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

Energy carried by an electric current

Demonstration

Electric current transfers energy from the battery or power station to a lamp.

Apparatus and materials

Lamp, 240 V 100 W

Lamp, 12 V 24 W

Lamp, 12 V 6 W

Lamp, 12 V 36 W and power supply

Stopclock

Demonstration meter with 0-5 amp DC and 0-5 amp AC ranges, 0-12 volt and 0-240 V AC ranges

Power supply, 12 V AC

Power supply, 240V

Socket complete with wattage and current meter (see Technical notes)

Health & Safety and Technical notes


Do not build circuits for connection to the mains.

The simplest demonstration just uses a safety pattern BC batten lamp holder wired via a 5 A double-insulated flex to a 13 A plug.

If ammeter and voltmeter(s) are used, all connections must use shrouded 4 mm connectors. The connections to the mains can be bared wires using a 'safebloc'.

The lamps can be purchased from Beecroft & Partners Ltd. or other scientific equipment suppliers.
 
An alternative is a device called 'cost plug', which is no longer on sale.
 
The socket with wattage and current meter is available from the Machine Mart website.

 

Procedure


a Connect a simple series circuit of a 12 volt 24 watt lamp and demonstration meter to a 12 volt supply. Switch on for 15 seconds. Discuss the energy transfer from the supply to heat and radiation from the lamp.

energy carried by an electric current equipment b Repeat using the 12 volt 36 watt and again with the 12 volt 6 watt lamp.
If an AC ammeter is available, repeat again, using the 12 volt AC terminals of the transformer.
 
c Connect the 240 volt 100 watt lamp using the commercial socket with in-built meters.

 

Teaching notes


1 The chemical energy released from the burning of fuel in a power station is carried by means of the electric current to the lamp. When the electric current passes through the filament, the filament warms up. Its temperature rises so that it glows red or white hot. The energy is transferred from the filament as electromagnetic waves of visible light and infrared radiation which spread out into the surroundings, so warming up the environment.
 
2 The 5 W, 24 W and 36 W lamps, operating from 12 V, glow with different brightness. The 36 W lamp is the brightest and so radiates most energy. The electric current in the circuit with the 36W lamp is also greatest. It appears that the energy radiated may be proportional to the current. (More experiments than this one need to be done before this can be substantiated!) Refer to the clock to emphasize that you are discussing energy and not power.
 
There is an advantage in setting up two circuits side by side e.g. 12 V 24 W bulbs, one on DC and one on AC. Students can see that AC makes very little difference.
 
3 The 240 V 100 W lamp and 12 V 6 W lamp carry the same current but more energy is radiated from the 240 V lamp and this is indicated by its 100 W rating.
 
4 You have shown that the energy radiated depends not only on the current but also on the potential difference. It will also depend on how long the current is flowing:
Energy transferred by the electric current, E = V x I x t = VQ
 
The volt can be defined as the energy transferred per coulomb passing from a power supply to a component. This definition is frequently used when students are learning electrical concepts in the early stages.
 
This experiment was safety-checked in December 2005