Answers · Magnets and magnetic fields

Magnetic materials — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. BQ14 · Practice

Model answer. A magnet attracts only magnetic materials: iron, steel, nickel and cobalt. So it attracts the steel paper clip, the iron washer and the strip of nickel. Copper, aluminium and brass are metals but non-magnetic, and plastic is non-magnetic too.

A full answer includes

I chose the steel paper clip, the iron washer and the strip of nickel.
I left out the copper pipe, the aluminium can, the brass key and the plastic comb.

Also accept

I chose the steel paper clip, the iron washer and the strip of nickel.
I left out the copper pipe, the aluminium can, the brass key and the plastic comb.

Watch for

“All metal objects are attracted, so the pipe, the can and the key too.” — Copper, aluminium and brass are metals but non-magnetic. A magnet does not attract them.
“Nickel is not attracted because it is not iron.” — Nickel and cobalt are magnetic materials too, as well as iron and steel.

2. BQ15 · Practice

Model answer. (a) A magnetic material is attracted by a magnet and can be made into a magnet. A non-magnetic material is not attracted and cannot be magnetised. (b) A magnet does not attract copper, so copper is a non-magnetic material, even though it conducts electricity very well. Conducting electricity and being magnetic are different properties: aluminium and gold conduct well and are non-magnetic too. Iron, steel, nickel and cobalt are magnetic materials.

A full answer includes

(a) I wrote that a magnetic material is attracted by a magnet and can be magnetised.
(a) I wrote that a non-magnetic material is not attracted by a magnet and cannot be magnetised.
(b) I wrote that a magnet does not attract copper, so copper is non-magnetic: conducting electricity is a different property.
(b) I identified iron, steel, nickel or cobalt.

Also accept

(a) I wrote that a magnetic material is attracted by a magnet and can be magnetised.
(a) I wrote that a non-magnetic material is not attracted by a magnet and cannot be magnetised.
(b) I wrote that a magnet does not attract copper, so copper is non-magnetic: conducting electricity is a different property.
(b) I identified iron, steel, nickel or cobalt.

Watch for

“Good conductors are magnetic.” — Copper, aluminium and gold are good conductors, and a magnet attracts none of them. Being magnetic is a different property.
“Magnetic means metal.” — Most metals are non-magnetic. Only iron, steel, nickel and cobalt are magnetic materials.

3. BQ16 · Practice

Model answer. Row B. Repulsion happens only between two magnets, when like poles meet, so J is a magnet: its repelled end is an S pole. L is attracted at both ends, which is what happens to an unmagnetised piece of iron or steel. K is not attracted at all, so it is non-magnetic.

A full answer includes

I chose row B.
I can explain that only a magnet can be repelled, so J is a magnet.
I can explain that L, attracted at both ends, is a magnetic material but unmagnetised, and K, not attracted, is non-magnetic.

Also accept

I chose row B.
I can explain that only a magnet can be repelled, so J is a magnet.
I can explain that L, attracted at both ends, is a magnetic material but unmagnetised, and K, not attracted, is non-magnetic.

Watch for

“L is a magnet because both its ends are attracted.” — A magnet attracts unmagnetised iron or steel at both ends too. Only repulsion proves a magnet.
“K is unmagnetised steel.” — Unmagnetised steel would still be attracted. No effect at all means a non-magnetic material.

4. BQ17 · Practice

Model answer. (a) If the bar were a magnet, the end she tested would be an N pole or an S pole, and one of the magnet’s poles would repel it, because like poles repel. It was attracted both times, so it is not a pole: the bar is unmagnetised steel, a magnetic material that a magnet attracts with either pole. (b) Bring each end of the bar in turn near one pole of a magnet. If one end is repelled, the bar is a magnet; if both ends are only attracted, it is unmagnetised.

