Answers · Magnets and magnetic fields

Poles and magnetic fields — answer key

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

1. BQ1 · Practice

Model answer. Row C. Two S poles are like poles, and like poles repel. Row A is wrong because two N poles also repel. Row B is wrong because an N pole and an S pole are unlike poles, which attract. Row D is wrong because there is a force across the gap before the magnets touch.

A full answer includes

I chose row C.
I used the rule: like poles repel, unlike poles attract.

Also accept

I chose row C.
I used the rule: like poles repel, unlike poles attract.

Watch for

“Two N poles attract because they are the same kind.” — The same kind of pole means repulsion: like poles push apart.
“An N pole and an S pole repel.” — N and S are unlike poles, and unlike poles attract.
“There is no force until the magnets touch.” — Magnetic forces act across a gap, and they grow stronger as the poles come closer.

2. BQ2 · Practice

Model answer. The middle magnet floats, so the bottom magnet pushes it up: the force is repulsion. Only like poles repel, so the face of the middle magnet that faces the N pole below is also an N pole. Its top face is then an S pole, and the bottom face of the top magnet must be an S pole to push the top magnet up too.

A full answer includes

I chose “N pole”.
I can explain my choice: the middle magnet is pushed up, so the facing poles repel, and only like poles repel.

Also accept

I chose “N pole”.
I can explain my choice: the middle magnet is pushed up, so the facing poles repel, and only like poles repel.

Watch for

“S pole, because the bottom magnet holds the middle one up.” — A pull from the bottom magnet would drag the middle magnet down onto it. Floating needs a push, and only like poles push.
“It could be either pole.” — If it were an S pole, the two magnets would attract and snap together. Floating needs repulsion, so it must be N.
“The face is not a pole.” — The flat faces of a ring magnet are its poles. That is why the rings push on each other.

3. BQ3 · Practice

Model answer. A magnetic field is a region in which a magnetic pole experiences a force. Field lines are only a drawing of the field, and the field is there between the lines too. Iron filings show the field only where it is strong enough to turn them. The field fills the space round the magnet, not only the inside.

A full answer includes

I chose “A region in which a magnetic pole experiences a force”.

Also accept

I chose “A region in which a magnetic pole experiences a force”.

Watch for

“The field is the lines drawn round the magnet.” — The lines are a picture of the field. The field fills the whole region, including the spaces between the lines.
“The field is where iron filings can be seen.” — Far from the magnet the field is too weak to turn filings, but a compass there still feels a force.
“The field is only inside the magnet.” — A compass beside the magnet is pushed round, so the field reaches out into the space around it.

4. BQ4 · Practice

Model answer. The direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point. The north-seeking end of the needle is an N pole, so the field at X points the way it points: towards the top of the page.

A full answer includes

I chose “Towards the top of the page”.
I can explain my choice: the north-seeking end of the needle is an N pole, and the field direction is the direction of the force on an N pole.

Also accept

I chose “Towards the top of the page”.
I can explain my choice: the north-seeking end of the needle is an N pole, and the field direction is the direction of the force on 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 it is the way the north-seeking end points.
“The field points to the nearer pole.” — The needle lines up with the field line through X, which need not point at either pole.
“A field has no direction.” — A field has a direction at every point: the way the force on an N pole acts there.

5. BQ5 · Practice

Model answer. Drawing C. Outside a bar magnet the lines leave the N pole, curve round and enter the S pole, so the arrows point from N to S, and the lines never cross. In A the arrows point from S to N. In B two lines cross, which would mean two directions at one point. In D the lines circle round the middle of the magnet instead of joining its poles.

A full answer includes

I chose drawing C.
I can explain my choice: the lines go from the N pole round to the S pole, with arrows from N to S, and do not cross.

Also accept

I chose drawing C.
I can explain my choice: the lines go from the N pole round to the S pole, with arrows from N to S, and do not cross.

Watch for

“Drawing A, because the arrows show the field going into the N pole.” — Field lines come out of the N pole and go into the S pole. The arrows in A point the wrong way.
“Drawing B is correct because its lines still join N and S.” — In B two lines cross. At the crossing the field would have two directions, and a compass can point only one way.
“Drawing D, because field lines go round a magnet.” — The lines must start at the N pole and end at the S pole. Closed loops round the middle belong to a different arrangement, not to a bar magnet.

6. BQ6 · Practice

Options as numbered on the pupil sheet: 1 — To the right; 2 — Up the page; 3 — Down the page; 4 — To the left.

