Magnetic Poles, Field Lines, and Earth’s Field

A magnet has two poles, north and south; like poles repel and unlike poles attract, and the magnetic field around a magnet is represented by field lines that leave the north pole, enter the south pole, and are closer together where the field is stronger. In brief: the direction of the field at a point is the direction in which the north pole of a small compass needle would point. The Earth behaves like a giant bar magnet with a field of about 25 to 65 microtesla at the surface. A compass needle is a small magnet that aligns with this field, pointing to the geographic north, which is near the Earth’s magnetic south pole.

Refrigerator magnets, compass needles and the auroras all involve magnetic fields. This guide describes magnetic poles, materials and field lines, the Earth’s magnetic field and compasses, compares magnetic materials in a table, works through a compass calculation and corrects common mistakes.

What are magnetic poles?

Every magnet has two poles, where the magnetic effect is strongest: a north-seeking pole (N) and a south-seeking pole (S). Poles always occur in pairs. If a bar magnet is cut in two, each piece has both a north and a south pole (no magnetic monopole has been found). Like poles repel and unlike poles attract. A magnet attracts magnetic materials (iron, nickel, cobalt and some alloys), which can be magnetized by stroking with a magnet or by placing them in a strong field. The magnetization of iron arises from small regions called domains, each acting as a tiny magnet: in unmagnetized iron the domains point in random directions, and in a magnet they are aligned. Heating or hammering a magnet disturbs the domains and demagnetizes it. Above the Curie temperature (770 °C for iron), a material loses its ferromagnetism. Soft magnetic materials (soft iron) are easily magnetized and demagnetized and are used in electromagnets and transformers. Hard magnetic materials (steel, neodymium alloys) keep their magnetism and are used for permanent magnets.

What are field lines?

The magnetic field is the region around a magnet in which another magnet or magnetic material feels a force. It is a vector quantity with the symbol B, measured in tesla (T). Field lines are drawn to show the field:

• they leave the north pole and enter the south pole outside the magnet (and continue inside from south to north, forming closed loops);
• the arrow gives the direction of the force on a north pole;
• the lines are closer together where the field is stronger (near the poles);
• lines never cross.

They can be shown by scattering iron filings around a magnet, or by plotting with a small compass. Between the opposite poles of two magnets, the field is almost uniform (parallel, equally spaced lines). Between two like poles, there is a point of zero field (a neutral point) where the lines are pushed apart.

What is the Earth’s magnetic field?

The Earth’s field resembles that of a bar magnet tilted about 11° from the axis of rotation, with the south pole of this “magnet” near the geographic North Pole (that is why the north pole of a compass is attracted to it). It is produced by electric currents in the liquid iron outer core (the geodynamo). At the surface, the field is about 25 µT near the equator and 65 µT near the poles. The field lines are horizontal at the magnetic equator and enter the ground vertically at the poles. The angle of the field below the horizontal is the dip (inclination), and the angle between magnetic north and true north is the declination, which varies from place to place and changes over time. A compass is a magnetized needle free to rotate, and its north end points toward the Earth’s magnetic pole in the north. The field extends into space as the magnetosphere, which deflects the solar wind. Charged particles that are trapped and guided along the field lines to the polar regions cause the auroras. The field reverses at irregular intervals of hundreds of thousands of years; the last full reversal was about 780,000 years ago.

Magnetic materials compared

Worked example: a compass near a magnet

A compass is placed on a table in a place where the horizontal component of the Earth’s field is 20 µT, pointing north. A bar magnet is placed to the east of the compass with its axis along the east-west line, so that at the compass its field of 10 µT is perpendicular to the Earth’s horizontal field.

1. Resultant field: the two fields are perpendicular: 20 µT north and 10 µT along the east-west line. B = √(20² + 10²) = √500 = 22.4 µT.
2. Deflection of the compass: tan θ = 10 ÷ 20 = 0.50, so θ = 26.6° away from north, toward the direction of the field of the magnet.
3. Field twice as strong (20 µT): tan θ = 1.0, θ = 45°.
4. Moving the magnet away: the field of a bar magnet falls off roughly as the cube of the distance. Doubling the distance reduces the field to 10 ÷ 8 = 1.25 µT, so tan θ = 1.25 ÷ 20 = 0.0625, and θ = 3.6°.

How was magnetism discovered?

Lodestone (magnetite) was known in ancient Greece and China. The Chinese used magnetic compasses for navigation by the 11th century. Pierre de Maricourt described poles in 1269, and William Gilbert’s De Magnete (1600) concluded that the Earth is a giant magnet. Hans Christian Ørsted discovered in 1820 that an electric current deflects a compass needle, linking electricity and magnetism. Paleomagnetic studies of rocks showed the reversals of the field and supported plate tectonics in the 1960s.

Which mistakes are common?

Mistake 1: Calling the Earth’s north magnetic pole a north pole of a magnet

The pole near the geographic North Pole is magnetically a south pole, since it attracts the north end of a compass.

Mistake 2: Believing that field lines are physical objects

They are a way of drawing the field. The field exists everywhere in the region.

Mistake 3: Thinking that all metals are attracted by magnets

Only a few are ferromagnetic. Aluminum, copper and gold are not.

Mistake 4: Expecting an isolated north or south pole

Cutting a magnet gives two smaller magnets, each with two poles.