A full answer includes

(a) I wrote that if the bar were a magnet, its end would be a pole and would be repelled by one of the magnet’s poles.
(a) I wrote that attraction by both poles shows that the bar is unmagnetised: a magnetic material, but not a magnet.
(b) I wrote: bring each end of the bar near one pole of a magnet; if one end is repelled, the bar is a magnet.

Also accept

(a) I wrote that if the bar were a magnet, its end would be a pole and would be repelled by one of the magnet’s poles.
(a) I wrote that attraction by both poles shows that the bar is unmagnetised: a magnetic material, but not a magnet.
(b) I wrote: bring each end of the bar near one pole of a magnet; if one end is repelled, the bar is a magnet.

Watch for

“Attraction proves that the bar is a magnet.” — A magnet attracts unmagnetised iron or steel too. Only repulsion proves that something is a magnet.

5. BQ18 · Practice

Options as numbered on the pupil sheet: 1 — S pole; 2 — N pole.

Model answer. The end nearest a pole always becomes the opposite pole. The head of the nail is next to the magnet’s S pole, so it becomes an N pole, and the point becomes an S pole. The top of the clip touches that S pole, so it becomes an N pole, and the bottom of the clip an S pole. Each joint in the chain is an N pole meeting an S pole, so each piece is attracted to the next.

A full answer includes

I matched the head of the nail to N, the opposite of the magnet’s S pole.
I matched the point of the nail to S.
I matched the top of the clip to N and the bottom of the clip to S.

Also accept

I matched the head of the nail to N, the opposite of the magnet’s S pole.
I matched the point of the nail to S.
I matched the top of the clip to N and the bottom of the clip to S.

Watch for

“The head of the nail becomes an S pole, the same as the magnet.” — The near end always becomes the opposite pole. That is why the nail is attracted.

6. BQ19 · Practice

Model answer. The clip is a magnetic material, so the magnet’s field makes it a magnet: this is induced magnetism. With the N pole near, the end of the clip nearest the magnet becomes an S pole, and unlike poles attract, so the clip slides towards the magnet. With the S pole near, the induced poles are reversed: the same end now becomes an N pole, which is again the opposite pole, so the clip is attracted again.

A full answer includes

I wrote that the clip becomes a magnet while it is in the magnet’s field (induced magnetism).
I wrote that with the N pole near, the near end of the clip becomes an S pole, and unlike poles attract.
I wrote that with the S pole near, the induced poles reverse: the near end becomes an N pole and is again attracted.

Also accept

I wrote that the clip becomes a magnet while it is in the magnet’s field (induced magnetism).
I wrote that with the N pole near, the near end of the clip becomes an S pole, and unlike poles attract.
I wrote that with the S pole near, the induced poles reverse: the near end becomes an N pole and is again attracted.

Watch for

“The clip is a magnet with fixed poles.” — A magnet with fixed poles would be repelled by one of the two poles. The clip is attracted both times because its poles are induced, opposite to whichever pole comes near.

7. BQ20 · Practice

Model answer. Row D. Soft iron is magnetised easily, but it loses most of its magnetism as soon as the magnet is removed, so it makes temporary magnets. Steel is harder to magnetise, but it keeps its magnetism, so it makes permanent magnets. Electromagnet cores are soft iron, not steel, so that they lose their magnetism when the current stops.

A full answer includes

I chose row D.
I wrote that soft iron is magnetised easily but loses most of its magnetism when the magnet is removed.
I wrote that steel keeps its magnetism, so it is used to make permanent magnets.

Also accept

I chose row D.
I wrote that soft iron is magnetised easily but loses most of its magnetism when the magnet is removed.
I wrote that steel keeps its magnetism, so it is used to make permanent magnets.

Watch for

“Soft iron keeps its magnetism because it is magnetised so easily.” — Easily magnetised does not mean it stays magnetised: soft iron loses most of its magnetism when the magnet is removed.
“Steel is used for electromagnet cores.” — A steel core would stay magnetised when the current is switched off, so the electromagnet could not let go.

