Senior 2 practice book · Unit 8 of 10

Magnetism

50 questions that complete the Senior 2 quiz for this unit: 30 core and 20 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

Every metal is attracted by a magnet.Only a few materials (iron, nickel, cobalt and alloys such as steel) are strongly attracted. Copper, aluminium, brass, gold and silver are metals but are not attracted noticeably.
If a bar is attracted by a magnet, it must itself be a magnet.Unmagnetised iron or steel is also attracted, because the magnet induces magnetism in it. Only repulsion proves that a bar is a magnet.
The middle of a bar magnet is not magnetised.All the domains along the bar are lined up. At the middle the effects of neighbouring domains cancel outside the bar, so the attraction is weak there, but the material is still magnetised.
A magnet can be made stronger without limit by stroking it more and more.Once all the domains point the same way the bar is saturated; extra stroking or extra current cannot make it any stronger.
The north pole of a compass needle points to the Earth's magnetic north pole.Unlike poles attract, so the needle's N pole points to a magnetic SOUTH pole, which lies near the geographic North Pole.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Magnetic and non-magnetic materials
Ferromagnetic, paramagnetic and diamagnetic materials
Properties of magnets: poles, law of poles, induced magnetism and the test for a magnet
Domain theory of magnetism and magnetic saturation
Methods of magnetising: stroking, electrical (DC solenoid) and induction
Methods of demagnetising and the Curie temperature
Hard and soft magnetic materials; storing magnets with keepers
Magnetic fields: field lines, plotting and neutral points
The Earth's magnetic field: declination and angle of dip
Magnetic shielding
Electromagnets: polarity of a solenoid and factors affecting strength
Uses of permanent magnets and electromagnets

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 20 harder ones are marked ★ Challenge.

1True or false
All metals are attracted by a magnet.
Show answer
Answer: False
Only a few metals (iron, nickel, cobalt and their alloys) are attracted; copper, aluminium, gold and silver are not.
Common mistakeLearners link magnets with 'metal' in general; magnetism belongs to only a few materials.
2True or false
A magnet that is cooled in a freezer loses its magnetism.
Show answer
Answer: False
Cooling does not knock domains out of line; it is strong heating that weakens or destroys magnetism.
Common mistakeLearners think any big change in temperature affects a magnet; only heating weakens it.
3True or false · ★ Challenge
Above its Curie temperature, iron behaves as a paramagnetic material.
Show answer
Answer: True
Above the Curie temperature the domains break up, so iron is only weakly attracted, like a paramagnetic material.
Common mistakeSome think iron becomes completely non-magnetic when hot; it is still very weakly attracted (paramagnetic).
4True or false
Soft iron is called a 'soft' magnetic material because it is easy to bend.
Show answer
Answer: False
'Soft' means it is easily magnetised and easily demagnetised; it describes magnetic behaviour, not hardness to bend.
Common mistakeLearners take 'soft' in its everyday meaning; in magnetism it describes how easily the magnetism comes and goes.
5True or false · ★ Challenge
In Rwanda, which is close to the equator, a dip needle points almost straight down.
Show answer
Answer: False
Near the equator the Earth's field is nearly horizontal, so the dip is small; a needle points almost straight down only near the magnetic poles.
Common mistakeSome think dip needles always point down; the angle of dip depends on where you are on the Earth.
6True or false
In a magnetised bar, the domains near the middle of the bar are not lined up.
Show answer
Answer: False
All the domains along the bar are aligned; at the middle the effects of neighbouring domains cancel outside the bar, so the attraction there is weak.
Common mistakeWeak attraction at the middle is wrongly taken to mean that the middle is unmagnetised.
7True or false · ★ Challenge
A magnet heated above its Curie temperature regains its magnetism by itself once it has cooled down.
Show answer
Answer: False
On cooling, domains re-form but point in random directions, so the bar is unmagnetised until it is magnetised again.
Common mistakeSome think magnetism 'comes back' like a material's colour; the alignment is lost for good.
8True or false
Magnetic field lines can begin or end in the empty space between two magnets.
