Magnet Mastery

It is all about the field

A magnet can pull a paperclip across a gap without touching it. The thing that reaches across the gap is the magnetic FIELD, the invisible region around a magnet where it can affect magnetic materials. Almost everything in this topic is about what makes a field and what shape it has. ⭐⭐ Every magnet has two poles, a north and a south, and the field is strongest there. Like poles repel and unlike poles attract, so two norths push apart while a north and a south pull together. The field lines run from the north pole round to the south, and where they are closest together the field is strongest. ⭐ There are two kinds of magnet. A PERMANENT magnet is always magnetic, whatever is around it. An INDUCED magnet is a material, such as iron, that becomes magnetic only while it sits in a field, and loses its magnetism again once the field is taken away. That is why a magnet can pick up a chain of paperclips: each one is induced in turn. ⭐⭐ A current makes a field too. Send a current through a wire and a magnetic field circles it; wind the wire into a coil and you get a strong field like a bar magnet. That is an electromagnet, and its great advantage is control: switch the current off and the magnetism vanishes, and turn the current up to make it stronger. This module is about magnets and their fields. The force on a current-carrying wire in a field, the motor effect, belongs to a different module. ⚠️ Carry one question the whole way through: what is making the field, and can I switch it off?

Three ways to be magnetic

The first column is a permanent magnet, the second an induced one, and the third an electromagnet.

Tap the two correct pole facts

Tap the TWO statements about magnetic poles that are correct.

• A north pole and a south pole attract each other.
• Two north poles pushed together repel each other.
• Two north poles pull together strongly.
• The field is weakest at the poles.

The paperclip that lets go

A steel paperclip sticks to a magnet, but falls off the moment the magnet is taken away. What does this show?

• The paperclip was an induced magnet: magnetic only while in the field
• The paperclip is a permanent magnet
• The magnet has no field
• The paperclip is an electromagnet

The language of magnets

Six terms this topic turns on. The first names the region, the rest name the kinds and the parts.

Match each to what it means

Two ways to strengthen it

Select the TWO changes that would make an electromagnet STRONGER.

• Increasing the current through the coil
• Winding more turns of wire onto the coil
• Switching the current off
• Painting the coil a darker colour

Answering a magnetism question

Magnetism questions reward precise language about fields and clear thinking about the three kinds of magnet. ⭐⭐ TELL PERMANENT FROM INDUCED. A permanent magnet is always magnetic and has fixed poles. An induced magnet is magnetic only while in a field, and is always attracted, never repelled. If something loses its magnetism when the magnet is removed, it was induced. ⭐ DESCRIBE THE FIELD PROPERLY. Field lines run from a north pole to a south pole, and the closer together they are, the stronger the field, which is why the field is strongest at the poles. Never say the lines cross; they do not. ⭐⭐ PRAISE THE ELECTROMAGNET FOR ITS CONTROL. Its advantage over a permanent magnet is that you can switch it on and off with the current, and change its strength by changing the current or the number of turns. That is why cranes and bells use one. KNOW A USE OR TWO. A scrapyard crane lifts and drops steel by switching an electromagnet on and off; an electric bell uses one to move a hammer. Being able to link the switchable field to the use earns marks. ⚠️ A wording habit. Say magnetic field, not magnetic force field, and remember an induced magnet is only ever attracted, so it cannot be pushed away. Keep the one question to hand: what is making the field, and can I switch it off?

Order how to plot a field

Put the steps of plotting the magnetic field around a bar magnet with a compass into a sensible order.

• Place the bar magnet on paper and draw round it
• Put a plotting compass near one pole
• Mark a dot at each end of the compass needle
• Move the compass along and repeat to trace a line
• Join the dots and add an arrow from north to south

Build the electromagnet advantage

Assemble one sentence about why a scrapyard crane uses an electromagnet rather than a permanent magnet.

The magnet run

Five quick decisions about magnets. Three lives.

Three kinds of heater

Leave physics aside for a screen. Think about how a room can be kept warm, and about the warmth spreading out around each heater. ⭐ Each heater throws out a region of warmth around it, an invisible reach you feel more strongly the closer you stand. That reach is the same idea whichever heater makes it. ⭐⭐ But the heaters differ in HOW they are on. An old storage heater is simply always on, giving out warmth all day whatever you do. A different heater has a sensor and only warms up when the room grows cold, then stops again once it does not need to; its warmth comes and goes with the conditions around it. And a third kind sits on a switch. You turn it on and the warmth builds; you turn it off and it fades away; turn the dial higher and the warmth reaches further. You are in complete control of when it works and how strongly. ⚠️ Hold that picture. The reach of warmth is the region a magnet influences, the always-on heater is a magnet that is magnetic all the time, the sensor heater that only warms when triggered is a material magnetic only while something acts on it, and the heater on a switch is the coil you can turn on, off and up at will.

Finish each magnetism line

A magnet that is always magnetic, whatever is around it, is a _____ magnet. A material that is magnetic only while it sits in a magnetic field is an _____ magnet. The region around a magnet where it affects magnetic materials is its magnetic _____. A coil of wire that becomes magnetic when a current flows, and can be switched off, is an _____. The two ends of a magnet, where the field is strongest, are its _____.

Three magnet decisions

Three situations arise. For each, choose the response that the magnetism rules support.

• A crane in a scrapyard needs to pick up steel and then drop it precisely into a truck.
• A student pushes the north pole of one magnet towards the north pole of another and is surprised they push apart.
• An iron nail becomes able to pick up other nails while touching a magnet, but drops them all when pulled away.

Explain permanent, induced and electromagnets

Explain the difference between a permanent magnet, an induced magnet and an electromagnet, using the idea of the magnetic field. Explain why an electromagnet is chosen for a scrapyard crane.

• Explain what a magnetic field is and where a magnet is strongest
• Explain what a permanent magnet is
• Explain what an induced magnet is and what happens when the field is removed
• Explain how an electromagnet is made and how it is switched and strengthened
• Explain why the crane uses an electromagnet rather than a permanent magnet