IGCSE Physics: Electromagnetic Effects Practice Questions
When a conductor and a magnetic field move relative to one another, an electromotive force is induced. When a current carrying conductor sits in a magnetic field, it experiences a force. These two effects underpin generators, motors and transformers.
Sub-topic 4.5 of Cambridge IGCSE Physics 0625 is the hardest part of the electricity section and carries substantial marks on Paper 4. Induction questions need the word relative and the word cutting, and transformer questions need the equation applied carefully. The questions below cover both.
What you need to know for Electromagnetic Effects
- Electromagnetic inductionAn electromotive force is induced when a conductor cuts magnetic field lines, or when the magnetic field through a coil changes. Movement or change is essential. A stationary magnet in a stationary coil induces nothing.
- Factors affecting the induced emfThe induced emf increases with the speed of relative movement, the strength of the magnetic field, and the number of turns on the coil.
- Direction of the induced effectThe induced current opposes the change producing it. Pushing a north pole into a coil induces a current that makes the near face of the coil a north pole, opposing the motion.
- The motor effectA current carrying conductor in a magnetic field experiences a force, provided the current is not parallel to the field. Fleming's left hand rule gives the direction: first finger field, second finger current, thumb motion.
- Transformer equationVp divided by Vs equals Np divided by Ns. A step up transformer has more turns on the secondary, and a step down transformer has fewer.
- Why transmission uses high voltageFor a given power, a higher voltage means a smaller current. Power loss in the cables is I2R, so reducing the current greatly reduces the energy wasted as heat.
IGCSE Physics Electromagnetic Effects questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A magnet is pushed into a coil connected to a sensitive ammeter and the needle deflects. State two ways the size of the deflection could be increased, and explain why the needle returns to zero when the magnet is held still inside the coil.
Show the worked answer
- Way 1: move the magnet into the coil more quickly, so field lines are cut at a greater rate.
- Way 2: increase the number of turns on the coil, or use a stronger magnet.
- When the magnet is held still inside the coil, there is no relative movement between the magnet and the coil.
- No field lines are being cut and the magnetic field through the coil is not changing, so no electromotive force is induced and no current flows. The needle returns to zero.
A transformer has 200 turns on its primary coil and 1200 turns on its secondary coil. The primary is connected to a 240 V supply. Calculate the secondary voltage and state whether this is a step up or step down transformer.
Show the worked answer
- Use Vp divided by Vs = Np divided by Ns.
- Rearrange: Vs = Vp x Ns divided by Np.
- Vs = 240 x 1200 divided by 200 = 240 x 6 = 1440 V.
- The secondary voltage is higher than the primary voltage, and there are more turns on the secondary, so it is a step up transformer.
Explain why electrical energy is transmitted across the country at very high voltage.
Show the worked answer
- The power delivered is given by P = IV, so for a fixed power a higher transmission voltage means a smaller current.
- Energy is wasted in the transmission cables because they have resistance, and the power dissipated is given by I2R.
- Because the loss depends on the square of the current, halving the current reduces the wasted power to a quarter.
- Transmitting at very high voltage therefore keeps the current small and greatly reduces the energy wasted as thermal energy in the cables. Transformers step the voltage up for transmission and back down for safe use.
A straight wire carrying a current is placed at right angles to a magnetic field and experiences an upward force. State two changes that would reverse the direction of the force, and name the rule used to predict it.
Show the worked answer
- Change 1: reverse the direction of the current in the wire.
- Change 2: reverse the direction of the magnetic field, for example by swapping the magnetic poles.
- Reversing both at once would leave the force in its original direction, so only one change should be made.
- The direction is predicted using Fleming's left hand rule: the first finger points along the field from north to south, the second finger points in the direction of the conventional current, and the thumb gives the direction of the force or motion.
Common mistakes in this topic
- Forgetting that induction requires relative movement or a changing field.
- Inverting the transformer turns ratio.
- Using Fleming's left hand rule for generators or the right hand rule for motors.
- Saying transformers work with direct current. They require an alternating current to produce a changing magnetic field.
- Explaining transmission losses using V rather than I2R.
Exam tips
- Use the words relative movement and cutting field lines in every induction answer.
- For transformers, check whether your answer is step up or step down against the turns ratio before moving on.
- Remember left hand for motors, right hand for generators. Motor and left both feature in the phrase motor effect uses left.
- In transmission questions, always write P = I2R explicitly. The squared term is where the mark is.
- State that a transformer needs alternating current, because a steady current produces a steady field and induces nothing in the secondary.
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Electromagnetic Effects FAQs
What is electromagnetic induction?
Electromagnetic induction is the production of an electromotive force when a conductor cuts magnetic field lines or when the magnetic field passing through a coil changes. Relative movement or a changing field is essential. A stationary magnet inside a stationary coil induces nothing.
How do I use the transformer equation?
Use the primary voltage divided by the secondary voltage equals the primary turns divided by the secondary turns. Rearrange for the unknown. Check the answer makes sense: more turns on the secondary always means a higher secondary voltage, which is a step up transformer.
Why is electricity transmitted at high voltage?
For a fixed power, a higher voltage means a smaller current. Energy wasted in the transmission cables is given by current squared multiplied by resistance, so reducing the current has a large effect. Halving the current cuts the wasted power to a quarter.
What is the difference between Fleming's left and right hand rules?
The left hand rule applies to the motor effect, predicting the direction of the force on a current carrying conductor in a magnetic field. The right hand rule applies to generators, predicting the direction of the induced current when a conductor moves through a field.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.