Magnetism – GCSE Combined Science Physics

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Think about the magnet on your fridge. It holds up a shopping list with no glue, no tape and no string. Something invisible is pulling on it, and that something is magnetism.

Magnetism is a force, and a force is just a push or a pull. A magnet can pull on some things and push on others without ever touching them. It can even do it through a sheet of paper or a thin piece of plastic.

Here is a picture to hold on to. Imagine a speaker at a party. You cannot see the music, but you know it is there because you can hear it close to the speaker, and it gets quieter as you walk away. A magnet is the same. The space around it where it can push or pull is called its magnetic field, and the field is strongest close to the magnet.

You meet magnetism every day. It is inside your phone speaker, the motor of a hairdryer, the doorbell, the crane that lifts old cars in a scrapyard and the little transformer in a laptop charger. Each one uses the same few ideas, and you will learn them one at a time.

By the end of this lesson you will be able to do three things. First, say what happens when magnets meet, and sketch the fields around a magnet, a wire and a coil. Second, explain how to build an electromagnet and make it stronger. Third, describe the motor effect. At Higher tier you will also use Fleming's left-hand rule, explain how an electric motor turns, and use two equations to find a force and a transformer output.

Poles and magnetic materials

Every magnet has two ends called poles. One is the north pole and the other is the south pole. The pulling and pushing is strongest at the poles, which is why a fridge magnet holds best at its edges.

Here is the rule to learn. Like poles repel, which means they push each other away. Opposite poles attract, which means they pull together. Two north poles push apart, and a north and a south snap together.

Try it with two fridge magnets. If one fights you when you push it towards the other, you are pushing like poles. Flip one magnet over and they jump together. The force acts across the gap, so the magnets do not need to touch. Attraction and repulsion between magnetic poles are non-contact forces.

Not every material is pulled by a magnet. Only a few metals are magnetic, and these are iron, steel, cobalt and nickel. Steel is mostly iron, which is why a steel paperclip sticks to a magnet. Copper, aluminium, plastic and wood are not magnetic, even though copper and aluminium are metals.

A permanent magnet always has its own magnetic field, like a bar magnet. An induced magnet is a piece of magnetic material that only acts as a magnet while it sits in another magnet's field. When a paperclip jumps to a magnet it has become an induced magnet, and it is always attracted, never repelled. Take the magnet away and the paperclip loses most of its magnetism.

Magnetic fields and field lines

A magnetic field is the region around a magnet where another magnet, or a magnetic material, feels a force. You cannot see it. What you can do is draw field lines, which are lines that show the shape of the field.

Outside a magnet, field lines run from the north pole to the south pole. Each line has an arrow on it to show that direction. The direction of the field at any point is the way a north pole would be pushed if you placed one there.

The lines also tell you how strong the field is. Where the lines are close together the field is strong, and where they spread out the field is weak. Around a bar magnet the lines crowd together at the two poles, so the field is strongest at the poles and gets weaker as you move away.

A magnetic material, such as iron, steel, cobalt or nickel, is always attracted to a magnet. The force between a magnet and a magnetic material is always one of attraction.

A compass is a tiny magnet on a pin that can spin freely. The Earth has its own magnetic field, as if a giant bar magnet sat inside it. A compass needle lines up with whatever field it is in, and near you that is mostly the Earth's, so one end points north.

Far from any other magnet, a compass still points north. That tells us the Earth itself makes a magnetic field. The field has to come from inside the Earth, so the Earth's core must be magnetic.

You can plot a field with a compass. Put the compass near the magnet and mark a dot at each end of the needle. Move the compass so its tail sits on the front dot, and mark again. Keep going, then join the dots to draw one field line, and repeat from a new start to build the whole pattern.

Electromagnets

An electric current makes a magnetic field. The field around a straight wire is a set of circles around the wire. The bigger the current, the stronger the field, and it gets weaker as you move away from the wire. A compass placed next to a wire carrying a large current is pushed off course.

To find the direction of the field, use the right-hand grip rule. Point your right thumb along the current. Your curled fingers show which way the circles of field go.

The field around a single straight wire is weak. To make it stronger, wind the wire into a coil called a solenoid. The fields from all the loops add together. Inside the solenoid the field is strong and uniform, and outside it has the same shape as the field of a bar magnet, with a north pole at one end and a south pole at the other.

Curl the fingers of your right hand the way the current goes round the coil. Your thumb points to the north pole of the coil.

An electromagnet is a solenoid with a core of soft iron inside it. The core makes the field much stronger. The big advantage is that you can switch it off, because with the current on it is a strong magnet and with the current off nearly all the magnetism goes.

There are three ways to make an electromagnet stronger. Wind on more turns of wire, increase the current, or put in an iron core. If you reverse the direction of the current, the north and south poles swap over.

Why soft iron and not steel? Soft iron is easy to magnetise and easy to demagnetise, which means it loses its magnetism quickly. Steel would stay magnetised after the current stopped. A scrapyard crane needs the magnet to switch off, otherwise the car would never drop.

The motor effect and electric motors

A wire with a current in it has its own magnetic field. Put that wire inside another magnetic field, for example between two magnets, and the two fields push on each other. The wire and the magnet exert a force on each other. This is called the motor effect.

