Permanent & induced magnets
By the end of this topic you'll explain attraction and repulsion, sketch a bar magnet's field, tell permanent from induced magnets, and plot a field with a compass.
Part 1Poles, attraction and repulsion
Every magnet has two poles — a north pole and a south pole. The poles are where the magnetic forces are strongest. When you bring two magnets together, the rule is simple: like poles repel (N–N or S–S push apart) and unlike poles attract (N–S pull together).
Attraction and repulsion between two poles is an example of a non-contact force — the magnets affect each other without touching. The force acts through the magnetic field that surrounds every magnet.
Words to nail
- Permanent magnet
- Produces its own magnetic field all the time — e.g. a bar magnet or a fridge magnet.
- Induced magnet
- A material that becomes a magnet only while it is in a magnetic field.
- Magnetic material
- Iron, steel, cobalt and nickel — these can be magnetised.
- Magnetic field
- The region around a magnet where another magnet or magnetic material feels a force.
Some materials are induced magnets. A piece of iron isn't a magnet on its own, but bring a magnet near it and the iron becomes magnetised while it's in the field. An induced magnet is always attracted to the magnet that induced it — never repelled. Take the magnet away and most of the magnetism quickly disappears.
⚠ Watch out — induced magnets only attract
An induced magnet (like a steel paperclip near a magnet) is always attracted, never repelled. Repulsion only ever happens between two magnets that already have their own poles. So if two objects repel, you know both must be permanent magnets — a useful test in the exam.
Two objects, X and Y, repel each other when brought close. What can you conclude?
- AX is a magnet and Y is an unmagnetised iron bar
- BBoth X and Y are permanent magnets
- CBoth X and Y are induced magnets
- DNeither object is magnetic
Show answer
Part 2The field of a bar magnet
A magnetic field is shown using field lines. The lines always point from north to south outside the magnet. Where the lines are closer together, the field is stronger — so the field is strongest at the poles. The lines never cross.
You can plot the field with a small plotting compass. A compass needle is a tiny magnet, so it lines up with the field and its (north) tip points along a field line. Move it around the magnet, mark the direction at each spot, and join the marks to draw the lines.
⚠ Watch out — direction and spacing
Field lines always have an arrow and always point from N to S (outside the magnet). Don't draw them touching or crossing. The spacing shows strength — packed lines mean a strong field, spread-out lines mean a weak one. Earth itself behaves like a giant bar magnet, which is why a compass points (roughly) north.
On a field-line diagram, what does it tell you when the lines are drawn close together?
- AThe field is weak there
- BThe field is strong there
- CThere is no field there
- DThe poles have swapped over
Show answer
State the rule for the force between two magnetic poles.
Like poles repel, unlike poles attract. It is a non-contact force.What is the difference between a permanent magnet and an induced magnet?
A permanent magnet makes its own field all the time. An induced magnet only becomes magnetic while it is in a magnetic field, and loses most of its magnetism when removed.Which way do magnetic field lines point, and what does their spacing show?
From north to south outside the magnet. Closer lines mean a stronger field.How would you use a plotting compass to find the shape of a magnet's field?
Place the compass near the magnet, mark the direction the needle points, move it on and repeat, then join the dots into field lines. The needle lines up with the field because it is itself a small magnet.
The magnetic field of a current
A current makes its own magnetic field — round a wire, and a strong, uniform one through a solenoid. Plus what makes electromagnets so useful.
Part 1The field around a straight wire
When a current flows through a wire, it creates a magnetic field around the wire. The field lines are concentric circles in flat rings around the wire, at right angles to it. The field gets weaker the further you go from the wire, so the circles are drawn further apart further out.
Two things make the field stronger: a bigger current, and being closer to the wire. Reverse the direction of the current and the field lines reverse direction too.
⚠ Watch out — circles, not straight lines
The field round a single straight wire is a set of circles wrapped around it, not a bar-magnet shape and not straight lines along the wire. People also forget the field has a direction: flip the current and every arrow flips too.
Part 2Solenoids and electromagnets
Wind the wire into a coil — a solenoid — and the fields from all the loops add together. Inside the solenoid this makes a strong, uniform field (the field lines are straight, evenly spaced and all in the same direction). Outside, the field looks just like a bar magnet's field, with a north pole at one end and a south pole at the other.
Put an iron core inside the solenoid and the field becomes much stronger — this is an electromagnet. The huge advantage of an electromagnet is that it can be switched on and off with the current, and its strength can be changed. That's why electromagnets are used in scrapyard cranes (pick up and drop cars), electric bells and relays (a small current switches a larger circuit).
⚠ Watch out — what makes an electromagnet
A solenoid with an iron core is an electromagnet. Don't confuse it with a permanent magnet: the electromagnet only works while current flows, and that's the whole point — you can turn it on and off. To make it stronger: more current, more turns on the coil, or an iron core.
Why is an electromagnet, not a permanent magnet, used in a scrapyard crane?
- AIt is cheaper to make than a permanent magnet
- BIt can be switched off to release the load
- CIt never needs any electricity
- DIt attracts every material, not just metals
Show answer
Describe the shape of the magnetic field around a long straight current-carrying wire.
A set of concentric circles in flat rings around the wire, getting weaker (further apart) the further you are from the wire.Give two ways to make the field around a wire stronger.
Increase the current, or get closer to the wire.Describe the field of a solenoid, inside and outside.
