A transformer is a device that changes alternating voltage using coils of wire and a shared magnetic field. It matters because electric power is easier and safer to transmit when voltage can be raised or lowered at different points in the grid. Power stations use step-up transformers to send electricity long distances with less energy loss.
Homes and devices use step-down transformers to reduce voltage to useful levels.
Understanding How Transformers Change Voltage
Inside a transformer, the two coils are electrically separate. They are usually wound around an iron core made from thin layers. Alternating current in the input coil produces a magnetic field that repeatedly grows, shrinks, and reverses direction.
The core gives this changing field an easy path to the second coil. As the magnetic field through the second coil changes, it pushes charges in that wire and creates an output voltage.
The coils do not need a direct wire connection. Energy crosses the gap through the changing magnetic field.
The number of turns matters because each loop of wire experiences the changing magnetic field. More loops mean more induced voltage. This is why a coil with many output turns can provide a higher voltage than the input coil.
There is an important trade off. In an ideal transformer, raising voltage reduces current by the matching amount. Lowering voltage allows a larger current.
The transformer does not create extra energy. Its job is to exchange one useful combination of voltage and current for another. This idea helps explain why high voltage cables can carry large amounts of power without requiring extremely large currents.
Real transformers are not perfect. Some energy becomes thermal energy when current flows through the resistance of the copper windings. This is called copper loss.
Energy is also lost in the iron core as it is magnetised first one way, then the other. Thin insulated layers in the core reduce unwanted circulating currents called eddy currents. Engineers choose core materials that waste little energy during magnetic reversal.
A transformer can hum because the core changes shape by a tiny amount as its magnetic field changes. Loose parts can make this vibration louder.
Students meet transformers in phone chargers, doorbells, laptop power supplies, plug in adapters, railway systems, and the local electricity network. Many modern chargers use a high frequency electronic circuit before a small transformer. A higher frequency makes the magnetic field change faster, so a smaller core can transfer the same power.
When studying circuit diagrams, pay close attention to which coil is the input and which is the output. Count turns carefully, then connect a voltage increase with a current decrease.
Remember that a steady direct current produces a magnetic field that stops changing after switch on. At that point it cannot keep inducing an output voltage, and the input coil may overheat.
Key Facts
- Ideal transformer voltage ratio: Vs/Vp = Ns/Np
- Step-up transformer: Ns > Np, so Vs > Vp
- Step-down transformer: Ns < Np, so Vs < Vp
- Ideal power relation: Pp = Ps, so VpIp = VsIs
- Changing magnetic flux induces voltage: E = -N ΔΦ/Δt
- Transformers require changing current, so they work with AC rather than steady DC
Vocabulary
- Primary coil
- The primary coil is the input winding connected to the source of alternating voltage.
- Secondary coil
- The secondary coil is the output winding where the changed voltage is induced.
- Magnetic flux
- Magnetic flux measures how much magnetic field passes through a surface or coil.
- Iron core
- The iron core guides and strengthens the changing magnetic field linking the two coils.
- Turns ratio
- The turns ratio is the comparison Ns/Np that determines how the output voltage compares with the input voltage.
Common Mistakes to Avoid
- Using the current ratio the same way as the voltage ratio is wrong because ideal transformer current changes inversely, so Ip/Is = Ns/Np.
- Forgetting that transformers need AC is wrong because a steady DC current produces no continuously changing magnetic flux after the initial switch-on moment.
- Assuming a step-up transformer creates extra energy is wrong because raising voltage lowers current in an ideal transformer, keeping power approximately constant.
- Ignoring coil turn counts is wrong because the voltage change depends directly on the ratio of secondary turns to primary turns.
Practice Questions
- 1 A transformer has 200 turns on the primary coil and 800 turns on the secondary coil. If the input voltage is 12 V AC, what is the output voltage for an ideal transformer?
- 2 An ideal step-down transformer changes 240 V AC to 24 V AC. If the primary coil has 1200 turns, how many turns should the secondary coil have?
- 3 A phone charger uses a transformer to reduce wall voltage before electronics convert it further. Explain why this transformer must be connected to an alternating current source rather than a steady direct current source.