A transformer has two coils wound on the same iron core. An alternating current in the first coil, the primary, produces a magnetic field that keeps changing direction. The iron core carries this changing field to the second coil, the secondary.
Because the field through the secondary is always changing, it induces a voltage in it, a case of electromagnetic induction. How large that voltage is depends on the number of turns. If the secondary has more turns than the primary, the voltage is stepped up; if it has fewer, the voltage is stepped down. The voltages are in the same ratio as the turns.
A transformer only works with alternating current, because a steady current gives a steady field and induces nothing. This is how the grid steps voltage up for efficient transmission across Malaysia and back down for safe use at home.
In SPM you should link the turns ratio to the voltage ratio and explain why alternating current is needed.
Common misconceptions
- A transformer can step up direct current voltage -> A steady d.c. gives an unchanging field and induces no voltage; transformers need alternating current.
- A transformer creates extra energy when it steps voltage up -> Stepping up voltage steps down current; energy is conserved, apart from small losses.
- The iron core carries the current from one coil to the other -> The coils are not electrically joined; the core carries the changing magnetic field.
The physics behind it
A transformer works by mutual induction. Alternating current in the primary coil sets up a magnetic field that keeps reversing, and the soft-iron core carries this changing field to the secondary coil, inducing a voltage across it.
For an ideal transformer the voltages share the same ratio as the number of turns: the primary voltage divided by the secondary voltage equals the primary turns divided by the secondary turns, all voltages in volts (V). If a primary of 1200 turns is fed 240 V and the secondary has 60 turns, then the secondary voltage = 240 V × (60 ÷ 1200) = 12 V, a step-down.
An ideal transformer also conserves power, so primary voltage × primary current = secondary voltage × secondary current, with current in amperes (A) and power in watts (W). Real transformers lose some energy to heating in the coils, eddy currents and hysteresis in the core, so efficiency = (output power ÷ input power) × 100% is always below 100%.
See it in daily life
The charger for a phone or laptop contains a transformer that steps the 240 V mains supply down to a few volts that the device can use safely. A doorbell and many low-voltage garden lights work the same way.
On a much larger scale, the national grid depends on transformers. Power stations generate at a moderate voltage, but a step-up transformer raises it to hundreds of kilovolts before the electricity travels along transmission lines across the country.
High voltage means smaller current for the same power, so far less energy is wasted heating the cables. Near towns and homes, step-down transformers bring the voltage back to the 240 V used in sockets.
Notice that a transformer only works on alternating current. A steady direct current gives a steady, unchanging magnetic field, and an unchanging field through the secondary induces no voltage at all, which is why chargers are built for the a.c. mains.
How this comes up in SPM
In Paper 2 you are commonly asked to state the function of a transformer, explain how a changing magnetic field induces a voltage in the secondary, and calculate an unknown voltage, turns number or current using the turns-ratio and power relationships. Remember there is no formula sheet, so recall these relationships yourself.
Questions often ask you to distinguish between a step-up and a step-down transformer from the turns given, and to explain why alternating current is necessary. Structured questions may also ask you to suggest ways to increase efficiency, such as using a laminated core or thick low-resistance windings.
This topic sits beside electromagnetic induction and the generation and transmission of electricity, so you should be ready to link the transformer to why high-voltage transmission reduces power loss in cables.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)