Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

AC power line at high voltage

Demonstration

This experiment shows clearly why the power transmission lines used for the National Grid operate at high voltage. A video demonstrating an experiment of this kind is freely available from the National STEM Centre e.library.

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 b 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.

Apparatus set-up

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 Electrosound, 30 Goulds Farm, Duckend Green, Rayne, Essex, CM77 6DF (tel: 0800 026 3405). The Electrosound 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. 

 

Procedure


mounted transformer at each end

a Connect the circuit to a 12 V AC supply as shown in figure 1a. 

b Reconnect the circuit, with a mounted transformer at each end, as shown in figure 1b but do not switch on yet.  

Boss stand and wooden rod

 

c 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. 
 
d Support the sleeved wires in the slots in the two rods held in boss heads in stands. 
 
e Connect up the two 12 V lamps and the supply as shown. 
 
f Switch on. 


Teaching notes


1 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. 

2 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. 
 
Using a high voltage reduces energy losses in the transmission cable. Heating loss in the transmission cable goes as I2R. 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 lost to heating the transmission wires. 
 
In step a, 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 c, the distant lamp should now be as bright as the one directly connected to the supply. 
 
3 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'. 
 
4 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. 
 
5 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-checked in July 2007

 

Related experiments


A step-up transformer

Model DC power line

 

Weblinks


Electrosound