Questions
1. Calculate: Vp = 240 V, Np = 1200 turns, Ns = 100 turns. Find the unknown using Vp / Vs = Np / Ns.
Vs = Vp × Ns / Np = 240 × (100 / 1200) = 20 V (step-down).
2. Calculate: An ideal transformer has Vp = 240 V, Ip = 0.5 A, Vs = 12 V. Find the unknown using VpIp = VsIs.
Is = VpIp / Vs = (240 × 0.5) / 12 = 10 A.
3. Calculate: Output power VsIs = 48 W, input power VpIp = 60 W. Find the unknown using efficiency = (VsIs)/(VpIp) × 100%.
efficiency = (VsIs)/(VpIp) × 100% = (48 / 60) × 100% = 80%.
4. State the manipulated, responding and constant variables for: To study the force that acts on a current-carrying conductor placed in a magnetic field and how it depends on the direction of the current and the field.
Manipulated: Direction of the current (and of the magnetic field). Responding: Direction in which the conductor moves. Constant: The magnetic field strength and the conductor used.
5. State the manipulated, responding and constant variables for: To investigate how the magnitude of the force on a current-carrying conductor in a magnetic field depends on the current.
Manipulated: Current in the conductor, I. Responding: Force on the conductor (shown by the change in the balance reading). Constant: The magnetic field strength and the length of the conductor in the field.
6. Explain: 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.
7. Explain: 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.
8. Explain: 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.
9. Define direct current.
An electric current that flows in one direction only.
10. Define alternating current.
An electric current that periodically reverses its direction of flow.
11. Define 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.
12. Define magnetic field.
The region around a magnet or current-carrying conductor in which a magnetic force can be detected.
13. Define 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.
14. Define 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.
Marking yourself
- Calculation: formula written, values substituted with units, final answer with unit and sensible significant figures.
- Variables: manipulated = what you change; responding = what you measure; constant = what you keep the same.
- Definition: one sentence, correct physical quantity, correct unit.
Exam tip
More Electromagnetism resources
- Chapter overview: Electromagnetism
- Revision Notes
- Common Mistakes
- 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)