Magnets and Magnetic Fields

An invisible map

On a clear winter night in much of Canada you may see the northern lights. Charged particles from the Sun are guided by Earth's magnetic field toward the polar regions, where they make the upper atmosphere glow. That same field turns a compass needle, and helps keep harmful charged particles away from the surface.

Magnetic fields are invisible, but we can map them. Before we connect magnetism to electricity in the next lessons, we need to describe magnets and their fields precisely.

Poles and the law of magnetic poles

Every magnet has two poles. If a bar magnet hangs freely, one end turns to point roughly north; we call it the north-seeking pole (N). The other is the south-seeking pole (S).

• Like poles repel; unlike poles attract.
• Poles always come in pairs. If you break a magnet in half, you get two smaller magnets, each with its own N and S. Nobody has ever found a single isolated pole.
• Magnets attract ferromagnetic materials (iron, nickel, cobalt and some alloys like steel) even if those materials are not magnets themselves.

Domain theory

Inside iron, the atoms behave like tiny magnets. Groups of atoms line up with each other in small regions called domains. In unmagnetized iron, the domains point in random directions and cancel out. When iron is placed in a strong magnetic field, the domains that point along the field grow and others rotate to line up. The iron becomes a magnet.

Domain theory explains a lot:

• Breaking a magnet gives two magnets, because each piece still has aligned domains.
• Heating or hammering a magnet can demagnetize it, because it shakes the domains back out of line. Above a temperature called the Curie point (770 °C for iron), iron cannot stay magnetized at all.
• "Soft" iron magnetizes and demagnetizes easily (good for electromagnets); "hard" steel keeps its magnetism (good for permanent magnets).

Magnetic fields and field lines

A magnetic field is the region around a magnet where a magnetic force acts. We give it the symbol B and the unit tesla (T). Earth's field at the surface is about 50 μT (0.000 05 T); a hospital MRI magnet is typically 1.5 T or 3 T.

We draw fields with field lines. The rules:

1. The direction of a field line at any point is the direction the N pole of a compass would point there.
2. Outside a magnet, lines go from N to S. Inside the magnet they continue from S to N, so every line is a closed loop. Field lines never start or stop in empty space.
3. Field lines never cross. (A compass can only point one way at a time.)
4. Where lines are closer together, the field is stronger. That is why the lines crowd together at the poles.
5. The field is three-dimensional. A drawing on paper shows only one slice; the real field surrounds the magnet in every direction, like the shape of an apple around its core.

Ways to map a field

• Compass plotting: place a small compass at many points around a magnet, mark the direction of its N end each time, and join the marks into lines.
• Iron filings: sprinkle fine iron filings on paper over a magnet and tap gently. Each filing becomes a tiny magnet and lines up with the field, showing the pattern (but not the direction) at a glance.
• Magnetic field sensors: a Hall-effect probe, or the magnetometer built into most smartphones, measures the strength (and often the direction) of B, so you can graph how the field changes with distance.

Earth's magnetic field

Earth behaves as if it had a huge bar magnet inside it. A compass's N pole points toward the north magnetic pole, which lies in the Arctic. Since unlike poles attract, that region must actually be a magnetic south pole! Earth's magnetic poles also move slowly: the north magnetic pole was in the Canadian Arctic for centuries and has been drifting toward Siberia.

Because magnetic north is not at the geographic North Pole, a compass usually points a little east or west of true north. This angle is the magnetic declination. Declination can be large in parts of Canada, so hikers, pilots and surveyors here must correct for it.