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 medium band mean?

Mid-level items usually need one rearrangement and a unit conversion.

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 (Medium), SPM Physics

How to approach

These need a rearrangement or two steps, often finding one quantity before the one asked for.

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 mid-level 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 VpIp = VsIs (Vp = primary voltage (V); Ip = primary current (A); Vs = secondary voltage (V); Is = secondary current (A)):

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

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: This assumes 100% efficiency (input power = output power).

Stepping down the voltage steps up the current. For mid-level items, expect one rearrangement and a unit conversion.

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