V=-N(dΦ/dt)
Electricity and Magnetism

AC power line at lower voltage

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Shows the basic principle of any transformer: a change in current in the primary coil will induce an e.m.f. (voltage) in the secondary coil.

Apparatus and Materials

  • 2 V a.c. power supply (fixed output) (‘Westminster pattern’)
  • 1.5 V 0.3 A bulbs in holders, 2
  • 20:1 turns ratio transformers (each made from 120 and 2400 turn coils, C-cores and clip), 2
  • Multimeter (used as a.c. voltmeter), 2
  • 2.0 m length of 28 swg Eureka resistance wire, 2
  • Wooden bar with two 4 mm terminals, 2
  • connecting wires with 4 mm plugs, 10

Health & Safety and Technical Notes

Using a power supply with a fixed, low-voltage output and transformers with a small turns-ratio ensures that no dangerously high voltages are produced. Check the maximum voltage that your equipment can produce is less than about 40 V by multiplying the output voltage of the supply by the turns-ratio of the transformer.

Ensure that the second (step-down) transformer is correctly connected, so that the voltage is not further stepped up.

Read our standard health & safety guidance


Procedure

  1. This method makes use of low voltage (1.5 V) bulbs and transformers constructed using standard lab coils.
  2. Show that, without the use of transformers, the ‘distant’ lamp is dim.
  3. Show that, when step-up and step-down transformers are included, the two lamps have approximately equal brightness.

Teaching Notes

  • Students are likely to be aware that electrical power is transmitted at high voltages. This demonstration shows that this results in less power loss.
  • The equipment models an electrical supply system. The power supply represents a power station; one lamp represents a consumer close to the power station while the other represents a consumer at a distance, at the end of long power lines.
  • The demonstration shows that the distant consumer receives less power (the available p.d. is reduced). This effect can be greatly reduced using step-up and step-down transformers.
  • After the demonstration, you can discuss the current in the circuit. Note that the distant consumer is in a series circuit with the two power lines. (It may look as though the two power lines are in parallel with each other, but tracing the path of the current from the supply shows that they are in series with each other, and with the distant consumer.) With transformers in the circuit, the current is reduced by a factor of 20 and so the power dissipated in the power lines is reduced by a factor of 202 = 400.
  • The approach followed in the film above shows that introducing transformers reduces power loss (as shown by the brightnesses of the bulbs). It then moves on from showing the phenomenon to explaining it. Alternatively, you may wish to include a discussion of the source of power loss (ohmic heating of the power lines) during the demonstration. Many students find this approach confusing because it involves the use of two expressions for electrical power: P = I × V when considering how the current changes when the p.d. of the supply is increased, and P = I 2 × R when considering the ohmic losses in the power lines.
  • Measuring currents in the circuit involves breaking into the circuit to add an a.c. ammeter.
  • A version using d.c., showing appreciable power losses (but without the use of transformers):

    Model DC power line


  • A version of this demonstration in which the voltage is stepped up to 240 V; more safety precautions required:

    AC power line at high voltage


  • This video shows how to set up and demonstrate a model electrical power line where voltages are kept to below 40V:

V=-N(dΦ/dt)

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