Transformers sit right at the end of the Electromagnetism section of the Singapore/Cambridge O-Level Physics syllabus (5059), and they show up almost every year in some form. The good news is that this topic is one of the most predictable in the whole paper. There are really only about five question types, and once you know the mark-scheme phrasing for each, the marks come quickly.
Here is how we teach it at Dojo, from tutors who sat these papers recently.
Start With the Physics, Not the Formula
Most students jump straight to Np/Ns = Vp/Vs. Examiners know this, so the explanation marks are where candidates lose the most. You must be able to describe mutual induction properly.
The standard four-step chain, in order:
- An alternating current flows in the primary coil.
- This produces a changing magnetic field, so there is a changing magnetic flux in the soft iron core.
- The core carries this changing flux to the secondary coil, so the flux linking the secondary coil changes.
- By Faraday's law, an alternating e.m.f. is induced in the secondary coil.
Write all four steps. Vague answers like "the magnetic field passes through and makes electricity" score nothing. The examiner is looking for the words changing, magnetic flux and induced e.m.f.
Why a Transformer Does Not Work With D.C.
This is a two-mark favourite. Say it like this: a steady direct current produces a constant magnetic flux in the core. Since there is no change in magnetic flux linking the secondary coil, no e.m.f. is induced, so there is no output voltage.
A nice bonus line if you want to sound sharp: an e.m.f. is only induced momentarily at the instant the d.c. supply is switched on or off, when the flux is changing.
Why Soft Iron, and Why Laminated
- Soft iron core: it is easily magnetised and demagnetised, and it concentrates the magnetic flux so that almost all the flux from the primary links the secondary.
- Laminated core: the thin insulated layers reduce eddy currents in the core, which reduces heat loss and improves efficiency.
Do not mix these two up. Lamination is about eddy currents. Soft iron is about magnetic properties.
The Calculations
The Turns Ratio
For an ideal transformer:
Vp / Vs = Np / Ns
Worked example. A transformer steps 240 V mains down to 12 V. The primary has 1000 turns. Find Ns.
240 / 12 = 1000 / Ns, so Ns = 1000 x 12 / 240 = 50 turns.
Exam tip: always check whether your answer makes sense. Step-down means fewer turns on the secondary. If your secondary number is bigger, you have flipped the ratio.
Power in an Ideal Transformer
For 100% efficiency, input power equals output power:
Vp Ip = Vs Is
So a step-down transformer steps voltage down and current up. Continuing the example above, if the 12 V output supplies 2.0 A, then output power = 24 W, so the primary current = 24 / 240 = 0.10 A.
When Efficiency Is Not 100%
Efficiency = (output power / input power) x 100%
If a transformer is 90% efficient with an input of 100 W, the output is 90 W, and the missing 10 W is lost mainly as heat in the coils (resistance) and in the core (eddy currents and repeated magnetisation).
Power Transmission Questions
This is the highest-value application, and it is worth practising until it is automatic.
Worked example. 20 kW of power is sent along cables of total resistance 0.50 Ω.
- At 400 V: I = P / V = 20000 / 400 = 50 A. Power lost = I²R = 50² x 0.50 = 1250 W.
- At 4000 V: I = 20000 / 4000 = 5.0 A. Power lost = 5.0² x 0.50 = 12.5 W.
The conclusion sentence examiners want: transmitting at high voltage means a smaller current for the same power, and since power loss = I²R, the heating loss in the cables is greatly reduced.
Two traps to avoid:
- Use I²R for the cable loss, not V²/R with the transmission voltage. The voltage across the cables is not the transmission voltage.
- Find the current using the transmission voltage first, then square it.
A Quick Checklist Before You Turn the Page
- Did you use turns ratio, not a guess, and check step-up versus step-down?
- Did you state "changing magnetic flux" and "induced e.m.f." in every explanation?
- Did you carry units and convert kW to W before using I²R?
- Did you mention both the smaller current and the I²R relationship in transmission questions?
Transformers reward precision in wording more than cleverness. Learn the four-step induction chain, drill three or four turns-ratio and transmission calculations, and this becomes a section you finish in five minutes with full marks.
Want a tutor to mark your electromagnetism answers line by line the way an examiner would? Start a free trial class with us on WhatsApp and bring your toughest transformer question along.