Model answer. The north-seeking end points along the field, and outside the magnet the field runs from N to S. P is just beyond the N end, so the needle points away from N: to the left. Q and T are beside the middle of the magnet, where the lines run parallel to it from N to S: to the right. R is beyond the S end, where the line runs into the S pole: to the left, towards the magnet.

A full answer includes

I matched P to “To the left”, away from the N pole.
I matched Q to “To the right”, along the magnet towards S.
I matched R to “To the left”, into the S pole.
I matched T to “To the right”.

Also accept

I matched P to “To the left”, away from the N pole.
I matched Q to “To the right”, along the magnet towards S.
I matched R to “To the left”, into the S pole.
I matched T to “To the right”.

Watch for

“At R the needle points to the right, away from the magnet.” — R is beyond the S end. The field there runs into the S pole, so the north-seeking end points left, towards the magnet.
“At Q the needle points down, at the magnet.” — At Q the line runs parallel to the magnet from N to S, so the needle lies along it, pointing right.

7. BQ7 · Practice

Options as numbered on the pupil sheet: 1 — Move the compass so that the south end of the needle is over that dot, and mark a new dot at the north end.; 2 — Join the dots with a smooth curve and draw an arrow on it pointing from N to S.; 3 — Repeat until the line reaches the S pole or the edge of the paper.; 4 — Put the compass near the N pole and mark a dot at the north-seeking end of the needle.; 5 — Draw round the magnet on a sheet of paper and mark its N and S ends..

Model answer. Draw round the magnet first, so it can be put back in the same place. Mark a dot at the north-seeking end of the needle near the N pole, then move the compass on so that the south end of the needle is over that dot and mark again. Repeat until the line reaches the S pole or the edge of the paper. Only then join the dots and add an arrow from N to S.

A full answer includes

I put drawing round the magnet first.
I put marking the first dot near the N pole before moving the compass on.
I put joining the dots and adding the arrow last.

Also accept

I put drawing round the magnet first.
I put marking the first dot near the N pole before moving the compass on.
I put joining the dots and adding the arrow last.

Watch for

“Join the dots before all of them are marked.” — Each dot is found from the compass position at the dot before it, so the curve can only be drawn once the chain of dots is complete.

8. BQ8 · Practice

Model answer. The north-seeking end of the needle points along the field, from N towards S. By putting that end on the last dot and marking at the south end, she steps backwards along the line each time, so her dots run back into the N pole. Putting the south end over the last dot and marking at the north end moves her forwards along the line, round to the S pole.

A full answer includes

I chose “Put the south end of the needle over the last dot and mark the new dot at the north end.”
I can explain my choice: the north-seeking end points along the field, so the next dot must be ahead of it.

Also accept

I chose “Put the south end of the needle over the last dot and mark the new dot at the north end.”
I can explain my choice: the north-seeking end points along the field, so the next dot must be ahead of it.

Watch for

“Her compass is broken.” — Her needle still lines up with the field. Putting the north end on the dot sends each step backwards along the line.
“Larger steps would fix it.” — Larger steps would still go backwards along the line. Which end leads is what matters.
“Moving the magnet would fix it.” — The magnet must stay where it was drawn round, or the dots no longer belong to one field line.
“Tapping the paper would fix it.” — Her needle already turns freely and lines up. Tapping cannot change which end she puts on the dot, so her steps would still go backwards.

9. BQ9 · Practice

Model answer. (a) The card 15 cm away is still in the magnet’s field, a region where a pole feels a force, but the field gets weaker with distance. There the force on each filing is too small to turn it against friction with the card, even when the card is tapped, so the filings stay where they fell. (b) Filings have no marked north end, so they cannot show direction. She places a plotting compass at the point; the field there points the way the north-seeking end of the needle points, and she draws the arrow that way.

A full answer includes

(a) I wrote that the card 15 cm away is still in the magnetic field (a compass needle there would still feel a force), but the field is much weaker there.
(a) I wrote that there the force on each filing is too small to turn it against friction with the card.
(b) I wrote that she places a plotting compass at the point.
(b) I wrote that the field direction is the way the north-seeking end of the needle points.

Also accept

(a) I wrote that the card 15 cm away is still in the magnetic field (a compass needle there would still feel a force), but the field is much weaker there.
(a) I wrote that there the force on each filing is too small to turn it against friction with the card.
(b) I wrote that she places a plotting compass at the point.
(b) I wrote that the field direction is the way the north-seeking end of the needle points.

Watch for

“Far from the magnet there is no field at all.” — The field gets weaker with distance but does not stop suddenly. It is just too weak there to turn filings against friction.
“The filings show direction if you look at which way they point.” — A filing has no marked north end, so it shows the line but not which way along it the field points.

