Answers · Magnets and magnetic fields

Magnets and magnetic fields — return questions key

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

1. BR1 · Return

Model answer. With S facing S the magnets push each other apart, and she feels the push grow as they come closer: repulsion. With N facing S they pull towards each other: attraction. The rule is that like poles repel and unlike poles attract.

A full answer includes

I wrote that with S facing S the magnets push apart: repulsion.
I wrote that with N facing S they pull together: attraction.
I wrote “like poles repel, unlike poles attract”.

Also accept

I wrote that with S facing S the magnets push apart: repulsion.
I wrote that with N facing S they pull together: attraction.
I wrote “like poles repel, unlike poles attract”.

Watch for

“Two S poles attract because they are the same kind.” — Poles of the same kind repel. Only an N pole and an S pole attract.

2. BR2 · Return

Model answer. Place the plotting compass at the point, away from other magnets and pieces of iron, and wait for the needle to settle. The direction of the field at a point is the direction of the force on an N pole there. The north-seeking end of the needle is an N pole, so it is pushed along the field: the field points the way the north-seeking end points.

A full answer includes

I wrote: place the compass at the point, away from other magnets and iron, and let the needle settle.
I wrote that the field at the point is in the direction the north-seeking end points.
I wrote that the field direction is the direction of the force on an N pole, and the north-seeking end is an N pole.

Also accept

I wrote: place the compass at the point, away from other magnets and iron, and let the needle settle.
I wrote that the field at the point is in the direction the north-seeking end points.
I wrote that the field direction is the direction of the force on an N pole, and the north-seeking end is an N pole.

Watch for

“The field points the way the south end of the needle points.” — The field direction is defined by the force on an N pole, so read it from the north-seeking end.

3. BR3 · Return

Model answer. (a) A magnetic field is a region in which a magnetic pole experiences a force. (b) Outside a bar magnet the lines run from the N pole round to the S pole. (c) At any point the field has only one direction. If two lines crossed, the field would have two directions there, and a compass at that point would have to point two ways at once.

A full answer includes

(a) I wrote “a region in which a magnetic pole experiences a force”.
(b) I wrote “from the N pole to the S pole”.
(c) I wrote that at any point the field has only one direction, so a compass there can point only one way.

Also accept

(a) I wrote “a region in which a magnetic pole experiences a force”.
(b) I wrote “from the N pole to the S pole”.
(c) I wrote that at any point the field has only one direction, so a compass there can point only one way.

Watch for

“(b) From the S pole to the N pole.” — The field direction is the direction of the force on an N pole. An N pole is pushed away from N and towards S, so outside the magnet the lines run from N to S.

4. BR4 · Return

Model answer. (a) When two magnets attract or repel, each one is in the field of the other. The force on each pole comes from the field it is in: a field is a region where a pole feels a force. Nothing has to touch the magnets, and nothing travels along the lines like a string. (b) The relative strength is shown by the spacing of the lines: where they are closest together the field is strongest, and where they spread out it is weaker.

A full answer includes

(a) I wrote that each magnet is in the magnetic field of the other.
(a) I wrote that the force on each magnet’s poles comes from the field they are in, not from anything touching them.
(b) I wrote that the field is strongest where the lines are closest together and weaker where they are further apart.

Also accept

(a) I wrote that each magnet is in the magnetic field of the other.
(a) I wrote that the force on each magnet’s poles comes from the field they are in, not from anything touching them.
(b) I wrote that the field is strongest where the lines are closest together and weaker where they are further apart.

Watch for

“(a) The force travels along a field line from one magnet to the other.” — The lines are a picture of the field. Each pole is pushed by the field where it is.

5. BR5 · Return

Model answer. Magnetic: any two of iron, steel (mostly iron), nickel and cobalt. Non-magnetic: any two other metals, such as copper, aluminium, brass, gold, silver, zinc or lead. Most metals are non-magnetic.

A full answer includes

I named two of iron, steel, nickel and cobalt as magnetic.
I named two metals that are not iron, steel, nickel or cobalt, such as copper, aluminium, brass, gold, silver, zinc or lead, as non-magnetic.

Also accept

I named two of iron, steel, nickel and cobalt as magnetic.
I named two metals that are not iron, steel, nickel or cobalt, such as copper, aluminium, brass, gold, silver, zinc or lead, as non-magnetic.

Watch for

“All metals are magnetic.” — Only iron, steel, nickel and cobalt are magnetic. Copper, aluminium and most other metals are not.

6. BR6 · Return

Model answer. Induced magnetism is magnetism produced in a magnetic material by the field of another magnet, without stroking. Near the N pole, the end of the clip nearest the magnet becomes an S pole and its far end becomes an N pole. The magnet’s N pole and the clip’s S pole are unlike poles, so the clip is attracted.

A full answer includes

I wrote that a magnetic material becomes a magnet when it is placed in the field of another magnet.
I wrote that the end of the clip nearest the N pole becomes an S pole and the far end an N pole.
I wrote that the clip is then attracted, because unlike poles attract.

Also accept

I wrote that a magnetic material becomes a magnet when it is placed in the field of another magnet.
I wrote that the end of the clip nearest the N pole becomes an S pole and the far end an N pole.
I wrote that the clip is then attracted, because unlike poles attract.

Watch for

“The near end of the clip becomes an N pole, matching the magnet.” — The near end always becomes the opposite pole: an S pole next to an N pole. That is why it is attracted.

7. BR7 · Return

Model answer. Row B. A compass needle must keep its magnetism for years, so it is made of steel, a permanent magnet. The core of a crane’s electromagnet must lose its magnetism when the current is switched off, so that the crane can drop its load, so it is made of soft iron.

A full answer includes

I chose row B.
I wrote that a compass needle must stay magnetised, so it is made of steel.
I wrote that an electromagnet’s core must lose its magnetism when the current stops, so it is made of soft iron.

Also accept

I chose row B.
I wrote that a compass needle must stay magnetised, so it is made of steel.
I wrote that an electromagnet’s core must lose its magnetism when the current stops, so it is made of soft iron.

Watch for

“Soft iron for the needle, because it is magnetised easily.” — A soft-iron needle would lose its magnetism and stop lining up with the field. A needle must keep its magnetism, so it is steel.
“Copper for the needle.” — Copper is non-magnetic. It cannot be magnetised at all.
“Steel for both.” — A steel core would stay magnetised when the current stops, so the crane could not drop the scrap.