A magnetic field is found in the area around magnetic materials and current carrying conductors, and has both a direction and a magnitude at each point. A magnetic field can either attract or repel other magnetic objects at a distance. A magnetic field differs in this respect from a gravitational field which is always attractive – and also in the fact that a magnetic field only affects some types of material. There is a close association between magnetic and electric fields, the study of which is known as electromagnetism.
Magnets have both North and South poles, which always occur in pairs. The North pole is defined as the pole of a magnetic compass which points towards the Earth’s Magnetic North.
A magnetic field may be represented by lines of magnetic flux which are closer together where the field is strongest and become further apart as it weakens. However, these lines of magnetic flux never cross.
It is found experimentally when two magnets are brought together that like poles repel (North–North and South–South) and opposite poles attract (North–South).
Lines of magnetic flux are considered to have direction, although nothing actually moves (unlike a flow of electric current for example). The orientation of the flux is given by the direction of force which would be experienced by a notional isolated North pole placed in the field. It follows that lines of magnetic flux are drawn leaving the North pole of a magnet and entering the South pole (since the isolated North pole would be repelled from the North pole of a magnet, but attracted to the South).
Although the above description talks of an isolated North pole, or monopole, magnetic poles always come in pairs. Cutting a magnet in half, for example, simply produces two smaller magnets. Hence it is not possible to isolate a single North or South pole. To understand this, consider that an electric current flowing in a circular path (as in a solenoid) generates a magnetic field equivalent to a bar magnet – having both North and South poles.
Activity 1: Investigating Magnetic Fields
To complete this activity you will need:
- Iron filings (a plotting compass may be used as an alternative).
- A pair of educational magnets (ideally with the ends painted red and blue to make identification of the North and South poles easier).
- A sheet of plain white paper or card.
- A digital camera to record your results (optional).
Use the magnets and iron filings to Identify the magnetic field patterns produced by:
- A single magnet.
- A pair of magnets with opposite poles facing inwards.
- Two magnets with like poles facing inwards.
Comment on your findings.
Show / hide answer
1. Magnetic field of a single bar magnet:
2. Magnetic field produced when two magnets with opposite poles experience attraction.
3. Magnetic field produced when two magnets with like poles experience repulsion.
The results obtained are in close agreement with those expected.