Magnetism is the second half of this module, and it starts with permanent magnets rather than with electricity, because the field is the object everything later depends on. By the end of this lesson you should be able to predict whether two magnets attract or repel from their poles alone, say which materials a magnet affects and which merely conduct electricity, read a field-line diagram for both the direction and the strength of the field, explain why an unmagnetised iron nail is attracted by either pole, and describe the shape of the Earth's own field.
Poles
Every magnet has two ends that behave differently, and they are called poles: a north pole and a south pole. The names come from navigation rather than from physics — a magnet free to turn settles with one particular end pointing roughly towards the Earth's geographic north, and that end is called the north pole. The rule for how two magnets interact is the same in form as the rule for charges from the first lesson of this module, and it is worth keeping the two separate in your head because they are different phenomena with the same shape: like poles repel, unlike poles attract. Two north poles brought together push apart, two souths push apart, and a north facing a south pulls together.
One difference from charge matters. A single positive charge can exist on its own — that is what a rubbed balloon has an excess of — but a single magnetic pole cannot. Cut a bar magnet in half and you do not get a north piece and a south piece; you get two shorter magnets, each with its own north and south. Cut those in half and the same thing happens again. Poles always come in pairs, which is a fact worth having because items sometimes offer the halves-of-a-magnet case as a way of producing an isolated pole.
Which materials a magnet affects
Only a few materials respond to a magnet at all, and the list is much shorter than most people expect. Iron is the important one, along with steel, which is mostly iron; nickel and cobalt behave the same way, and so do alloys made from those metals. Everything else you are likely to be offered does nothing: copper, aluminium, brass, lead, tin, zinc, gold and silver are not attracted, nor are glass, plastic, wood, water or any of the non-metals. Magnetic materials in this sense are collectively called ferromagnetic, from the Latin for iron.
That last point is worth stating as sharply as possible, because it is where the distractors live: being a good conductor of electricity and being magnetic are unrelated properties. Copper is the second-best electrical conductor there is and a magnet ignores it completely; aluminium is used for power lines and aircraft structure and is not attracted either. The two properties come from different things — electrical conduction from electrons being free to drift through the lattice, magnetism from the way the electrons inside each atom line up with their neighbours — and knowing that a wire conducts tells you nothing at all about whether a magnet will pick it up.
There is also a distinction between a permanent magnet and a material that merely responds to one. A permanent magnet keeps its own field with nothing done to it, because the atomic-scale magnetism inside it is locked into alignment; hard magnetic materials such as steel hold that alignment once it has been set. Soft magnetic materials, of which pure iron is the standard example, line up readily when a field is applied and then lose most of it again when the field is taken away. That is why an iron nail is attracted by a magnet but does not usually stay magnetic afterwards, and it is why anything that has to be magnetic only while it is switched on is built around an iron core rather than a steel one.
Fields and field lines
A magnet exerts its force without touching anything, and the region around it in which that force acts is called its magnetic field. A field is awkward to talk about because it has a value everywhere in a volume of space, so it is drawn instead, and the convention for drawing it is fixed and carries two separate pieces of information. Lines are drawn so that the direction of the field at any point is along the line through that point, with an arrowhead to say which way; by convention the arrows run out of the north pole, round the outside of the magnet, and back in at the south pole. And the lines are spaced so that where they crowd together the field is strong, and where they spread apart it is weak.
Read the figure with both conventions in mind and it tells you something a sentence cannot. The lines converge on the poles, which is why a magnet picks things up at its ends and hardly at all in the middle. They form closed loops rather than starting and stopping in mid-air, which is the graphical version of poles always coming in pairs. And they never cross one another anywhere, for the simple reason that a crossing point would have to have two different field directions at the same time. Between two unlike poles facing each other the lines run straight across the gap from one to the other; between two like poles they curve away from each other and leave a region in the middle where they cancel and the field is nearly zero.
Why an unmagnetised nail is attracted either way round
Here is the case that separates understanding the field from remembering the pole rule. Bring the north pole of a magnet near an iron nail that is not magnetised and the nail is attracted. Turn the magnet round and bring the south pole instead, and the nail is attracted again. That looks like a contradiction of the like-repels-unlike-attracts rule until you notice that the rule applies to two magnets, and the nail is not one. What happens is that the magnet's field reaches the nail first and magnetises it: the atomic-scale magnetism inside the iron lines up with the applied field, and it does so in whatever direction the applied field happens to point. The result is that the nail always develops the opposite pole on the end nearest the magnet, whichever pole is presented, and opposite poles attract.
Two consequences follow, and both come up. A magnetised object can repel, but an unmagnetised magnetic material can only ever be attracted — so if a question describes an object being repelled by one end of a magnet, that object must be a magnet itself. And a nail hanging from a magnet will pick up a second nail below it, because the first nail is now temporarily a magnet with a free pole at its lower end, and the chain continues until the induced magnetism has become too weak.
The Earth as a magnet
The Earth has a magnetic field of its own, generated by the movement of molten iron in its outer core, and to a first approximation it has the shape of the field around a bar magnet buried in the planet and tilted slightly from the axis of rotation. That field is what a compass responds to: a compass needle is a small magnet free to turn, and it settles along the local direction of the field, which over most of the world is roughly north–south.
There is a piece of vocabulary here that reads like a trick and is not. Since a compass needle's north pole is attracted towards the Earth's northern regions, and unlike poles attract, the magnetic pole up there must be a magnetic south. The naming is historical: the needle's end was called north because it points north, and that forces the Earth's polarity to come out the other way round. Two further honest points. The magnetic poles are not in the same places as the geographic poles, so the direction a compass gives differs from true north by an angle that depends on where you are, and the magnetic poles wander slowly over years. And the field is not horizontal except near the equator; it dips into the ground at an angle that steepens as you go towards the poles, which is why a compass is built to be sensitive only to the horizontal part of it. How those effects are corrected for in an aircraft instrument is a navigation matter rather than a physics one, and it is dealt with in the aviation-knowledge material.
Why it matters in aviation
The magnetic compass is still a required instrument, and the reason is the one thing it does not need: power. It responds to the Earth's field directly, so it keeps working when nothing electrical does. Everything about using one follows from the properties of that field. The horizontal direction it settles along is not true north, because the magnetic and geographic poles are in different places, so the angle between them differs from place to place and has to be applied as a correction that depends on where the aircraft is.
The aircraft's own magnetism deflects the needle further, since a structure containing ferromagnetic parts and current-carrying wires has a field of its own that the needle cannot distinguish from the Earth's. That is what swinging a compass is for: the remaining error is measured on a series of headings and recorded on a correction card kept with the instrument. And because the field dips ever more steeply into the ground as you approach the poles, leaving little horizontal part for the needle to line up with, the instrument becomes unreliable at high latitudes.
What the test asks
The reliable distractor in this topic is the belief that all metals are magnetic. Copper and aluminium are the two most often used to catch it, because both are excellent electrical conductors, and an item may put that in the stem specifically to make the wrong answer look reasonable — conduction and magnetism are unrelated. Expect an item on which pairings attract and which repel, where the case to think about is the unmagnetised piece of iron or steel, which is attracted by either pole and never repelled by either. Expect a field-line item where the wrong diagram has the arrows running into the north pole, or has lines crossing, or has them stopping in mid-air. And expect the Earth item, where the two available traps are forgetting that the magnetic pole near geographic north is a magnetic south, and assuming that a compass points at the geographic pole exactly.