The wire has to cross the field lines. The force is biggest when the wire is at right angles to the field. If the wire lies along the field, parallel to the lines, there is no force at all.

The force acts at right angles to both the wire and the field. Reverse the current and the force reverses. Reverse the magnets and the force reverses. Reverse both together and the force goes the same way as before.

Higher tier: Fleming's left-hand rule shows how the force, the current and the field are arranged. Hold your left thumb, first finger and second finger at right angles. The First finger points along the Field from north to south. The seCond finger points along the Current from positive to negative. The thuMb shows the Motion, which is the direction of the force.

Higher tier: a coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor. The coil sits between two magnets. The current runs one way along one side of the coil and the opposite way along the other side. By the left-hand rule the two forces point in opposite directions, so the coil turns.

Higher tier: a split-ring commutator is joined to the ends of the coil. It touches two fixed carbon brushes that are connected to the battery. Every half turn, the split ring swaps which brush touches which half. That reverses the current in the coil, so the forces keep pushing the coil round the same way instead of rocking it back.

Higher tier: a motor turns faster with a bigger current, stronger magnets or more turns on the coil. Reverse the current, or the magnets, and the motor spins the other way.

Higher tier: the force on the wire comes from an interaction between two magnetic fields. The current in the wire makes its own magnetic field, and this interacts with the field of the magnet. The two fields push against each other, which makes a force on the wire, and by Newton's third law an equal and opposite force acts on the magnet.

The size of the force (Higher tier)

At Higher tier you also work out how big the force on the wire is. It depends on three things. These are how strong the field is, how big the current is, and how much of the wire is inside the field.

Magnetic flux density is the number that tells you how strong the field is. It is measured in tesla, with the symbol T. A bigger number means a stronger field, so a strong lab magnet has a bigger value than a fridge magnet.

In words: force = magnetic flux density × current × length. In symbols: F = B × I × L. The force F is in newtons, N. The flux density B is in tesla, T. The current I is in amperes, A, and the length L is in metres, m.

Use the equation when the wire is at right angles to the field. The length is the part of the wire that is inside the field. If the length is given in centimetres, change it to metres first by dividing by 100.

Your method is the same each time. Write the equation, put the numbers in, work it out and add the unit. Because it is a product, doubling the current doubles the force, and doubling the length doubles it too.

Transformers (Higher tier)

When a magnetic field changes near a coil of wire, a potential difference is made in the coil. Potential difference, also called voltage, is the push that drives a current round a circuit. Making a potential difference in this way is called induction.

A transformer has two coils wound on one iron core. The primary coil is the input and the secondary coil is the output. An alternating current in the primary makes a changing field in the core, and that changing field induces an alternating potential difference in the secondary.

Alternating current, a.c., keeps swapping direction, so its field keeps changing. Direct current, d.c., flows one way, like the current from a battery, so its field is steady. A steady field induces nothing, and that is why a transformer needs a.c.

A step-up transformer has more turns on the secondary than on the primary, so the output potential difference is bigger. A step-down transformer has fewer turns on the secondary, so the output is smaller. In words: secondary p.d. = primary p.d. × secondary turns ÷ primary turns, or Vs = Vp × Ns ÷ Np, in volts.

The national grid sends electricity at a very high potential difference. For the same power this means a smaller current, so less energy is wasted as heat in the cables. Step-up transformers raise the voltage at the power station, and step-down transformers lower it near your home.

Induction also happens when a wire moves through a magnetic field, or a magnet moves in or out of a coil. The wire cuts the field lines, which induces a potential difference. A generator, or dynamo, works this way. The induced potential difference is bigger with a stronger magnet, with more turns on the coil and with faster movement. It reverses if the direction of movement is reversed or the magnet is turned the other way round. A complete circuit is needed to make a current.

Recap

Magnets have a north and a south pole. Like poles repel and opposite poles attract. Only iron, steel, cobalt and nickel are magnetic. A permanent magnet has its own field, while an induced magnet only acts as a magnet in another magnet's field and is always attracted. Field lines run from north to south, closest together where the field is strongest. A compass shows that the Earth's core is magnetic.

A current makes a magnetic field. A straight wire has circular field lines and a coil has a field like a bar magnet. A coil with a soft iron core is an electromagnet, and more turns, more current or an iron core makes it stronger. It switches off when the current stops, which is why cranes and relays use it.

A wire with a current in a magnetic field feels a force, called the motor effect. Reverse the current or the field and the force reverses. Higher tier: Fleming's left-hand rule gives the direction, a split-ring commutator keeps a motor turning, and F = B × I × L gives the size of the force.

A transformer uses a changing field to change a potential difference. At Higher tier, Vs = Vp × Ns ÷ Np, and the supply must be alternating. The national grid uses a high voltage so the current and the wasted heat are small.

How this is asked in an exam: a describe question wants one clear fact per mark, such as add more turns of wire. A calculation wants the equation, the substitution, the answer and the unit. An explain question about a transformer needs the chain: a.c., then a changing field, then an induced potential difference.