Interactive PhET Worksheet

Transformers

A transformer is a device used to change the size of an alternating voltage or current. Use the embedded PhET simulation to observe how changing magnetic fields transfer energy from a primary coil to a secondary coil without a direct wire connection.

Primary coil = power source side Secondary coil = output side No direct connection between coils

Core idea

In this activity, you are looking for evidence that electrical energy can be transferred between coils through a changing magnetic field. Pay special attention to what happens when the input current is AC compared with DC.

Primary coil The coil connected to the power source.
Secondary coil The coil where the output voltage appears.
Transformer rule Changing magnetic field first, induced current second.
1

Getting familiar with the simulation

Observe the wires, coils, and space between the coils.

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Step-by-step

  1. Use the embedded PhET simulation.
  2. Search/open “Faraday’s Electromagnetic Lab” if you use the full PhET site.
  3. Select the Transformer tab.
  4. Turn on Show magnetic field and Show electrons.
  5. Click AC power supply.
Look for the small charged particles moving back and forth in the coil wires.
Is there a wire connecting the two coils?
Use what you observed in the space between the coils. The key idea involves a magnetic field that changes.
2

What is happening in the first coil?

Focus only on the primary coil on the left side.

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Step-by-step

  1. Focus only on the primary coil on the left side.
  2. Watch the electrons.
  3. Watch the magnetic field as the electrons move.
Are the electrons moving in one direction or changing direction?
What happens to the magnetic field?
3

What happens in the second coil?

Look for evidence of induction in the secondary coil.

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Step-by-step

  1. Now look at the secondary coil on the right side.
  2. Watch carefully when the magnetic field changes.
  3. Compare the electron motion in the secondary coil with the magnetic field changes.
Does current appear even without a wire connection?
Watch for moments when the magnetic field is growing, shrinking, or reversing quickly.
4

Test DC vs AC

This is the key test for understanding why transformers use AC.

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Step-by-step

  1. Switch from AC to DC.
  2. Observe what happens in both coils.
  3. Then switch back to AC and compare.
A transformer needs a changing magnetic field. AC naturally changes direction over and over.
5

Changing number of turns

Keep the primary coil turns the same. Change the secondary coil turns.

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Step-by-step

  1. Keep the primary coil turns the same.
  2. Change the secondary coil turns.
  3. Try 5, then 10, then 20 loops.
  4. Watch the brightness or output voltage.
6

Changing input voltage

Increase the voltage of the AC source and observe the output.

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Step-by-step

  1. Increase the voltage of the AC source.
  2. Observe the secondary coil output.
  3. Decide whether the output change is small, large, or proportional.
Is the change small or large?
7

Build a full explanation

Use evidence from the simulation to explain how transformers work.

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Sequence your answer: AC in primary → changing magnetic field → induced current/voltage in secondary.
Finish check: Your final explanation should include AC, changing magnetic field, induced current/voltage, secondary turns, step-up, and step-down.

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