Electricity and magnetism, Year 11: magnets, electromagnets and motors

An induced magnet

HintAn iron nail can pick up a paperclip while it hangs from something else, and drops it soon after it is taken away.

The whyA permanent magnet has a magnetic field of its own all the time. This other kind is a piece of magnetic material, such as iron, that is magnetic only because it is sitting in another magnet's field. Take the field away and it loses most or all of its magnetism quickly.

attraction

HintA fridge magnet never pushes the fridge door away, whichever way round you hold it.

The whyAn induced magnet's poles are set by the field it is in, so they always form in the arrangement that pulls it towards the permanent magnet. Turning the permanent magnet round simply induces the opposite poles. Only two permanent magnets can repel each other.

It loses most or all of its magnetism quickly

HintThe nail was only borrowing its pull from the field it sat in.

The whyThe nail was an induced magnet: magnetic only while it was in the bar magnet's field. Once that field is removed it stops being a magnet almost at once, and the paperclip falls. A permanent magnet, by contrast, keeps its magnetism because it produces its own field.

Non-contact forces

HintThe other group includes friction and air resistance.

The whyLike poles repel and unlike poles attract, and they do so across a gap: the magnets need not touch. Forces that act at a distance like this are called non-contact forces. The poles are the places where a magnet's forces are strongest.

Place a plotting compass near the wire: its needle deflects when the current is switched on and swings back when it is switched off

HintUse the small instrument that maps the field of a bar magnet.

The whyA compass needle is a tiny magnet, so it turns when a magnetic field acts on it. Switching the current on and off shows that the field comes from the current and not from the wire itself. Reversing the current makes the needle deflect the other way.

Concentric circles around the wire, in the plane at right angles to it, getting further apart with distance from the wire

HintPicture the rings spreading out when a stone is dropped in a pond.

The whyThe field lines are closed circles centred on the wire. They are drawn closer together near the wire because the field is strongest there: its strength depends on the size of the current and on the distance from the wire. Reversing the current reverses the direction of the circles.

Anticlockwise

HintGrip the wire with your right hand, thumb pointing the way the current goes, and watch your fingers.

The whyThe right-hand grip rule gives the direction: point the thumb of your right hand along the current and your fingers curl the way the field goes. With the current coming up towards you, the fingers curl the opposite way to the hands of a clock. Seen from below, the same field would appear to go the other way.

uniform; bar magnet

HintInside, the lines are straight, parallel and evenly spaced; outside, think of the first field pattern you ever plotted.

The whyInside the coil the field lines run straight along its length, parallel and evenly spaced: the same strength and direction everywhere. Outside, the lines loop round from one end to the other, so one end of the solenoid behaves like a north pole and the other like a south pole.

The fields from all the turns line up and add together, concentrating the field inside the coil

HintEach loop makes its own small contribution, and within the tube they all point one way.

The whyEvery turn of the coil produces its own field. Inside the solenoid these all point in the same direction, so they reinforce one another. Adding an iron core makes the field stronger still, and a solenoid with an iron core is called an electromagnet.

The motor effect

HintIt is named after the machine that puts it to use.

The whyA current has its own magnetic field, and this interacts with the field of the magnet, so each pushes on the other. The equation for the size of the force applies when the wire is at right angles to the field. Electric motors use this force to produce rotation.

force; magnetic field; current

HintTwo of the three share an initial with their finger: First and seCond. The thumb gives what is left.

The whyHold the thumb, first finger and second finger of the left hand at right angles to one another. First finger = Field (north to south), seCond finger = Current (positive to negative), thuMb = Motion, the direction of the push on the wire. The rule shows that the three directions are all at right angles.

down

HintLeft hand: first finger to the right, second finger pointing away from you.

The whyFleming's left-hand rule: point the first finger along the field (left to right) and the second finger along the current (away from you). The thumb then points towards the floor, which is the direction of the force. Reversing either the current or the field would reverse the force.

0.09 N

HintThree quantities multiply; one of them needs its unit changing first.

The whyForce = magnetic flux density × current × length, or F = B × I × l, with F in newtons (N), B in tesla (T), I in amperes (A) and l in metres (m). Here l = 0.15 m, so F = 0.3 × 2 × 0.15 = 0.09 N. AQA gives this equation on the Physics equation sheet; it applies when the wire is at right angles to the field.

current; field; length

HintLook at what is multiplied together in the equation for this force.

The whyF = B × I × l, so the force is directly proportional to each of the three: flux density, current and the length of conductor in the field. Doubling any one of them doubles the force. AQA expects these factors to be recalled even though the equation itself is given.

The current flows in opposite directions along the two sides of the coil, so the forces on them are opposite: one side is pushed up and the other down

HintFollow the charge round the loop: which way is it heading on the far edge compared with the near edge?

The whyGoing round the coil, the current crosses the field one way on one side and the opposite way on the other. Fleming's left-hand rule then gives forces in opposite directions on the two sides. A pair of opposite forces on either side of an axle produces rotation, which is the basis of the electric motor.

current; field

HintFleming's left-hand rule has two inputs; turn either finger round and the thumb flips.

The whyThe direction of the force on each side of the coil depends on the direction of the current and of the field. Reversing one of them reverses every force, so the coil turns the other way. Reversing both at once changes nothing: the two reversals cancel.

current; magnet; turns

HintEach long edge feels F = B × I × l, and extra loops add extra edges.

The whyThe force on each side of the coil is F = B × I × l, so a bigger current or a greater flux density gives a bigger force. Extra turns mean extra lengths of wire, each feeling that force. A bigger turning force makes the motor spin faster or drive a heavier load.

Anticlockwise: the left-hand side is pushed down and the right-hand side up

HintUse Fleming's rule on each long edge in turn, then picture which edge rises.

The whyBy Fleming's left-hand rule the left-hand side, with its current flowing away from you, is pushed towards the floor. The current in the right-hand side flows the opposite way, so the force on it is reversed. One side going one way and the other side the opposite way is what makes the coil rotate.

An electric motor

HintIt is inside a fan, a drill and a washing machine.

The whyA current-carrying coil placed between the poles of a magnet tries to turn, because the forces on its two sides act in opposite directions. This machine uses that turning to do useful work. On AQA Combined Science this topic is Higher tier only.