What is a magnetic field?
Short answer: a magnetic field is the area around a magnet (or a current-carrying wire) in which magnetic forces act. It is described with field lines that run outside the magnet from the north pole to the south pole. Its strength is expressed in Tesla (T) or Gauss (G). The field weakens quickly with distance: at some distance from a magnet roughly with the third power. Twice as far away, only an eighth of the field remains.
Properties of a magnetic field
- Direction: outside the magnet from north to south, inside from south to north. The lines are always closed loops.
- Density: where the lines are close together (at the poles and edges), the field is strongest.
- Range: a field has no hard boundary but drops off quickly. That is why a magnet works much more strongly directly on steel than at 5 mm distance.
- Shielding: only ferromagnetic materials such as steel divert the field. Wood, glass and plastic let it through. See do magnets work through wood, glass and plastic?
How do you make a magnetic field visible?
- Iron filings: sprinkle them on a sheet of paper above a magnet and tap gently. The grains form the field lines.
- Compass: the needle turns with the direction of the field.
- Magnetic viewing film (green film): shows the poles and pole patterns of a magnet.
- Gaussmeter: measures the strength at a point. See what are Gauss and Tesla?
Field strengths in everyday life
| Source | Field strength (approx.) |
|---|---|
| Earth (Netherlands) | 50 µT (0.5 G) |
| Fridge magnet | 5 – 10 mT (50 – 100 G) |
| Neodymium magnet at the surface | 0.2 – 0.65 T (2,000 – 6,500 G) |
| MRI scanner | 1.5 – 3 T |
The earth's magnetic field
The earth itself has a magnetic field, generated by currents of liquid iron in the outer core. A compass needle is a small magnet that aligns with it. Physically speaking, the earth's magnetic north pole is a south pole: it attracts the north pole of the compass.
Magnetic fields and technology
Electric motors, generators, transformers, loudspeakers and sensors all work with magnetic fields. A moving field induces a current in a wire (induction); a current in a wire creates a field. Almost all electrical technology runs on that interplay.
Frequently asked questions
Is a magnetic field harmful?
Static fields from ordinary magnets are not harmful to healthy people. They can, however, interfere with pacemakers, ICDs, bank cards with a magnetic stripe, mechanical watches and compasses. Read working safely with magnets.
Can I shield a magnetic field?
Yes, with a steel plate or special mu-metal. The field is then diverted through the steel. A steel pot uses this principle to concentrate the field on one side. See why are pot magnets so strong?
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