Simple Phenomena of Magnetism Worked ExamplesIGCSE Physics 0625

(a) Only a magnet can be repelled by a magnet, because repulsion needs two like poles facing each other. Bar K is repelled by the N pole, so the end of K that was tested must itself be an N pole. Bar K is the magnet.

Attraction on its own proves nothing: a magnet attracts both a magnet (unlike poles) and an unmagnetised piece of magnetic material. That is why the repulsion is the deciding result.

(b) Bar J is attracted by both poles. It has no poles of its own, so it is an unmagnetised magnetic material (for example iron). Each pole of the magnet induces an opposite pole in the near end of J, so J is always attracted.

(c) Bar L feels no force from either pole, so it is a non-magnetic material, for example copper, aluminium or brass.

Soft iron is a magnetic material. Near the N pole, the end of the rod closest to the magnet becomes an induced S pole. Unlike poles attract, so the rod is attracted.

Near the S pole, the near end becomes an induced N pole instead. Again unlike poles face each other, so the rod is attracted. The rod is attracted by both poles: B.

A and C would need the rod to have fixed poles of its own, which an unmagnetised rod does not have. D describes a non-magnetic material such as copper, not iron.

(a) A magnetic material is attracted by a magnet (it can be magnetised); a non-magnetic material is not attracted by a magnet. Aluminium is non-magnetic, so the aluminium cans are left behind.

(b) The magnet induces magnetism in each steel can: the end of the can nearest the magnet becomes a pole opposite to the pole of the magnet.

Opposite (unlike) poles attract, so the can is pulled up to the magnet. This works whichever pole of the magnet is used.

(c) Steel keeps some of its induced magnetism after the magnet is removed (it is the material used for permanent magnets). The cans have become weak magnets, so they attract each other.

Recall the two properties. Soft iron is easily magnetised but loses its magnetism easily — it makes a temporary magnet. Steel is harder to magnetise but keeps its magnetism — it makes a permanent magnet.

(a) The core must stop being a magnet as soon as the current stops, so the door is released. Choose soft iron, because it loses its magnetism when the current is switched off.

A steel core would stay magnetised after the current was switched off, so the lock would keep holding the door.

(b) The holder has no electrical supply and must grip the phone all the time. Choose steel, because it stays magnetised — it is a permanent magnet.

(a) Field lines point away from an N pole and towards an S pole. The arrows leave ends 1 and 4, so these are N poles. The arrows go into ends 2 and 3, so these are S poles.

(b) Ends 2 and 3 face each other and are both S poles. Like poles repel, so the magnets repel. The diagram agrees: the lines from the two magnets bend away from each other in the gap instead of joining across it.

(c) The strength of a magnetic field is shown by the spacing of the field lines: the closer together the lines, the stronger the field.

At Q, just beyond the end of a magnet, the lines are closest together, so the field is strongest at Q.

At P, midway between the two S poles, the lines are furthest apart — the fields of the two magnets cancel there — so the field is weakest at P. (R is weak too, but lines still pass near it.)

The arrows on the field lines point away from end F and into end E. The direction of a magnetic field is the direction of the force on an N pole, so field lines leave an N pole and enter an S pole. F is an N pole. This rules out C and D.

The field is strongest where the lines are closest together. That is near the poles — point W, just beyond end F. X and Y are far from the magnet where the lines are widely spaced. The answer is A.

B is tempting because X lies on a large, smooth loop, but widely spaced lines mean a weak field. D is tempting because Z is close to the magnet, but the middle of a magnet is where the field is weakest near the magnet — the poles are at the ends.

(a) Draw round the magnet so it can be replaced in the same position. Place the plotting compass close to the N pole and mark a dot at the end the needle's N end points to (dot 1).

Move the compass so that the tail (S end) of the needle is over the last dot, then mark a new dot at the needle's N end. Repeat — this gives dots 2, 3, 4, 5.

Continue until the line reaches the S pole (or the edge of the paper), then join the dots with a smooth curve. Start again at a different point near the N pole to plot further lines.

(b) Draw an arrow on the line in the direction the needle's N end points — from the N pole towards the S pole.

(c) Iron filings show only the shape of the pattern; they do not show the direction of the field (they have no N end to point along it).

(a) At position 2 the N end of the needle points away from F. Like poles repel, so the N end of the needle is being pushed away by an N pole. F is the N pole (and E is the S pole).

Check with position 1: the N end of the needle points towards E, because it is attracted to an S pole. This agrees.

(b) The direction of a magnetic field at a point is the direction of the force on the N pole of a magnet placed at that point.

(c) Outside the magnet, field lines run from the N pole to the S pole. Above the middle of the magnet the line runs parallel to the magnet, from F towards E — to the left in the diagram.

(a) The magnet induces magnetism in the test bar, so the bar becomes a magnet itself. Its lower end is a pole that induces magnetism in the first clip, which induces magnetism in the next clip, and so on down the chain.

(b)(i) Soft iron holds more clips while in contact ($11$ compared with $6$), so it is magnetised more easily, and it holds none ($0$) once the magnet is removed, so it loses its magnetism. Soft iron is better for a temporary magnet.

(b)(ii) Steel still holds $5$ of its $6$ clips after the magnet is removed, so it keeps its magnetism. Steel is better for a permanent magnet.

(c) Any two of: the same bar magnet (and the same pole); test bars of the same size and shape; the same type/size of paper clip; the bar touching the magnet in the same way for the same time.

An electromagnet can be switched on and off, and its strength can be changed by changing the current. A permanent magnet needs no electrical supply and is magnetised all the time.

(a) Electromagnet — the crane must release the car over the pile, which happens when the current is switched off.

(b) Permanent magnet — the door must stay closed all the time without using electrical energy (and still work in a power cut).

(c) Electromagnet — the strength of the force can be adjusted by changing the current.

(d) Permanent magnet — it is small, needs no power supply and holds the note continuously.

(a) The field is strongest where the lines are closest together. In the gap at A the lines are close and nearly parallel. At C, near the outer N pole, the lines are spread out more. At B, far above the gap, they are furthest apart. Order: A, C, B.

(b) The direction of the field is the direction of the force on an N pole. In the gap the field goes from the N pole of the right-hand magnet to the S pole of the left-hand magnet, so the force on the N pole at A is to the left (towards the left-hand magnet).

(c) At B the field line points to the left (along the loop from the right-hand N pole back to the left-hand S pole). An S pole feels a force opposite to the field direction, so the force at B is to the right.

(d) Each magnet is in the magnetic field of the other. The field of each magnet exerts a force on the poles of the other magnet: the magnetic forces are due to the interaction between the two magnetic fields. With unlike poles facing, the resultant force pulls the magnets together.

Repulsion cannot be used: whichever way round the bars are held, a magnet and a piece of iron always attract. We need a test that makes one arrangement attract strongly and the other weakly.

The field of a magnet is strong at its poles (the lines are close together at the ends) and weak at its middle (few lines emerge there).

Suppose P is the magnet. Its end is a pole with a strong field, which induces magnetism in the iron at the middle of Q, so there is a strong attraction. When the end of Q (iron) touches the middle of P, the field there is weak, so there is little or no attraction.

Conclusion: the bar whose end strongly attracts the middle of the other bar is the magnet. If the end of Q attracts the middle of P strongly instead, Q is the magnet.