Is this formula given in the exam?

No. SPM Physics papers provide no formula sheet, so this must be recalled.

What does an ideal transformer assume?

It assumes 100% efficiency, so no energy is lost to heat in the core or coils. Input power VpIp then equals output power VsIs exactly.

If the voltage is stepped up, what happens to the current?

The current is stepped down by the same factor, because the power stays constant. Doubling the voltage halves the current in an ideal transformer.

Why is high voltage used to transmit electricity?

Stepping up the voltage lets the same power flow at a much smaller current. Smaller current means far less energy wasted as heat in the transmission cables (from the I²R heating effect).

Ideal transformer (power) Formula, SPM Physics

Ideal transformer (power)

VpIp = VsIs

Not given in the exam, recall it.

What it is for

Ideal transformer (power) is used in Electromagnetism (SPM Physics Form 5). Keep the SI units in every line of working. Electromagnetism

Symbols and SI units

Symbol	Meaning	SI unit
`Vp`	primary voltage	V
`Ip`	primary current	A
`Vs`	secondary voltage	V
`Is`	secondary current	A

Rearrangements

Is = VpIp / Vs
Ip = VsIs / Vp
Vs = VpIp / Is

Worked example

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

Common trap

This assumes 100% efficiency (input power = output power). Stepping down the voltage steps up the current.

Understanding Ideal transformer (power)

For an ideal transformer, input power equals output power: VpIp = VsIs. Here Vp and Vs are the primary and secondary voltages in volts (V), while Ip and Is are the primary and secondary currents in amperes (A).

The relationship holds because an ideal transformer wastes no energy, so all the power delivered to the primary appears at the secondary. This means voltage and current trade off: if the voltage is stepped down, the current is stepped up in the same proportion, and vice versa.

To rearrange, isolate the unknown: Is = VpIp / Vs finds the secondary current, Ip = VsIs / Vp finds the primary current, and Vs = VpIp / Is finds the secondary voltage.

A worked example, step by step

An ideal transformer has a primary voltage Vp = 240 V, a secondary voltage Vs = 48 V, and a secondary current Is = 2 A. Find the primary current Ip.

Because input power equals output power, rearrange for Ip:

Ip = VsIs / Vp = (48 V × 2 A) / 240 V = 96 / 240 = 0.4 A.

The voltage was stepped down from 240 V to 48 V (a factor of 5), so the current is stepped up from 0.4 A to 2 A (also a factor of 5), confirming the trade-off.

Check the power: input = 240 × 0.4 = 96 W and output = 48 × 2 = 96 W. The primary current is 0.4 A.

Common mistakes and how this is tested

A frequent slip is pairing the wrong voltage with the wrong current, for example writing VpIs. Always multiply a coil's own voltage by its own current.

Another error is assuming voltage and current both rise together; in a step-down they move in opposite directions. A third is forgetting this assumes 100% efficiency, which real transformers never reach.

Under calculate, substitute with units and state the answer to sensible significant figures. Under define, state that for an ideal transformer the input power equals the output power.

Under explain, you may be asked why high-voltage transmission reduces power loss in cables, using the small current that a stepped-up voltage allows.

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

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