8. BQ21 · Practice

Model answer. (a) X keeps 9 ÷ 12 × 100 = 75 % of its clips. Y keeps 1 ÷ 15 × 100 = 6.666… % = 6.7 % to 2 significant figures. (b) Y is soft iron. It held more clips while touching, so it is easily magnetised, and it kept almost none when the magnet was removed, so it is a temporary magnet. X kept most of its clips: it is steel, now a permanent magnet.

Checked automatically

(a) Rod X keeps: 75 % (2 significant figures)
- Common wrong answer 9 : That is the number of clips kept. Divide by the 12 it held and multiply by 100.
(a) Rod Y keeps: 6.7 % (2 significant figures)
- Common wrong answer 1 : That is the number of clips kept. Divide by the 15 it held and multiply by 100.
- Common wrong answer 7 : Give 2 significant figures: 6.7 %.

A full answer includes

(a) I wrote X: 9 ÷ 12 × 100 = 75 %.
(a) I wrote Y: 1 ÷ 15 × 100 = 6.7 %, to 2 significant figures.
(b) I wrote that Y is soft iron: it picked up more clips (easily magnetised) and kept almost none (a temporary magnet).

Also accept

(a) I wrote X: 9 ÷ 12 × 100 = 75 %.
(a) I wrote Y: 1 ÷ 15 × 100 = 6.7 %, to 2 significant figures.
(b) I wrote that Y is soft iron: it picked up more clips (easily magnetised) and kept almost none (a temporary magnet).

Watch for

“X is soft iron because it kept its clips.” — Keeping its magnetism is how steel behaves. Soft iron loses most of its magnetism when the magnet is removed, as Y did.

9. BQ22 · Practice

Model answer. A lifting magnet must let go of each plate when it is in place, so it needs a magnet that can be switched off: an electromagnet. Switch off the current and its soft-iron core loses its magnetism, so the plate drops. A note on a fridge, a compass needle and a phone held in a car holder need a magnet that works all the time with no power supply: a permanent magnet.

A full answer includes

I chose “A lifting magnet that must drop each steel plate when it is in place”.
I wrote that an electromagnet can be switched off, so it can let go; the other jobs need a magnet that works all the time.

Also accept

I chose “A lifting magnet that must drop each steel plate when it is in place”.
I wrote that an electromagnet can be switched off, so it can let go; the other jobs need a magnet that works all the time.

Watch for

“A compass needle, because it must be strong.” — A compass needle is a small permanent magnet. It must keep its magnetism all the time, with no power supply.
“The fridge magnet, because it holds on hard.” — It must hold the note all the time with no power supply: a permanent magnet does that.
“The phone holder, because it is in a car with a battery.” — The holder must grip the phone all the time. A permanent magnet does that with no current at all.

10. BQ23 · Practice

Options as numbered on the pupil sheet: 1 — Electromagnet; 2 — Permanent magnet.

Model answer. A permanent magnet works all the time with no power supply, so it suits the knife rack, the magnet holding a poster on a noticeboard and the compass needle. An electromagnet can be switched on and off, so it suits the lifter, which must put each plate down on the lorry, and the electric bell, whose magnet must pull the hammer and let it go many times a second.

A full answer includes

I matched the knife rack, the noticeboard magnet and the compass needle to permanent magnets: they must work all the time with no power supply.
I matched the plate lifter and the bell to electromagnets: they must be switched on and off.

Also accept

I matched the knife rack, the noticeboard magnet and the compass needle to permanent magnets: they must work all the time with no power supply.
I matched the plate lifter and the bell to electromagnets: they must be switched on and off.

Watch for

“The bell needs a permanent magnet, because it rings again and again.” — To pull the hammer and then let it go, the magnet must switch on and off many times a second. Only an electromagnet can do that.
“The knife rack needs an electromagnet to be strong enough.” — A permanent magnet holds the knives all the time with no power supply. Strength is not the reason to choose an electromagnet; switching it off is.