Show answer
Answer: False
Field lines are continuous closed loops: outside a magnet they go from N to S and they continue inside from S to N.
Common mistakeDrawing lines that stop in mid-air is a frequent mistake; a neutral point is where fields cancel, not where lines end.
9True or false
A coil with no iron core still produces a magnetic field when a current flows in it.
Show answer
Answer: True
Any current produces a magnetic field; the iron core only makes the field much stronger.
Common mistakeLearners think the iron is what makes the magnetism; the current makes the field and the iron strengthens it.
10Multiple choice · ★ Challenge
At the Earth's magnetic equator, how does a dip needle settle?
1. AIt keeps spinning round
2. BVertical (dip 90°)
3. CAt 45° to the horizontal
4. DHorizontal (dip 0°)
Show answer
Answer: D. Horizontal (dip 0°)
At the magnetic equator the Earth's field lines are parallel to the ground, so the dip is 0°; the dip is 90° at the magnetic poles.
Common mistake'Vertical' describes the magnetic poles, where the field lines go straight into or out of the ground.
11True or false
A sheet of aluminium placed between a magnet and a steel pin stops the magnet attracting the pin.
Show answer
Answer: False
Aluminium is non-magnetic and does not divert the field; only magnetic materials such as soft iron give shielding.
Common mistakeLearners think any metal blocks magnetism; shielding needs a magnetic material.
12Multiple choice · ★ Challenge
A long bar magnet is carefully cut LENGTHWISE (along its length) into two thin bars. What is each piece?
1. AUnmagnetised, because cutting it has destroyed its domains
2. BA magnet with an N pole at both ends
3. CA magnet, with its N pole at the same end as before
4. DA magnet with its poles in the middle
Show answer
Answer: C. A magnet, with its N pole at the same end as before
The domains in each piece still point along the bar the same way, so each thin piece is a magnet with N and S poles at the same ends as the original.
Common mistake'Unmagnetised' assumes cutting destroys the domains; cutting only divides the aligned domains between two pieces.
13Fill in the blank
Brass, copper and wood are not attracted by a magnet; they are called ______ materials.
Show answer
Answer: non-magnetic
Materials that a magnet does not attract (or affects only extremely weakly) are non-magnetic.
Common mistakeSome write 'paramagnetic' for every non-attracted material; in everyday use these are simply non-magnetic.
14Multiple choice · ★ Challenge
In the right-hand grip rule for a solenoid, the fingers curl round in the direction of the current. The thumb then points:
1. ATowards the S pole end
2. BAt right angles to the coil axis
3. CTowards the N pole end
4. DIn the direction of the current
Show answer
Answer: C. Towards the N pole end
Gripping the solenoid with the fingers along the current, the thumb points along the field inside the coil, towards the N pole.
Common mistake'Towards the S pole' reverses the rule; inside the coil the field points towards the N end.
15Fill in the blank
Magnetic poles always exist in ______; no one has ever found a single N pole on its own.
Show answer
Answer: pairs
Every magnet has both an N and an S pole; cutting a magnet produces new pairs of poles.
Common mistakeSome think a magnet can be cut to separate the N pole from the S pole, but each piece gets both poles.
16Fill in the blank
A long coil of insulated wire that becomes a magnet when a current flows in it is called a ______.
Show answer
Answer: solenoid
A current-carrying solenoid has a field like a bar magnet; with an iron core it is an electromagnet.
Common mistakeSome write 'transformer'; a transformer has two coils and needs alternating current.
17Multiple choice · ★ Challenge
A student magnetises steel bars in a solenoid connected to a DC supply. Which change gives a stronger magnet (before saturation)?
1. AReversing the current halfway
2. BUsing a copper bar instead
3. CUsing a larger alternating current
4. DIncreasing the direct current
Show answer
Answer: D. Increasing the direct current
A larger direct current makes a stronger field in the solenoid, so more domains line up in the steel.
Common mistakeA larger AC is tempting, but AC reverses direction and leaves the domains random.
18Fill in the blank
Materials such as copper, bismuth and water are very weakly REPELLED by a strong magnet; they are called ______ materials.
