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Transformer Principle (asset code : E-Transf-1)

Applicable Levels : GCSE

Applicable Examination Boards : AQA, OCR21C , WJEC


To investigate how the output voltage of a transformer depends upon the number of turns on the secondary coil.


Electrical power supply set at 2 V a.c.
Connecting leads
2 a.c. voltmeters [it is possible to do this experiment with 1 voltmeter]
Iron C-core with 30 turns of wire on it.
Access to various iron C-cores with 10, 20, 30, 40, 50 and 60 turns of wire


Always switch off the power supply before making any changes to the circuit.


Connect up the apparatus as shown in the diagram. Don’t switch on until you are ready to take measurements. Make sure that the coil connected to the power supply has 30 turns and that the power supply is set to 2V.
Using the 10 turn secondary coil measure the input and output voltages. Switch off.
Repeat the measurements using C-cores with different numbers of turns of wire on the secondary coil. Keep the primary coil with 30 turns.
Record all your results in a table.


Primary Turns Secondary Turns Input Voltage /V Output Voltage /V
30 10 2.16 0.71
30 20 2.16 1.45
30 30 2.16 2.18
30 40 2.16 2.93
30 50 2.16 3.50
30 60 2.16 4.38


The coils could be wound on cardboard tubes which slide over the C-cores.

It is important that the two C-cores clip firmly together ensuring an iron to iron contact. A layer of enamel or paint will reduce the magnetic field considerably and consequently the output voltage. A similar effect can be had by placing a sheet of paper between the two C-cores.

For a striking demonstration, use a transformer with 60 turns as primary and 1200 as secondary. A couple of bits of wire on the 1200 turn side (not held by hand!) produced a spark. With a 1.5V cell on the 'primary' I could get no obvious spark on making the circuit, but on breaking it could get a spark to jump across a gap of up to 4mm (ish). The primary coil should have a high inductance, and a large current flowing through it before breaking the circuit.
Swapping to an EHT supply I needed to go off the top of the 5kV calibrated range to get a spark with only a 3mm gap, and got no spark at all with the 4mm I had been able to bridge (and again bridged, just to check) using the 1.5V cell.
This implies an output p.d. of around 6kV or more with the 1.5V cell.


Step-up transformers National Grid X-Ray machine
Step-down transformers National Grid Phone charger
Isolating transformer Shaving socket (same voltage)


PowerPoint presentation is available to download for use with this class experiment.


AQA P3 13.9
OCR21C P5.4
OCR21C P3 2b


  • 1.   Write up your experiment with a diagram, a brief account of the experiment, and the results.
  • 2.  The method includes safety precautions. Say what these are and why they are necessary.
  • Switch ON only after checking circuit each time. Check range of voltmeter and supply voltage is set to 4V ac.
  • 3.  State the independent variable, the dependent variable and all the control variables
  • independent variable = Number of turns on Secondary coil,
    dependent variable = Output voltage
    control variables = Supply voltage set to 4V ac and Number of turns on Primary coil kept at 60 turns.
  • 4.  What kind of variable is the independent variable? Choose the word that best describes it; CATEGORIC, CONTINUOUS, DISCRETE, ORDERED.
  • Discrete.
  • 5.  What was the range of values that you used for the independent variable? Was this a sensible range? Give a reason for your answer.
  • 20 turns to 120 turns. Sensible YES, less than 20 output voltage too low, above 120 output voltage too high.
  • 6.  In your experiment you would have used at least one kind of voltmeter.
    • a.  Was the RANGE over which this meter was capable of measuring suitable? Give a reason for your answer.
    • Expected range 0 - 15V
    • b.   Was the SENSITIVITY of this meter suitable? Give a reason for your answer.
    • Yes, meter read to 0.01V
  • 7.  From your graph or the results describe carefully how the Output Voltage depends on the Number of turns on the Secondary Coil.
  • . Output voltage is directly proportional to number of turns on secondary coil, and output voltage found from equation below.
  • 8.   Explain carefully why a transformer will not work if the supply voltage is not alternating.
  • To generate a voltage the secondary coil needs a varying magnetic field. This is produced by the varying, a.c. current in the primary coil.
  • 9.  For transformers, which are 100% efficient, the output voltage can be calculated using this equation. Hold cursor here.
    • a.  Use this equation to calculate the expected output voltage for 80 turns on the secondary coil with the input voltage you used.   How does this answer compare with your results?
    • Output voltage = 80/60 x 4 = 5.33V
      The experimental result is 10% less.
      Actual output voltage is 4.9V
    • b.   If the output voltage is less than the calculated voltage, the transformer is less than 100% efficient.
      Use your results to investigate the efficiency of this transformer set-up.
    • Output voltage is 10% less so output power is 20% less as power = voltage2/R
  • 10.  Transformers can have a low efficiency if they produce a lot of heat. The higher the input voltage, the more heat they produce. This suggests the question, “Does the input voltage affect the efficiency of a transformer?”
    Plan an experiment to test how whether the efficiency of the transformer is affected by the input voltage.
  • Independent variable – input voltage, dependent variable – output voltage, control variables – number of turns on primary and secondary coils.
    Secondary coil must be connected to a load eg. a lamp, to produce a current and heating.
    Plot graph of output voltage (y-axis) against input voltage (x-axis)
    If graph is a straight line with gradient = 1, then Transformer is 100% efficient.
    If graph is a straight line with gradient < 1, the efficiency is constant but less than 100%.
    If graph is a curve then efficiency varies with input power.