10. BQ10 · Practice

Model answer. (a) The magnets are identical, so their fields are equally strong at equal distances, and they point in opposite directions between the facing poles. They cancel midway: 12.0 cm ÷ 2 = 6.0 cm from each S pole. (b) At the neutral point there is no field to line the needle up, so it stays pointing whichever way it happens to be left.

Checked automatically

(a) Distance from each S pole: 6.0 cm (2 significant figures)
- Common wrong answer 12 : This is the whole gap. With identical magnets the neutral point is midway.
- Common wrong answer 24 : The neutral point lies between the poles, so it is less than the gap, not more.

A full answer includes

(a) I wrote that identical magnets give a neutral point midway between the poles.
(a) I divided the gap by 2: 12.0 cm ÷ 2.
(a) My answer is 6.0 cm, to 2 significant figures.
(b) I wrote that the two fields cancel there, so nothing turns the needle and it points no particular way.

Also accept

(a) I wrote that identical magnets give a neutral point midway between the poles.
(a) I divided the gap by 2: 12.0 cm ÷ 2.
(a) My answer is 6.0 cm, to 2 significant figures.
(b) I wrote that the two fields cancel there, so nothing turns the needle and it points no particular way.

Watch for

“There is no neutral point between two S poles; only N poles have one.” — Between any two like poles of equal magnets the fields cancel midway. Two S poles repel and have a neutral point just as two N poles do.

11. BQ11 · Practice

Model answer. Each magnet sits in the field of the other, and a magnetic pole in a field experiences a force. The field of the second magnet points into its S pole, so the N pole of the first magnet is pushed towards it; in the same way the S pole is pulled towards the N pole. The combined pattern shows this: lines run straight across the gap from N to S, joining the two poles. The force is due to the interaction of the two fields.

A full answer includes

I wrote that each magnet is in the magnetic field of the other.
I wrote that the force on each pole comes from the field it is in: the N pole is pushed along the other magnet’s field, towards its S pole.
I wrote that the field lines run straight across the gap from the N pole to the S pole, joining the two magnets.

Also accept

I wrote that each magnet is in the magnetic field of the other.
I wrote that the force on each pole comes from the field it is in: the N pole is pushed along the other magnet’s field, towards its S pole.
I wrote that the field lines run straight across the gap from the N pole to the S pole, joining the two magnets.

Watch for

“The magnets pull because they are joined by the air between them.” — Air plays no part: the magnets attract in a vacuum too. Each pole is pushed by the field it is in.
“The force travels from one magnet to the other along one field line.” — The lines are a picture of the combined field. Each pole feels a force from the field where it is, not from a line acting like a string.

12. BQ12 · Practice

Model answer. The relative strength of a field is shown by the spacing of its lines: the field is strongest where the lines are closest together, which is near the poles, and weaker where they spread out. The strength changes along a line, because the lines spread apart as they curve away from the poles. Beside the middle of the magnet the lines are far apart, so the field there is weak.

A full answer includes

I chose “The field is strongest where the lines are closest together.”

Also accept

I chose “The field is strongest where the lines are closest together.”

Watch for

“The field is strongest where the lines are furthest apart.” — Wide spacing means a weak field. The lines crowd together near the poles, where the pull is strongest.
“The strength is the same all along one line.” — A line starts where the lines are crowded, near a pole, and spreads out as it curves away, so the field along it gets weaker.
“The field is strongest beside the middle.” — A magnet pulls hardest at its poles. Beside the middle the lines are far apart, so the field is weak.

13. BQ13 · Practice

Model answer. P. The relative strength of a field is shown by the spacing of its lines: where they are closest the field is strongest. The lines crowd together at the poles, and P is just beyond the N end. Q and T are beside the middle, where the lines are further apart, and R is further from the S pole than P is from the N pole.

A full answer includes

I chose P.
I wrote that the field lines are closest together at P, and closer lines mean a stronger field.

Also accept

I chose P.
I wrote that the field lines are closest together at P, and closer lines mean a stronger field.

Watch for

“Q, because it is beside the middle of the magnet.” — The pull of a magnet is weakest near its middle. The lines are closest together near the poles.
“R, because R is on a line and the others are not.” — Strength is shown by how close the lines are, not by whether a point lies on one. The field is there between the lines too.
“T, because more lines pass round the outside.” — At T the lines are far apart, so the field there is weak.

14. BT1 · Apply

Model answer. Row B. Field lines leave N poles and enter S poles. The arrows point from Y to X, so Y is an N pole and X is an S pole. Lines that run straight across the gap from one pole to the other join unlike poles, and unlike poles attract. Between two like poles the lines would turn away from the gap and leave a neutral point.