Show answer
Answer: diamagnetic
Diamagnetic materials are weakly repelled from strong magnetic fields.
Common mistakeWriting 'paramagnetic' is the usual slip; paramagnetic materials are weakly attracted.
19Multiple choice
Which list contains ONLY magnetic materials?
1. ANickel, brass, iron, cobalt
2. BIron, nickel, cobalt, steel
3. CSteel, aluminium, cobalt, iron
4. DIron, copper, nickel, steel
Show answer
Answer: B. Iron, nickel, cobalt, steel
Iron, nickel and cobalt, and alloys of iron such as steel, are magnetic; copper, aluminium and brass are not.
Common mistakeLists with copper, aluminium or brass are tempting because these are metals, but most metals are not magnetic.
20Multiple choice · ★ Challenge
A steel paper clip on a thread is held up in the air by a magnet above it, without touching. Sliding a sheet of material X into the gap makes the clip fall. What is X most likely to be?
1. ASoft iron
2. BGlass
3. CAluminium
4. DPaper
Show answer
Answer: A. Soft iron
Soft iron draws the field lines into itself, so they pass through the sheet instead of reaching the clip: magnetic shielding.
Common mistakeAluminium is tempting because it is a metal, but it is non-magnetic and lets the field pass straight through.
21Multiple choice
Which of these is NOT a property of a bar magnet?
1. AHung freely, it settles north–south
2. BIt attracts iron and steel
3. CIt attracts copper and aluminium
4. DIts poles come in pairs, N and S
Show answer
Answer: C. It attracts copper and aluminium
A magnet attracts magnetic materials such as iron and steel, not copper or aluminium.
Common mistakeLearners who think magnets attract all metals miss that the copper and aluminium statement is false.
22Multiple choice · ★ Challenge
Three small rods X, Y and Z are hung between the poles of a very strong magnet. X lines up strongly along the field; Y lines up weakly along the field; Z turns weakly to lie across the field. Which is correct?
1. AX ferro-, Y para-, Z diamagnetic
2. BX ferro-, Y dia-, Z paramagnetic
3. CX dia-, Y para-, Z ferromagnetic
4. DX para-, Y ferro-, Z diamagnetic
Show answer
Answer: A. X ferro-, Y para-, Z diamagnetic
Ferromagnetic rods are pulled strongly along the field, paramagnetic rods weakly along it, and diamagnetic rods are weakly repelled and settle across it.
Common mistakeSwapping Y and Z mixes up weak attraction (paramagnetic) with weak repulsion (diamagnetic).
23Multiple choice
Which of these does NOT depend on magnetism to work?
1. AAn electric kettle's heating element
2. BA fridge-door seal
3. CA loudspeaker
4. DAn MRI scanner in a large hospital
Show answer
Answer: A. An electric kettle's heating element
A kettle element simply heats up when current flows through its resistance; the others use magnets or magnetic fields.
Common mistakeThe fridge-door seal is easy to miss: it contains a flexible permanent magnet strip.
24Multiple choice
Which change would NOT make an electromagnet any stronger?
1. AIncreasing the current
2. BAdding a soft-iron core
3. CAdding more turns of wire
4. DReversing the current
Show answer
Answer: D. Reversing the current
Reversing the current swaps the poles but leaves the strength the same.
Common mistakeLearners sometimes think a soft-iron core weakens the coil, but iron makes the field much stronger.
25Multiple choice · ★ Challenge
Why do bar magnets stored in pairs with keepers keep their magnetism longer than magnets stored alone?
1. AKeepers give the magnets extra domains
2. BThe field forms closed loops, so end domains stay aligned
3. CKeepers stop the magnets getting warm
4. DKeepers cancel the magnets' field completely, so nothing can change
Show answer
Answer: B. The field forms closed loops, so end domains stay aligned
With unlike poles side by side and keepers across the ends, the field lines form closed loops through iron, so there are no free poles to turn the end domains out of line.
Common mistake'Keepers cancel the field' is wrong: they guide the field round a loop; they do not destroy it.
26Multiple choice
Which of these will NOT magnetise a steel needle permanently?