Inside: a strong, uniform field (straight, evenly spaced lines). Outside: like a bar magnet, with a N and S pole at the ends.Give one use of an electromagnet and state why a permanent magnet could not do the job.
A scrapyard crane (or bell/relay). A permanent magnet couldn't be switched off to release the load, and you couldn't change its strength.
The motor effect
Put a current in a magnetic field and the wire feels a force. The force you can calculate, the rule that gives its direction, and how to make it bigger.
Part 1A force on a wire
A wire carrying a current has its own magnetic field. Place that wire in another magnetic field (say between the poles of a magnet) and the two fields interact. The result is a force on the wire that pushes it out of the field. This is the motor effect.
The force is biggest when the wire is at 90° (right angles) to the magnetic field. If the wire lies along the field (parallel to it), the force is zero. You can make the force larger by increasing the current or using a stronger magnetic field.
Equation
- F = B I L given
- force on the conductor (N) = magnetic flux density (T) × current (A) × length of wire in the field (m). The field is measured in tesla (T). This only applies when the wire is at right angles to the field.
⚠ Watch out — right angles matter
The motor-effect force is only a maximum when the wire is at 90° to the field. Line the wire up parallel to the field and there's no force at all. Also: in F = B I L, the length L is just the bit of wire that's actually inside the field, and B is in tesla (T) — don't mix it up with the current.
Part 2Finding the direction and size
Higher tier — Fleming's left-hand rule & calculating F = B I L
To find the direction of the force, use Fleming's left-hand rule. Hold the thumb and first two fingers of your left hand at right angles to each other: First finger = Field (N→S), seCond finger = Current, and the thuMb = Motion (the force). Point the first finger along the field and the second finger along the current, and your thumb shows which way the wire is pushed.
Higher-tier questions also expect you to rearrange and use F = B I L — for example finding the field strength B from a measured force, or the force from B, I and L. Watch your units: force in newtons, B in tesla, current in amps, length in metres.
Worked example — force on a wire (Higher)
A wire of length 0.05 m carries a current of 3 A at right angles to a magnetic field of flux density 0.2 T. Calculate the force on the wire.
A 0.1 m length of wire carries 4 A at right angles to a 0.5 T field. What is the force on it?
- A0.2 N
- B2 N
- C0.02 N
- D20 N
Show answer
What is the motor effect?
When a current-carrying wire is placed in a magnetic field, the two fields interact and the wire experiences a force.State the equation for the force on a conductor and the unit of magnetic flux density.
F = B I L. Magnetic flux density B is measured in tesla (T).How can you make the force on the wire bigger?
Increase the current, use a stronger field (bigger B), and keep the wire at 90° to the field.A 0.2 m wire carries 5 A at right angles to a 0.3 T field. Calculate the force. (Higher)
F = B I L = 0.3 × 5 × 0.2 = 0.3 N.
The electric motor
Take the motor effect, put it on a coil, and you get continuous spinning. The forces, the trick that keeps it turning, and how to speed it up.
Part 1Forces that turn a coil
An electric motor uses the motor effect to make a coil spin. The coil sits between the poles of a magnet. When a current flows, each side of the coil is a current-carrying wire in a field, so each side feels a force (from F = B I L).
Here's the clever part: current flows in opposite directions on the two sides of the coil. So one side is pushed up and the other is pushed down. These two opposite forces make the coil rotate — they create a turning effect (a moment) about the axle.
⚠ Watch out — the coil would stall
Without help, the coil would only swing half a turn and stop, because the forces would then hold it still. A motor fixes this with a split-ring commutator: it swaps the current direction in the coil every half turn, so the force on each side keeps pushing it the same way round. That's what makes the spinning continuous.
What is the job of the split-ring commutator in a d.c. motor?
- AIt increases the size of the magnetic field
- BIt reverses the current in the coil every half turn to keep it spinning the same way
- CIt stops the coil from getting too hot
- DIt changes the coil into a permanent magnet
Show answer
Part 2Speeding it up and changing direction
Because the turning force comes from F = B I L acting on the coil, you can make the motor spin faster (turn with more force) by increasing the current, using a stronger magnet, or adding more turns to the coil.
To make the motor turn the other way, you reverse the direction of the current or reverse the magnetic field (swap the poles). Reversing the force direction reverses the rotation.
Worked example — force on one side of a motor coil (Higher)
One side of a motor coil is 0.04 m long and carries a current of 2.5 A in a field of 0.30 T. Calculate the force on that side.
Which change would make a simple d.c. motor spin in the opposite direction?
- AIncrease the current
- BAdd more turns to the coil
- CReverse the direction of the current
- DUse a longer piece of wire for the coil
Show answer
Why do the two sides of a motor coil experience forces in opposite directions?
The current flows in opposite directions on the two sides, so the motor-effect force on one side is up and on the other is down — creating a turning effect.What does the split-ring commutator do?
It reverses the current in the coil every half turn so the coil keeps rotating the same way instead of stopping.Give three ways to make a motor turn faster (with a greater turning force).
Increase the current, use a stronger magnetic field, and add more turns to the coil.State two ways to reverse the direction a motor spins.
Reverse the current direction, or reverse the magnetic field (swap the magnet's poles).A motor coil side of length 0.05 m carries 3 A in a 0.4 T field. Find the force on it. (Higher)
F = B I L = 0.4 × 3 × 0.05 = 0.06 N.