How Magnets Create Forces at a Distance

A bar magnet has a north-seeking pole and a south-seeking pole. Opposite poles attract, while matching poles repel. The force becomes weaker with distance, although the exact pattern depends on the magnets' shapes and strengths.

If a bar magnet is cut in half, each piece forms its own north and south poles. Ordinary magnets do not separate into one isolated north pole and one isolated south pole. The magnetic behaviour comes from organised effects throughout the material.

Iron, nickel and cobalt are strongly magnetic because groups of atoms can form regions called domains. In an unmagnetised piece, many domains point in different directions. When enough align, their effects reinforce one another and the object becomes magnetised.

Magnetic field lines are a model used to show field direction and relative strength. Outside a bar magnet, the conventional direction runs from the north pole towards the south pole. Closely spaced lines represent a stronger field.

Field lines are not physical threads filling space. They help you predict the direction a small compass would point at different locations. A compass needle is itself a tiny magnet, so it turns until it aligns with the local field.

When two magnetic fields overlap, their combined pattern explains attraction or repulsion. Unlike poles produce connecting field lines and pull together. Like poles create a region where the fields oppose one another, and the magnets push apart. The model gives the invisible interaction some visual manners.

Moving electric charges create magnetic fields. A current in a straight wire produces a circular field around the wire. Coiling the wire concentrates the field, creating an electromagnet with north and south poles.

Placing an iron core inside the coil can make the field much stronger because domains in the iron align. The electromagnet can be controlled by changing the current, the number of coil turns or the core material. Switching off the current removes most of the magnetic effect.

Electromagnets appear in relays, speakers, motors, lifting cranes and many other devices. Remember the core relationships:

• •Opposite poles attract.
• •Matching poles repel.
• •Closer field lines show stronger fields.
• •Electric current creates a magnetic field.
• •A coil and iron core strengthen an electromagnet.

A compass works almost anywhere on Earth because the planet has its own magnetic field. The field is generated deep inside, where molten iron flows in the outer core. Moving electric charges in that flowing metal produce magnetic fields, the same principle behind an electromagnet, and the motion keeps the field going in a process called the geodynamo. This leads to a surprising detail. The north pole of a compass needle points north, and opposite poles attract, so the magnetic pole in the Arctic is, in magnetic terms, a south pole.

Earth's field is not fixed. The magnetic poles wander over the years, which is why maps and navigation systems need regular updates. Over much longer periods the whole field has reversed many times, swapping north and south. Scientists know this because some rocks record the field at the moment they formed. As lava cools on the sea floor at mid-ocean ridges, magnetic minerals line up with the field, leaving stripes of alternating direction parallel to the ridges. These stripes became key evidence that the sea floor spreads, linking magnetism to plate tectonics.

A permanent magnet works because its domains stay lined up, so anything that jostles the atoms can spoil it. Heat is the clearest example. Above a temperature called the Curie point, named after the physicist Pierre Curie, the thermal motion of atoms becomes strong enough to destroy the alignment, and the material stops behaving as a magnet. For iron, the Curie point is about 770 degrees Celsius. Dropping a magnet repeatedly or hammering it can also shake domains out of line and weaken it over time.

Engineers choose materials for how easily their domains move. Soft magnetic materials, such as pure iron, magnetise easily and lose their magnetism easily, which makes them ideal for electromagnet cores and transformers that must switch on and off. Hard magnetic materials keep their alignment, so they make good permanent magnets. Modern neodymium magnets, made from an alloy of neodymium, iron and boron, are extremely strong for their size and are used in headphones, electric motors and wind turbines, though they lose strength if they get too hot.

A common belief is that magnets attract all metals, but most metals are barely affected. Aluminium, copper, brass, gold and silver do not cling to a fridge magnet, because their atoms do not form domains that line up together. Even steel can surprise you. The stainless steel in many kitchen sinks contains plenty of iron, yet its atoms are arranged in a crystal pattern that prevents strong magnetism, so a magnet hardly grips it. Recycling centres use this difference: powerful magnets lift steel cans out of mixed waste, leaving aluminium cans behind to be separated by other methods.

Yet a non-magnetic metal can still push back against a moving magnet. Drop a strong magnet down a vertical copper pipe and it drifts down surprisingly slowly, as if falling through syrup. As the magnet moves, its changing field drives swirling electric currents, called eddy currents, in the copper around it. Those currents create their own magnetic field, which opposes the magnet's motion, a rule known as Lenz's law. Drop an ordinary unmagnetised weight of the same size and it falls straight through. The same effect provides smooth, contact-free braking on some trains and fairground rides, with no pads to wear out.

Magnetic field strength is measured in a unit called the tesla, named after the inventor Nikola Tesla. One tesla is a very strong field, so everyday values are usually tiny fractions of it. Earth's magnetic field at the surface is only around 25 to 65 millionths of a tesla, depending on where you are, which is enough to turn a delicately balanced compass needle but not to lift anything. A typical fridge magnet produces a field of a few thousandths of a tesla close to its surface, roughly a hundred times stronger than Earth's, but that strength falls away quickly with distance.

Hospital MRI scanners use fields of 1.5 or 3 tesla, tens of thousands of times stronger than Earth's. Such fields come from electromagnets with coils of superconducting wire, which carries current without resistance only when cooled with liquid helium to a few degrees above absolute zero. Once established, the current can circulate for years, so the magnet is normally left on all the time. That is why strict safety rules apply in scanner rooms. Steel objects such as scissors, oxygen cylinders or chairs can be pulled towards the scanner with great force, so everything that enters is checked carefully first.

Magnets exert forces through magnetic fields, with opposite poles attracting and matching poles repelling. Magnetic domains explain how some materials become magnets, while electric currents create controllable magnetic fields. Use field direction and strength to describe the interaction, and action at a distance stops feeling quite so mysterious.