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Electromagnetic Induction

Transformer power calculations

  • 4 things to know
  • Paper 6

Overview

The power equation for a 100% efficient transformer, calculating an output current, an output voltage or a primary current, and why stepping the voltage up lowers the current.

What you need to know

Answer each one before you reveal it.

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Key term

State the power equation linking the voltages and currents in the two coils of a transformer that is 100% efficient.

Numbers

A transformer takes 2.0 A from a 230 V supply and gives an output of 23 V. Calculate the output current.

Numbers

A transformer takes 5.0 A from a 12 V supply and delivers a current of 0.50 A. Calculate the output voltage.

Numbers

A transformer supplies 3.0 A at 400 V and is connected to a 100 V supply. Calculate the current in its primary coil.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the power equation linking the voltages and currents in the two coils of a transformer that is 100% efficient.

    Model answer

    Primary voltage × primary current = secondary voltage × secondary current.

  2. 2

    State the assumption made about a transformer when its power equation is used.

    Model answer

    It is assumed to be 100% efficient, so the output power equals the input power.

  3. 3

    A transformer takes 2.0 A from a 230 V supply and gives an output of 23 V. Calculate the output current.

    Model answer

    Input power = 230 × 2.0 = 460 W, so output current = 460 ÷ 23 = 20 A.

  4. 4

    A transformer takes 5.0 A from a 12 V supply and delivers a current of 0.50 A. Calculate the output voltage.

    Model answer

    Input power = 12 × 5.0 = 60 W, so output voltage = 60 ÷ 0.50 = 120 V.

  5. 5

    A transformer supplies 3.0 A at 400 V and is connected to a 100 V supply. Calculate the current in its primary coil.

    Model answer

    Output power = 400 × 3.0 = 1200 W, so primary current = 1200 ÷ 100 = 12 A.

  6. 6

    A transformer runs a 6.0 V lamp from a 240 V supply and draws 0.50 A from that supply. Calculate the current in the lamp.

    Model answer

    Input power = 240 × 0.50 = 120 W, so the lamp current = 120 ÷ 6.0 = 20 A.

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