09. Interacting fields

Electrical and magnetic fields are both caused by the behaviour of electrons. However, the two fields are not only related - in fact they produce one another. This almost magical connection was first identified by accident by the Danish experimenter Hans Christian Ørsted in 1820. Ørsted was giving a public lecture on the heating effect of electricity when he suddenly thought to see if there was a magnetic field in the vicinity of his experiment. As he had a compass to hand, he placed it next to the wire - and discovered the electromagnetic interaction.

ACTIVITY: Spiralling fields

ATL - Critical thinking skills: Practice observing carefully in order to recognise problems.

Safety: As this activity involves relatively high electric currents, it should be a demonstration.

Equipment:

• • Low-voltage, high current power supply (at least 10 A)

  • Copper wire of at least 0.5 mm diameter and about 1 m in length

  • Small plotting compasses x 4

  • Lab stand and three clamps

  • Piece of card with a 1 cm diameter hole cut in the middle

Method

• 1. Set up the equipment as shown in the photo.

  2. Position the plotting compasses around the wire at right angles as shown.

  3. Turn on the current.

Write down your observations of the effect of the current on the compasses.

Turn off the current, then change the position of the compasses to obtain other readings for the effect.

Change the height of the card and repeat until you have an outline for the effect around the wire.

Now reverse the connections from the power supply to the wire (so that the current is flowing in the opposite direction to before). Repeat your observations.

Summarise your observations. What is happening around the current carrying wire? What is the shape of the effect produced?

Sketch a diagram showing the effect.