1. APlacing it in a solenoid carrying AC
2. BStroking it many times with one pole of a magnet
3. CStroking it from the middle with two magnets
4. DPlacing it in a solenoid carrying DC
Show answer
Answer: A. Placing it in a solenoid carrying AC
Alternating current reverses many times each second, so the domains are left random when it stops.
Common mistakeThe double-touch method (two magnets from the middle) may look odd, but it does magnetise the needle.
27Multiple choice · ★ Challenge
A magnet is demagnetised by hammering. Why should it lie in the EAST–WEST direction while it is hammered?
1. ASo it is not heated by the hammering
2. BSo that the hammer does not stick to the magnet while hitting it
3. CSo its poles end up facing east and west
4. DSo the Earth's field cannot re-align its loosened domains
Show answer
Answer: D. So the Earth's field cannot re-align its loosened domains
Hammering loosens the domains; lying east–west, the Earth's north–south field cannot line them up again.
Common mistake'So its poles face east and west' misses the point: the aim is to have NO poles, with random domains.
28Fill in the blank
A point where the magnetic fields of two magnets (or of a magnet and the Earth) cancel, so a compass has no preferred direction, is called a ______ point.
Show answer
Answer: neutral
At a neutral point the resultant field is zero.
Common mistakeSome call it the 'pole'; at a pole the field is strongest, the opposite of a neutral point.
29Multiple choice
A bar magnet is dipped into iron filings and lifted out. Where do most of the filings cling?
1. AEvenly along the whole bar
2. BOnly at the north-pole end
3. CNear both of its ends
4. DMostly in the middle
Show answer
Answer: C. Near both of its ends
The magnetic force is strongest at the poles, which are near the two ends of the bar.
Common mistake'Only at the north pole' forgets that both poles attract magnetic materials equally.
30Short answer · ★ Challenge
Use the domain theory to explain why a magnet loses its magnetism when it is heated strongly.
Show answer
Model answer: Heating makes the atoms vibrate more strongly. The vibrations knock the domains out of line so they point in random directions, and their magnetic effects cancel. Above the Curie temperature the domains are destroyed completely, and on cooling they form again in random directions, so the magnetism is lost.
Common mistakeA common answer says 'the heat melts the magnetism'; the explanation must mention domains becoming random.
31Multiple choice
Iron filings sprinkled round a bar magnet show the field pattern. What can the filings NOT show?
1. AThe direction of the field lines
2. BWhere the field is strongest
3. CThe shape of the field lines around it
4. DWhere the poles are
Show answer
Answer: A. The direction of the field lines
Filings line up along the field but have no N or S end marked, so a plotting compass is needed to show the direction.
Common mistake'Where the field is strongest' can be seen from where the filings crowd together, so it is not the answer.
32Short answer · ★ Challenge
Explain why alternating current (AC) cannot be used to make a permanent magnet in a solenoid.
Show answer
Model answer: AC reverses direction many times a second (50 times in Rwanda's mains), so the field in the solenoid keeps reversing and the domains are flipped back and forth. When the current is switched off, the field could be at any point in its cycle, so the domains are not left all pointing one way and no reliable permanent magnet is made.
Common mistakeSome say AC is 'too weak'; the real problem is that its direction keeps reversing.
33Multiple choice
The angle between the direction of geographic north and the direction in which a compass needle points is called:
1. AThe angle of dip
2. BThe declination
3. CThe latitude
4. DThe Curie angle
Show answer
Answer: B. The declination
Magnetic declination (variation) is the angle between true (geographic) north and magnetic north.
Common mistakeThe angle of dip is the angle with the HORIZONTAL, not with geographic north.
34Short answer · ★ Challenge
Explain how a magnetic relay allows a small current from a switch to turn on a large water pump.
Show answer
Model answer: A small current from the switch flows through the relay's coil, which becomes an electromagnet. It attracts a soft-iron armature, which pivots and pushes two contacts together in a separate circuit, so the large pump current flows. When the small current stops, the soft iron loses its magnetism and a spring opens the contacts again.
Common mistakeA common error is to think the large current passes through the coil; the two circuits are separate.
