Investigating the Output of a Transformer (WJEC GCSE Physics)

Aim of the Experiment

• The aim of the experiment is to investigate the relationship between the number of turns of wire on a transformer's secondary core and the secondary voltage (the voltage output from that secondary core)

Variables:

• Independent variable = Number of turns on the secondary core, N₂
• Dependent variable = Secondary voltage, V₂
• Control variables:
  • Number of turns on the primary core, N₁
  • Primary voltage, V₁
  • Frequency of a.c. power supply
  • Temperature of wires and transformer (turning off the power supply in between readings)

Equipment List

Equipment	Purpose
Two iron C-cores	C-shaped pieces of iron (around which wires are wrapped) that are held in contact to form the transformer core
Two long connecting wires	To wrap around the C-cores and vary the number of turns
Two a.c. voltmeters	To record the average voltage supplied by the source and the voltage across the secondary coil
a.c. power supply	To provide a source of alternating potential difference with which to operate the transformer

Method

Equipment Set Up

Two C-cores placed together to form the transformer's iron core. Wires are coiled around these C-cores

1. Connect one end of a wire to an a.c. power supply which is turned off
2. Wrap 100 turns of this wire around the primary C-core and connect the other end back to the a.c. power supply
3. Connect a voltmeter across this wire as shown in the diagram
4. Wrap 20 turns around the secondary C-core using another wire and place this C-core in contact with the primary C-core
5. Connect the second wire on the secondary C-core to the second voltmeter
6. Switch on the a.c. supply and record primary and secondary voltages as well as N₁  and N₂
7. Switch off the a.c. supply
8. Increase the number of turns on the secondary core by 20
9. Repeat the procedure for 40, 60, 80 and 100 turns on the secondary core

• An example of a suitable table of results might look like this:

Relationship Between Number of Secondary Coils and Secondary Voltage

Number of turns on the secondary coil	Secondary voltage / V
20	3.01
40	6.94
60	8.09
80	13.50
100	13.89

Analysis of Results

• Plot a graph of turns on the secondary coil (x axis) against secondary voltage (y axis)
• The graph should be a straight line passing through the origin
  • This indicates that, for a constant N₁  and constant V₁ , N₂  and V₂  are directly proportional
• The transformer equation predicts this relationship
  • This becomes clear if it is rearranged to make V₂  the subject

• This shows that the gradient of the graph is equal to , which are both constant quantities

A Graph of Secondary Voltage Against Number of Secondary Turns

The gradient of this graph is the primary voltage divided by the number of primary turns

Evaluating the Experiment

Systematic Errors:

• Ensure that, when the a.c. power source is turned off, the voltmeters read zero, to avoid zero error in the readings

Random Errors:

• In practice, the voltmeters and wires will have some resistance, therefore the voltages and currents displayed may be slightly inaccurate
• The temperature of the equipment could affect its resistance - this must be controlled carefully
• Taking multiple readings of the voltage and calculating an average for each experiment for a given number of secondary turns will provide a more accurate result and reduce uncertainties

Safety Considerations

• When there is a high current and a thin wire, the wire will become very hot
  • Make sure never to touch the wire directly when the circuit is switched on
• Switch off the power supply right away if burning is smelled
• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment
• Disconnect the power supply in between readings to avoid the components heating up too much