Electricity and Magnetism

AC power line at high voltage

Practical Activity for 14-16 PRACTICAL PHYISCS


This experiment shows clearly why the power transmission lines used for the National Grid operate at high voltage.

Apparatus and Materials

  • Mounted transformers
  • Power supply, 12 V AC
  • SBC sockets fitted with 12 V 24 W lamps, mounted, 2
  • Leads, 4 mm, 6
  • Stands fitted with boss heads, 2
  • Multimeters, 2 (optional)
  • Resistance wire 28 SWG (dia 0.376 mm) e.g. constantan

Health & Safety and Technical Notes

The power line in step 2 will be at 240 volts. Insulation at the transformer boxes and sleeved wire will offer sufficient protection and is essential. Several teachers have ended up in hospital through trying to manage without it.

Read our standard health & safety guidance

Cut two 1.5-metre lengths of resistance wire. This will produce a significant power loss in the low voltage line. Then cut two lengths of transparent sleeving, about 1 cm shorter. Slide them onto the wires.

Cut two lengths of wooden rod as shown. Make slots to take the sleeved resistance wire.

To prevent the wire sagging too much between supports, use elastic bands to attach it to a strip of wood or coil it round wooden rods.

A safe version of this experiment, ready prepared, is available from Irwin Science Education. The Irwin Science Education apparatus allows the same wires (representing long power lines) to be used first at low voltage and then quickly changed to operate at high voltage.


  1. Connect the circuit to a 12 V AC supply as shown in figure 1a.
  2. Reconnect the circuit, with a mounted transformer at each end, as shown in figure 1b but do not switch on yet.
  3. Open the transformer boxes. Connect the two lengths of sleeved resistance wire between the two terminal blocks and close the boxes, gripping the sleeved wire between the bottom and the lid.
  4. Support the sleeved wires in the slots in the two rods held in boss heads in stands.
  5. Connect up the two 12 V lamps and the supply as shown.
  6. Switch on.

Teaching Notes

  • Before doing this demonstration, students should understand step-up and step-down transformers. Knowing the turns ratio of the transformers, they will then be able to calculate the voltage in the high tension part of the circuit and at the distant lamp.
  • Transmission lines connect power stations to consumers. This is convenient, but there is a cost to pay in energy terms. An electric current warms up the transmission cables and so there are energy losses as the cables warm the atmosphere. Energy is dissipated so that it is stored thermally in the surroundings.
  • Using a high voltage reduces the energy dissipated in the transmission cables. Energy dissipated in the transmission cable goes as I 2R. Because the product of current I times voltage V is constant (equals electrical power), stepping up the voltage reduces the current. This dramatically reduces energy dissipated by heating the transmission wires.
  • The lamp distant from the power supply should look much dimmer (if it glows at all) than the one directly connected to the supply. In step 3, the distant lamp should now be as bright as the one directly connected to the supply.
  • You could explain that voltmeters measure the energy transfer in joules per coulomb. By connecting across the power supply at the power station, you measure the energy transfer there. The students can then calculate the power used by the consumer and 'the power used by the 'power line + consumer'.
  • You could also connect an AC ammeter (reading at least 2 amps) into the circuits with the lamps. Without the transformers, the current to the power station lamp is greater than that to the consumer lamp.
  • A lower voltage alternative is to use a 4 V AC input at the power station end, with transformers that have a turn ratio of 8:1\. This will produce a transmission voltage of about 30 V. Use 1 2 V lamps, which will just glow at 4 V.

This experiment was safety-tested in July 2007

  • A video showing a demonstration that can be used to illustrate power transmission:


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