transformer, or swapping step-up and step-down, loses marks. Physics only · 2P
The turns ratio has no units, but the two voltages must be in the same unit. Mixing kV and V in \(V_p/V_s\) gives an answer 1000 times too big or too small. Physics only · 2P
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| A magnet sitting inside a coil induces a current. | Induction needs relative movement or a changing field. | No movement, no change → no induced voltage. |
| Electromagnetic induction makes a current, whatever the circuit. | A current needs a complete circuit. | A voltage is induced; it drives a current only in a complete circuit. |
| A transformer works with d.c. from a battery. | A steady current gives a steady field, which induces nothing. | Transformers need a.c., which gives a constantly changing field. |
| A step-up transformer gives out more power. | Energy is conserved. | Voltage goes up but current goes down; power out ≤ power in. |
| High-voltage transmission lowers the resistance of the cables. | Resistance depends on the cables, not the voltage. | High voltage means low current, so less heating. |
Key equations
| Equation | Use it to find | Route | On the Equation List? |
|---|---|---|---|
| \(\dfrac{V_p}{V_s}=\dfrac{N_p}{N_s}\) | an output voltage, an input voltage or a number of turns | Physics only · 2P | yes — Physics 2P only |
| \(V_pI_p=V_sI_s\) (100% efficiency) | the current on one side of a transformer | Physics only · 2P | yes — Physics 2P only |
| \(P=I\times V\) | the current in transmission lines for a given power | All (from 2(b)) | yes — all papers |
| \(f=\dfrac{1}{T}\) | the frequency of a generator's output from its period | Double · Physics (from 3(b)) | yes — all papers |
There are no equations for 6.15 and 6.16 — those statements are tested by description and explanation, and by reading voltage–time graphs.
Check your understanding
1. State three ways of increasing the voltage induced when a magnet is pushed into a coil. Double · Physics
Push the magnet in faster; use a stronger magnet; use a coil with more turns. (An iron core in the coil is also accepted.)
2. A coil is connected to a meter. Explain why the meter reads zero while a magnet rests inside the coil. Double · Physics
There is no relative movement, so no field lines are cut and the field through the coil is not changing. No voltage is induced, so there is no current.
3. The generator in Worked example 2 (peak 3.0 V, period 0.04 s) is turned at half its original speed. State the new peak voltage and period. Double · Physics
Peak voltage 1.5 V; period 0.08 s.
4. A transformer has 50 turns on its primary coil and 200 turns on its secondary. The input is 12 V a.c. Calculate the output voltage and name the type of transformer. Physics only · 2P
\(V_s=12\times\dfrac{200}{50}=\) 48 V; a step-up transformer.
5. A 100% efficient transformer takes 0.50 A from a 230 V supply and gives an output of 23 V. Calculate the output current. Physics only · 2P
\(230\times0.50=23\times I_s\Rightarrow I_s=\dfrac{115}{23}=\) 5.0 A
6. A local step-down transformer changes 11 kV to 230 V. Its secondary coil has 46 turns. Calculate the number of turns on its primary coil. Physics only · 2P
\(N_p=N_s\times\dfrac{V_p}{V_s}=46\times\dfrac{11\,000}{230}=\) 2200 turns (convert 11 kV to 11 000 V first).
7. Explain why a transformer does not work when its primary coil is connected to a battery. Physics only · 2P
A battery gives a direct (steady) current, so the magnetic field in the core is constant. The field through the secondary does not change, so no voltage is induced in it.
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