Answers · Magnets and magnetic fields

Magnets and magnetic fields — final check key

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

1. BK1 · Final check

Model answer. (a) The magnets push each other apart: repulsion, because like poles repel. (b) Yes. A magnetic field is a region in which a magnetic pole experiences a force. The needle turns when the magnet does, so its poles still feel a force 30 cm away: the compass is in the field there, although the field is weak. (c) East. The direction of a field at a point is the direction of the force on the N pole of a magnet there, and the north-seeking end of the needle is an N pole.

A full answer includes

(a) I wrote that they push apart: repulsion, because like poles repel.
(b) I wrote “yes”: the needle turns, so its poles experience a force there.
(b) I wrote that a magnetic field is a region in which a magnetic pole experiences a force.
(c) I wrote “east”, because the field direction is the direction of the force on an N pole, and the north-seeking end of the needle is an N pole.

Also accept

(a) I wrote that they push apart: repulsion, because like poles repel.
(b) I wrote “yes”: the needle turns, so its poles experience a force there.
(b) I wrote that a magnetic field is a region in which a magnetic pole experiences a force.
(c) I wrote “east”, because the field direction is the direction of the force on an N pole, and the north-seeking end of the needle is an N pole.

Watch for

“(a) Two N poles attract.” — Like poles repel: two N poles push apart.

2. BK2 · Final check

Model answer. (a) Draw round the magnet. Put the compass near the N pole and mark a dot at the north-seeking end of the needle. Move the compass so that the south end of the needle is over that dot, and mark the north end again. Repeat until you reach the S pole, join the dots with a smooth curve and add an arrow; start again from other points for more lines. (b) The sketch shows a rectangle labelled N and S with closed curves round it: each line leaves the N pole, curves round and enters the S pole, with an arrow pointing from N to S; the lines never cross and are drawn closest together near the poles. (c) Filings show the shape of the whole pattern at once, but not its direction, because a filing has no marked north end.

A full answer includes

(a) I wrote: put the compass near the N pole and mark a dot at the north-seeking end of the needle.
(a) I wrote: move the compass so its south end is over the dot, mark again, repeat, then join the dots.
(b) My sketch shows lines that leave the N pole, curve round and enter the S pole, with arrows from N to S.
(b) In my sketch no two lines cross.
(b) In my sketch the lines are closest together near the poles.
(c) I wrote that filings show the shape of the pattern but cannot show its direction.

Also accept

(a) I wrote: put the compass near the N pole and mark a dot at the north-seeking end of the needle.
(a) I wrote: move the compass so its south end is over the dot, mark again, repeat, then join the dots.
(b) My sketch shows lines that leave the N pole, curve round and enter the S pole, with arrows from N to S.
(b) In my sketch no two lines cross.
(b) In my sketch the lines are closest together near the poles.
(c) I wrote that filings show the shape of the pattern but cannot show its direction.

Watch for

“(b) The arrows point from S to N.” — The field direction is the way the north-seeking end of a compass points. Outside the magnet that is from N to S.

3. BK3 · Final check

Model answer. (a) Each magnet is in the field of the other, and magnetic forces are due to this interaction of the fields. Near an N pole the field points away from it, so the N pole of each magnet is pushed away from the other: they repel. The fields are equal and opposite midway between identical magnets, so the neutral point is 10.0 cm ÷ 2 = 5.0 cm from each pole. (b) A. The relative strength of a field is shown by the spacing of the lines, and the lines are closer at A.

A full answer includes

(a) I wrote that each magnet is in the field of the other, and the force on each pole comes from the field it is in.
(a) I wrote that each N pole is pushed along the other magnet’s field, away from it, so the magnets repel.
(a) I wrote that the neutral point is midway, 5.0 cm from each pole, where the two fields cancel.
(b) I wrote “A”, because closer lines mean a stronger field.

Also accept

(a) I wrote that each magnet is in the field of the other, and the force on each pole comes from the field it is in.
(a) I wrote that each N pole is pushed along the other magnet’s field, away from it, so the magnets repel.
(a) I wrote that the neutral point is midway, 5.0 cm from each pole, where the two fields cancel.
(b) I wrote “A”, because closer lines mean a stronger field.

Watch for

“(b) B, because wider spacing means a bigger field.” — Closer lines mean a stronger field. The lines are closer together at A.

