Electromagnetism: Common Mistakes (Form 5 SPM Physics)

The mistakes, and the fix

1. Vp / Vs = Np / Ns, The voltage ratio equals the turns ratio. More turns on the secondary means a step-up transformer.
2. VpIp = VsIs, This assumes 100% efficiency (input power = output power). Stepping down the voltage steps up the current.
3. efficiency = (VsIs)/(VpIp) × 100%, It is output over input, so efficiency is always ≤ 100%. Use power (V × I) values, not voltage alone.
4. In the The effect on a current-carrying conductor in a magnetic field practical, skipping this: Switch the current on only in short bursts to avoid overheating the wire.
5. In the The effect on a current-carrying conductor in a magnetic field practical, skipping this: Make sure the wire hangs freely so it can move.
6. In the Factors affecting the force on a current-carrying conductor practical, skipping this: Tare the balance to zero before each set of readings.
7. In the Factors affecting the force on a current-carrying conductor practical, skipping this: Switch on the current only briefly for each reading to avoid heating the wire.
8. Mixing up Faraday's law of electromagnetic induction and Magnetic field. Faraday's law of electromagnetic induction: The magnitude of the induced electromotive force is directly proportional to the rate of change of magnetic flux linking the conductor. Magnetic field: The region around a magnet or current-carrying conductor in which a magnetic force can be detected.
9. Mixing up Electromagnet and Force on a current-carrying conductor. Electromagnet: A magnet made by passing an electric current through a coil, usually wound around a soft iron core, whose magnetism can be switched on and off. Force on a current-carrying conductor: The force experienced by a current-carrying conductor placed in a magnetic field, at right angles to both the current and the field.
10. Mixing up Electromagnetic induction and Induced current. Electromagnetic induction: The production of an electromotive force in a conductor when there is a changing magnetic field linking it. Induced current: The current that flows in a closed conductor as a result of electromagnetic induction.
11. Mixing up Transformer and Step-up transformer. Transformer: A device that uses electromagnetic induction to change the voltage of an alternating current. Step-up transformer: A transformer with more turns on the secondary coil than the primary, which increases the alternating voltage.
12. Mixing up Step-down transformer and Direct current. Step-down transformer: A transformer with fewer turns on the secondary coil than the primary, which decreases the alternating voltage. Direct current: An electric current that flows in one direction only.
13. Dropping the unit on the final line, an answer without its SI unit is incomplete.
14. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.
15. Answering "describe" with a definition, describe means say what happens; define means say what it is.
16. Forgetting that direction matters for vector quantities in this chapter, state the sign convention before substituting.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Electromagnetism resources

• Chapter overview: Electromagnetism
• Revision Notes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)