A full answer includes

I chose row B.
I wrote that field lines point from N to S, so the arrows from Y to X show that Y is an N pole and X is an S pole.
I wrote that lines running across the gap join unlike poles, which attract.

Also accept

I chose row B.
I wrote that field lines point from N to S, so the arrows from Y to X show that Y is an N pole and X is an S pole.
I wrote that lines running across the gap join unlike poles, which attract.

Watch for

“X is an N pole because the arrows point towards it.” — Field lines come out of N poles and go into S poles. The arrows point into X, so X is an S pole.
“The magnets repel because the lines are crowded in the gap.” — Lines that run across the gap from pole to pole join unlike poles, which attract.
“Both ends are N poles.” — Between two N poles the lines turn away from the gap; they do not run across it.

15. BT2 · Apply

Model answer. (a) E is an S pole. (b) The north-seeking end of the needle is an N pole, and it is pulled towards E. Unlike poles attract, so E must be an S pole. In field terms: the needle shows that the field just beyond E points into E, and outside a magnet field lines go into the S pole.

A full answer includes

(a) I chose “An S pole”.
(b) I wrote that the north-seeking end of the needle is an N pole, and it is pulled towards E.
(b) I wrote that unlike poles attract, so E is an S pole, or that the field points into E and field lines go into S poles.

Also accept

(a) I chose “An S pole”.
(b) I wrote that the north-seeking end of the needle is an N pole, and it is pulled towards E.
(b) I wrote that unlike poles attract, so E is an S pole, or that the field points into E and field lines go into S poles.

Watch for

“An N pole, because the north-seeking end points at it.” — The north-seeking end is an N pole itself. It is pulled towards E, and unlike poles attract, so E is an S pole.
“Either pole, because a compass needle is attracted by both poles.” — A magnetised needle is pushed away from an N pole and pulled towards an S pole. Only an unmagnetised piece of iron is attracted by both.
“Neither: the needle only lines up along the magnet.” — The needle does line up with the field, and which of its ends points at E tells you the pole: its north-seeking end is an N pole, pulled towards an S pole.

16. BT3 · Apply

Model answer. (a) Field lines are a picture of a field, not the field itself. The field fills the whole region round the magnet: a compass placed between two of her lines still lines up along the field, and she could draw another line through that point. (b) The relative strength is shown by the spacing: where her lines are closest together, near the poles, the field is strongest; where they spread out, it is weaker.

A full answer includes

(a) I wrote that field lines are a picture of the field, and the field fills the whole region round the magnet.
(a) I wrote that a compass placed between two lines still lines up, so there is a field there.
(b) I wrote that the field is strongest where the lines are closest together, which is near the poles.

Also accept

(a) I wrote that field lines are a picture of the field, and the field fills the whole region round the magnet.
(a) I wrote that a compass placed between two lines still lines up, so there is a field there.
(b) I wrote that the field is strongest where the lines are closest together, which is near the poles.

Watch for

“The field exists only along the lines.” — A compass placed between two drawn lines still turns, so there is a field there. Six lines are only a choice of how many to draw.

17. BT4 · Apply

Model answer. Each needle is a small magnet with its own field, and each sits in the field of the other. A few millimetres from a needle its field is much stronger than the Earth’s weak field, so the other needle’s field decides which way each needle points. The force on each pole comes from the field it is in: the N pole of one needle is pushed along the other needle’s field, towards its S pole. So the needles turn until unlike poles are next to each other, and unlike poles attract. Move the compasses apart and the Earth’s field wins again: both point north.

A full answer includes

I wrote that each needle is a small magnet with its own magnetic field, and each needle is in the field of the other.
I wrote that so close, the other needle’s field is stronger than the Earth’s field, so it decides how each needle lines up.
I wrote that the poles of each needle are pushed by the field they are in, so the needles line up with unlike poles next to each other, which attract.

Also accept

I wrote that each needle is a small magnet with its own magnetic field, and each needle is in the field of the other.
I wrote that so close, the other needle’s field is stronger than the Earth’s field, so it decides how each needle lines up.
I wrote that the poles of each needle are pushed by the field they are in, so the needles line up with unlike poles next to each other, which attract.

Watch for

“The compass cases attract each other.” — The cases are plastic, a non-magnetic material. The needles turn because each needle is a magnet in the field of the other.
“The needles still point north; only the cases moved.” — The needles have turned away from north. The field of the other needle, so close, is stronger than the Earth’s field.