11. BT5 · Apply

Model answer. (a) Copper is non-magnetic, so a coin made only of copper would not be picked up. The coin is probably mostly steel, a magnetic material, with only a thin coating of copper on the outside; many coins are made like this. (b) She is not right. A magnet attracts unmagnetised steel too, so the attraction shows only that the coin contains a magnetic material. To show that it is a magnet, she would need to find that one side of it is repelled by a pole of the magnet.

A full answer includes

(a) I wrote that copper is non-magnetic, so a coin made only of copper would not be attracted.
(a) I suggested that the coin is mostly a magnetic material, such as steel, with a thin coating of copper.
(b) I wrote that she is not right: attraction shows only that it contains a magnetic material; to show it is a magnet, part of it would have to be repelled.

Also accept

(a) I wrote that copper is non-magnetic, so a coin made only of copper would not be attracted.
(a) I suggested that the coin is mostly a magnetic material, such as steel, with a thin coating of copper.
(b) I wrote that she is not right: attraction shows only that it contains a magnetic material; to show it is a magnet, part of it would have to be repelled.

Watch for

“Copper is magnetic because it is a metal.” — Copper is a metal but non-magnetic. The attraction must come from a magnetic material inside the coin.

12. BT6 · Apply

Model answer. Touch one end of the first rod to the N pole of the magnet, dip its other end into the clips, lift it out and count the clips it holds; the rod holds them by induced magnetism. Then take the magnet away and count the clips that stay. Repeat with the second rod, using the same magnet and pole, the same clips and the same depth of dipping, so that only the material changes. The rod that holds more clips but drops almost all of them when the magnet is removed is soft iron, a temporary magnet. The rod that keeps most of its clips is steel, a permanent magnet.

A full answer includes

I described touching one end of a rod to the magnet, dipping its other end into the clips and counting the clips it holds.
I described taking the magnet away and counting the clips that stay on the rod.
I described repeating this with the other rod using the same magnet, the same pole, the same clips and the same way of touching.
I wrote that soft iron picks up more clips but drops almost all of them, while steel keeps most of its clips.

Also accept

I described touching one end of a rod to the magnet, dipping its other end into the clips and counting the clips it holds.
I described taking the magnet away and counting the clips that stay on the rod.
I described repeating this with the other rod using the same magnet, the same pole, the same clips and the same way of touching.
I wrote that soft iron picks up more clips but drops almost all of them, while steel keeps most of its clips.

Watch for

“The rod that picks up more clips is steel, because steel makes stronger magnets.” — Soft iron is magnetised more easily, so it usually picks up more clips while touching. The test that separates them is what happens when the magnet is removed.

13. BT7 · Apply

Model answer. (a) The current in the coil magnetises the core. When the current stops, the core is no longer a magnet, so it no longer attracts the steel plate and the door swings shut. (b) Soft iron loses most of its magnetism as soon as the current stops. Steel keeps its magnetism, so a steel core would stay magnetised and keep holding the door open in a fire. (c) The alarm system can release all the doors at once without anyone touching them; and if the power fails, the doors close, which is the safe way for a fire door to fail.

A full answer includes

(a) I wrote that with no current the core is no longer magnetised, so it stops attracting the steel plate.
(b) I wrote that soft iron loses its magnetism when the current stops, but a steel core would stay magnetised and keep holding the door.
(c) I suggested an advantage such as: the alarm can release every door at once, or the door also closes if the power fails.

Also accept

(a) I wrote that with no current the core is no longer magnetised, so it stops attracting the steel plate.
(b) I wrote that soft iron loses its magnetism when the current stops, but a steel core would stay magnetised and keep holding the door.
(c) I suggested an advantage such as: the alarm can release every door at once, or the door also closes if the power fails.

Watch for

“The door is released because the steel plate stops being magnetic.” — The steel plate is still a magnetic material. It is the core that stops being a magnet when the current stops.