4. BK4 · Final check

Model answer. (a) A magnetic material is attracted by a magnet and can be magnetised; a non-magnetic material is not attracted and cannot be magnetised. The iron nail and the cobalt disc are magnetic. The silver spoon and the zinc sheet are metals too, but non-magnetic. (b) Bring each end of each bar near one pole, say the N pole, of the labelled magnet. Both ends of the unmagnetised iron bar show attraction. One end of the magnetised bar shows repulsion, because like poles repel; repulsion only happens between two magnets.

A full answer includes

(a) I wrote that a magnetic material is attracted by a magnet and can be magnetised, and a non-magnetic material is not attracted and cannot be magnetised.
(a) I identified the iron nail and the cobalt disc as magnetic, and the silver spoon and the zinc sheet as non-magnetic.
(b) I wrote: bring each end of each bar near one pole of the labelled magnet.
(b) I wrote that the bar with an end that shows repulsion is magnetised, and the bar that shows only attraction at both ends is unmagnetised.

Also accept

(a) I wrote that a magnetic material is attracted by a magnet and can be magnetised, and a non-magnetic material is not attracted and cannot be magnetised.
(a) I identified the iron nail and the cobalt disc as magnetic, and the silver spoon and the zinc sheet as non-magnetic.
(b) I wrote: bring each end of each bar near one pole of the labelled magnet.
(b) I wrote that the bar with an end that shows repulsion is magnetised, and the bar that shows only attraction at both ends is unmagnetised.

Watch for

“(b) The bar that is attracted is the magnet.” — Both bars are attracted. Only the magnet can also be repelled.

5. BK5 · Final check

Model answer. (a) An S pole at the top of each rod, because the end nearest a pole becomes the opposite pole. (b) Each rod becomes a magnet by induced magnetism, with an N pole at its bottom end. Each clip that touches it is magnetised in turn and holds the next one, because at every joint an N pole meets an S pole. (c) B is steel: it kept 5 of its 6 clips. Soft iron is magnetised more easily (A held more clips), and it loses most of its magnetism when the magnet is removed (a temporary magnet), while steel keeps its magnetism (a permanent magnet).

A full answer includes

(a) I wrote “an S pole”, because the near end becomes the opposite pole.
(b) I wrote that each rod is magnetised by induced magnetism, and each clip becomes a magnet too and holds the next, because unlike poles attract.
(c) I wrote that B is steel.
(c) I wrote that soft iron is magnetised more easily than steel.
(c) I wrote that soft iron loses most of its magnetism when the magnet is removed, while steel keeps it.

Also accept

(a) I wrote “an S pole”, because the near end becomes the opposite pole.
(b) I wrote that each rod is magnetised by induced magnetism, and each clip becomes a magnet too and holds the next, because unlike poles attract.
(c) I wrote that B is steel.
(c) I wrote that soft iron is magnetised more easily than steel.
(c) I wrote that soft iron loses most of its magnetism when the magnet is removed, while steel keeps it.

Watch for

“(c) A is steel because it picked up more clips.” — Picking up more clips shows that A is magnetised easily, like soft iron. A dropped them all, so it is soft iron; B kept its clips, so it is steel.

6. BK6 · Final check

Model answer. (a) For example, a permanent magnet in a cupboard catch, and an electromagnet in a scrapyard crane. (b) The catch must hold the door shut all the time without a power supply, so a permanent magnet suits it. The crane must let go of the steel over the skip, so it needs a magnet that can be switched off. (c) A simple electromagnet is a coil of insulated wire wound round a soft-iron core; the core is a strong magnet while a current flows in the coil and loses its magnetism when the current stops.

A full answer includes

(a) I gave a use of a permanent magnet, such as a door catch, a compass needle, a fridge door seal or the magnet in a loudspeaker or motor.
(a) I gave a use of an electromagnet, such as a scrapyard crane, a fire-door holder, an electric bell or a relay.
(b) I wrote that the permanent magnet must work all the time with no power supply, and the electromagnet must be switched on and off.
(c) I wrote “a coil of insulated wire round a soft-iron core”, magnetised when a current flows in the coil.

Also accept

(a) I gave a use of a permanent magnet, such as a door catch, a compass needle, a fridge door seal or the magnet in a loudspeaker or motor.
(a) I gave a use of an electromagnet, such as a scrapyard crane, a fire-door holder, an electric bell or a relay.
(b) I wrote that the permanent magnet must work all the time with no power supply, and the electromagnet must be switched on and off.
(c) I wrote “a coil of insulated wire round a soft-iron core”, magnetised when a current flows in the coil.

Watch for

“(b) An electromagnet is chosen because it is stronger.” — The reason is control: an electromagnet can be switched on and off. A permanent magnet cannot be switched off.