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How does a transformer work?

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Quick answer

A transformer transfers electrical energy between two coils through a changing magnetic field. Alternating current in the primary coil creates a field that induces voltage in the secondary coil. The turns ratio sets the voltage change: more secondary turns step voltage up, fewer step it down, while power stays roughly constant.

A transformer changes alternating current from one voltage to another using electromagnetic induction between two coils. An alternating current in the primary coil creates a changing magnetic field that induces a voltage in the secondary coil, with the ratio of turns determining the voltage change.

Step by step

  1. Alternating current from a power source flows into the primary coil.
  2. This current creates a magnetic field that reverses direction with each cycle.
  3. The iron core carries the changing magnetic field to the secondary coil.
  4. The changing field induces an alternating voltage in the secondary coil.
  5. The ratio of turns between coils determines the output voltage.
  6. The secondary voltage drives current through an external circuit.

Key things to understand

  1. A transformer has two coils, called the primary and secondary, wound around a shared iron core.
  2. When alternating current flows through the primary coil, it creates a magnetic field that constantly changes direction.
  3. The changing magnetic field is guided through the iron core to the secondary coil.
  4. The changing field induces an alternating voltage in the secondary coil by electromagnetic induction.
  5. The ratio of turns in the two coils determines whether voltage is stepped up or stepped down.

Labelled diagram

  1. Primary coil

    Receives alternating current, creating a changing magnetic field around it.

  2. Iron core

    Guides the magnetic field from the primary coil to the secondary coil.

  3. Secondary coil

    The changing field induces an alternating voltage across this coil.

  4. Turns ratio

    Number of turns on each coil sets whether voltage steps up or down.

  5. Output circuit

    The induced voltage drives current through connected devices or wires.

How a transformer changes voltage

Real-world examples

  • On electricity poles near homes, step-down transformers reduce high distribution voltages to the 120 or 230 volts used by appliances.
  • Inside phone chargers, small transformers step down household voltage to a low safe level for the device.
  • At power stations, step-up transformers raise generator voltage to hundreds of thousands of volts for long-distance transmission.
  • In welding machines, transformers lower voltage while increasing current to melt metal joints.

Common mistake

✗ Myth: Transformers create energy, so they can increase voltage without any trade-off.

✓ Fact: Transformers conserve energy. When they step up voltage, they step down current by the same ratio, so the product of voltage and current stays roughly the same. Small losses occur as heat, but no energy is created.

People also ask

How does a transformer step up voltage?
A step-up transformer has more turns on its secondary coil than on its primary. The changing magnetic field from the primary induces a higher voltage in the secondary because each turn contributes a small voltage. The output voltage equals the input voltage multiplied by the ratio of secondary to primary turns.
Why do transformers hum?
The hum comes from the iron core. As the alternating magnetic field changes, the core material expands and contracts slightly, a effect called magnetostriction. This makes the core vibrate at the same frequency as the current, usually 50 or 60 hertz, producing a steady hum.
What is a transformer used for?
Transformers are used to change voltage levels in electrical systems. They step up voltage at power plants for efficient long-distance transmission, then step it down at substations and in chargers for safe household use. They also isolate circuits and match impedances in audio equipment.
Why do transformers only work with alternating current?
Transformers rely on a changing magnetic field to induce voltage. Alternating current reverses direction many times per second, so the magnetic field constantly changes. Direct current flows steadily, producing a constant field that induces no voltage in the secondary coil.
What is the difference between a step-up and a step-down transformer?
A step-up transformer has more turns on the secondary coil than the primary, so it increases voltage. A step-down transformer has fewer secondary turns, so it decreases voltage. Both types conserve power, so when voltage increases, current decreases proportionally.
Why do transformers get warm during operation?
Some energy is always lost as heat due to resistance in the coils and tiny currents induced in the iron core. These losses are reduced by using thick copper wire and laminated cores, but they cannot be eliminated entirely.
Can a transformer change direct current voltage?
No. A transformer needs a changing magnetic field to work. Direct current produces a steady field, so no voltage is induced in the secondary coil. To change DC voltage, electronic circuits called converters are used instead.

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