Magnetic fields and field lines

Earth's field at the surface is about 25 to 65 microtesla; the field inside a hospital MRI solenoid is tens of thousands of times stronger.

A map of forces

A magnet pushes or pulls on another magnet without touching it. To keep track of that action at a distance, physicists describe the space around a magnet with a magnetic field, the B field. At every point the field has a direction and a strength. The direction is defined by a small compass: the field points the way the compass needle's north end points. A compass is small enough not to disturb the field it measures.

Put compasses all around a bar magnet and join up the directions they show, and you get field lines. OpenStax sums up four rules for them. First, the field at a point is along the line there, so a compass lines up with it. Second, the strength is shown by how close the lines are: crowded lines mean a strong field. Third, field lines never cross, because the field has only one direction at each point. Fourth, they form closed loops with no start or end: outside the magnet they run from the north pole to the south pole, and inside they carry on from south back to north.

Field lines are a drawing tool, not threads in space. You can see their pattern, though, by shaking iron filings onto paper over a magnet. Each filing becomes a tiny magnet and turns along the field, so the filings settle into curves that match the lines.

Measuring field strength: the tesla

The SI unit of magnetic field strength is the tesla (T). One tesla is a strong field: a wire 1 m long carrying 1 A across a 1 T field feels a force of 1 N, so 1 T = 1 N/(A·m). Everyday fields are much smaller, so you often see microtesla (μT, millionths of a tesla) and nanotesla (nT, billionths). An older unit, the gauss, is still common in geology and on magnet labels: 1 gauss = 10⁻⁴ T = 100,000 nT.

Some reference values help. NOAA's National Centers for Environmental Information give Earth's field at the surface as roughly 25,000 to 65,000 nT, which is 25 to 65 μT or 0.25 to 0.65 gauss. OpenStax often rounds it to 5.0 × 10⁻⁵ T, 50 μT. Its worked examples use 1.50 T for a strong laboratory magnet and 2.01 T inside a large solenoid of the kind used in MRI scanners. That is about 40,000 times Earth's field.

Two magnets: the fields add

When two magnets are near each other, the field at any point is the sum of the two fields, added as arrows. Put a north pole facing a south pole and the lines run straight across the gap from one magnet into the other; the field in the gap is strong, and the magnets pull together. Put two north poles face to face and the lines from each are turned aside. They squeeze sideways out of the gap, and halfway between the poles there is a spot where the two fields cancel. A compass placed exactly there has no preferred direction and only Earth's weak field steers it.

The same adding explains why a compass on a desk can mislead you. A steel desk frame, a speaker magnet or a current in nearby wiring makes its own field, and near it that field can be stronger than Earth's 25 to 65 μT. The needle then points along the total, not toward the north. Hold a compass away from metal objects and electrical equipment when you want a true reading.