Electromagnetism: Revision Notes (Form 5 SPM Physics)

What this chapter covers

• 4.1 Force on a Current-carrying Conductor in a Magnetic Field
• 4.2 Electromagnetic Induction
• 4.3 Transformer

Key ideas explained

• How an electric motor turns, A current-carrying coil in a magnetic field feels a turning force. Because the two sides of the coil are pushed in opposite directions, the coil spins, and this drives the motor.
• How a transformer changes voltage, A changing current in one coil creates a changing magnetic field that induces a voltage in a second coil. The ratio of turns on the two coils sets how much the voltage is stepped up or down.
• How a generator induces current, Moving a coil in a magnetic field, or moving a magnet near a coil, induces a voltage. A generator spins a coil so the magnetic field through it keeps changing, driving a current.

Formulas to memorise (none are given in the exam)

• Vp / Vs = Np / Ns, Transformer turns ratio, Vp = primary voltage (V); Vs = secondary voltage (V); Np = number of primary turns (no unit) (); Ns = number of secondary turns (no unit) ()
• VpIp = VsIs, Ideal transformer (power), Vp = primary voltage (V); Ip = primary current (A); Vs = secondary voltage (V); Is = secondary current (A)
• efficiency = (VsIs)/(VpIp) × 100%, Transformer efficiency, Vs = secondary voltage (V); Is = secondary current (A); Vp = primary voltage (V); Ip = primary current (A)

Experiments the DSKP names

• 4.1.1 · Study the effect of a current-carrying conductor in a magnetic field
• 4.1.3 · Investigate the factors affecting the magnitude of the force on a current-carrying conductor in a magnetic field

Terms you must define precisely

• 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.
• 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.
• Alternating current, An electric current that periodically reverses its direction of flow.
• 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.
• 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.

A revision order that works

1. Derive each formula once from its meaning, then practise rearranging it before substituting numbers.
2. For each experiment, write the manipulated, responding and constant variables from memory, then the graph you would plot.
3. Finish with a timed set of calculations with units on every line.
4. Read the 3 standards above and tick the ones you can already explain aloud without notes.
5. Rewrite every definition in one sentence with the correct unit; definitions are the cheapest marks in Paper 2.

More Electromagnetism resources

• Chapter overview: Electromagnetism
• Common Mistakes
• 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)