Are formulas given in the exam?

No. You must recall SPM Physics formulas.

Is a formula sheet given for these calculations?

No. SPM Physics (4531) provides no formula sheet, recall each formula and write units on every line.

What does the easy band mean?

Foundation items use the formula directly; marks come from showing the method.

Can a tutor mark my working?

Yes, in a one-to-one lesson the teacher checks each line and fixes the exact step that loses marks. From RM50/hr, paid trial available.

Electromagnetism Calculations (Easy), SPM Physics

How to approach

These are direct plug-in problems: identify the formula and substitute. Get these automatic first.

The method

List what is given, with units.
Choose the formula and rearrange for the unknown.
Substitute with units on every line.
Round to the correct significant figures.

Worked examples

Transformer turns ratio

Vp / Vs = Np / Ns

Vp = 240 V, Np = 1200 turns, Ns = 100 turns. Vs = Vp × Ns / Np = 240 × (100 / 1200) = 20 V (step-down).

Ideal transformer (power)

VpIp = VsIs

An ideal transformer has Vp = 240 V, Ip = 0.5 A, Vs = 12 V. Is = VpIp / Vs = (240 × 0.5) / 12 = 10 A.

Transformer efficiency

efficiency = (VsIs)/(VpIp) × 100%

Output power VsIs = 48 W, input power VpIp = 60 W. efficiency = (VsIs)/(VpIp) × 100% = (48 / 60) × 100% = 80%.

Where marks are lost

Skipping units, rounding too early, and writing only the final answer (which loses method marks in Paper 2).

Practise these

Try each formula above with your own numbers, showing every line. Bring anything you get stuck on to a one-to-one lesson.

Formulas you use for Electromagnetism calculations

These foundation problems draw on the 3 relationships in Electromagnetism. None is printed in the exam, so recall each and write its symbols with units before substituting:

Vp / Vs = Np / Ns, Transformer turns ratio
VpIp = VsIs, Ideal transformer (power)
efficiency = (VsIs)/(VpIp) × 100%, Transformer efficiency

A worked example, with units on every line

Using Vp / Vs = Np / Ns (Vp = primary voltage (V); Vs = secondary voltage (V); Np = number of primary turns (no unit) (); Ns = number of secondary turns (no unit) ()):

Vp = 240 V, Np = 1200 turns, Ns = 100 turns.
Vs = Vp × Ns / Np = 240 × (100 / 1200) = 20 V (step-down).

Where the marks are, and the common mistakes

In Paper 2, calculation marks are awarded line by line: the formula, the substitution with units, and the final answer with its unit and sensible significant figures. Watch this trap: The voltage ratio equals the turns ratio.

More turns on the secondary means a step-up transformer. For foundation items, the numbers are direct, the marks are in showing the method, not just the answer.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)

Written by the spmphysics.com.my editorial team.· Updated 5 Sept 2026