(ii) techniques and procedures used to investigate transformers.

The idea

A transformer has two coils on a shared laminated iron core. An alternating current in the primary produces a changing magnetic flux in the core, which links the secondary and induces an alternating emf. To investigate this, connect an a.c. supply to the primary and measure primary and secondary voltages with a.c. voltmeters, or use a signal generator and oscilloscope. Vary the number of turns on each coil and record the ratio of secondary to primary voltage, comparing it with the turns ratio. Keep frequency constant when testing turns, and keep turns constant when testing frequency. Laminated cores reduce eddy currents; the core links flux between coils.

What earns the marks

• A transformer requires a changing magnetic flux, so the primary must carry alternating current; a steady direct current induces no continuous secondary emf.
• The laminated iron core links magnetic flux from the primary coil to the secondary coil, increasing the induced emf.
• Secondary voltage depends on the ratio of secondary turns to primary turns, so changing turns changes the output voltage.
• Measurements use a.c. meters or an oscilloscope; a signal generator can supply a variable-frequency alternating input.
• Keeping other variables constant, such as frequency when varying turns, makes the investigation a fair test.

Common mistakes and how to put them right

• Thinking a transformer works with direct current.

  Put it right: A steady d.c. gives constant flux and no induced secondary emf, so an alternating supply is needed.

• Assuming the core carries current between the coils.

  Put it right: The core links magnetic flux, while current flows in the coils themselves.

• Ignoring the turns ratio and expecting output voltage to depend only on input voltage.

  Put it right: The ratio of secondary to primary turns controls the voltage change.

Exam tips

• State clearly that the input must be alternating, because electromagnetic induction needs changing flux.
• Describe the measurement method: a.c. voltmeters across each coil, or an oscilloscope to compare input and output waveforms.
• When comparing results, identify the controlled variable, such as keeping frequency constant while changing the number of turns.