35Multiple choice
A magnetised needle is pivoted so it can turn in a vertical plane. The angle it makes with the horizontal is called:
1. AThe angle of reflection
2. BThe neutral angle
3. CThe declination
4. DThe angle of dip
Show answer
Answer: D. The angle of dip
The Earth's field is tilted to the horizontal at most places; a dip needle measures this angle of dip.
Common mistakeDeclination is tempting, but it is measured in the horizontal plane, between two directions of north.
36Multiple choice
A sensitive instrument is placed inside a closed box made of soft iron. What is the magnetic field from outside like inside the box?
1. AReversed in direction
2. BVery weak
3. CThe same as outside
4. DMuch stronger than outside
Show answer
Answer: B. Very weak
The field lines prefer to travel through the iron walls, so very little field reaches the space inside the box.
Common mistake'Much stronger' confuses shielding with an iron core inside a coil, where iron concentrates the field in itself, not in the space it surrounds.
37Short answer · ★ Challenge
Describe the magnetic field between the poles of a horseshoe (U-shaped) magnet and explain why this shape is useful in instruments such as moving-coil meters.
Show answer
Model answer: Between the poles the field lines run almost straight and parallel from the N pole to the S pole, so the field is strong and nearly uniform. Bending the magnet brings the two poles close together, giving a strong field in a small gap where a coil can be placed.
Common mistakeLearners often draw lines going from S to N across the gap; outside a magnet, lines go from N to S.
38Multiple choice
Materials such as aluminium and platinum are attracted only VERY weakly by a strong magnet and cannot be made into magnets. They are called:
1. ANon-metallic
2. BDiamagnetic
3. CFerromagnetic
4. DParamagnetic
Show answer
Answer: D. Paramagnetic
Paramagnetic materials are weakly attracted into a strong field and lose this effect as soon as the field is removed.
Common mistake'Diamagnetic' is tempting, but diamagnetic materials are weakly REPELLED, not attracted.
39Short answer · ★ Challenge
A hiker in Nyungwe Forest wants to walk towards a point due GEOGRAPHIC north using a magnetic compass. Explain why she cannot simply follow the needle, and what extra information she needs.
Show answer
Model answer: The compass needle points to magnetic north, which is not exactly the same as geographic (true) north; the angle between them is the declination. She needs the value of the declination for that place (from a map) and must turn her direction by that angle east or west of the needle.
Common mistakeMany think compasses point exactly to the North Pole; they line up with the Earth's MAGNETIC field.
40Multiple choice
According to the domain theory, why does a magnet attract an unmagnetised iron nail?
1. AIt gives the nail an electric charge
2. BIts field lines up the nail's domains, giving an unlike pole nearest
3. CThe nail's domains are already aligned
4. DIts own domains jump across the gap into the iron nail
Show answer
Answer: B. Its field lines up the nail's domains, giving an unlike pole nearest
The magnet's field turns the domains of the nail so the nail becomes a magnet with an unlike pole next to the magnet, and unlike poles attract.
Common mistake'The nail's domains are already aligned' describes a magnet, not an unmagnetised nail; the alignment is induced.
41Multiple choice
Why does the hammer of an electric bell keep striking the gong while the switch is pressed?
1. AA permanent magnet pushes it back and forth
2. BThe gong repels the hammer after each strike it makes
3. CEach stroke breaks the circuit, so a spring pulls it back
4. DThe current reverses direction each time
Show answer
Answer: C. Each stroke breaks the circuit, so a spring pulls it back
When the electromagnet attracts the armature the contact opens, the current stops, the magnet switches off and the spring returns the armature, closing the circuit again; this repeats.
Common mistake'The current reverses' is tempting, but a bell works on DC; it is the make-and-break contact that keeps it ringing.
42Short answer · ★ Challenge
You are given two identical-looking steel bars. One is a magnet and the other is not, and you have nothing else. How can you find out which is the magnet?
Show answer
Model answer: Hold bar A and touch one end of it against the MIDDLE of bar B. If they attract strongly, A is the magnet (its pole attracts B). If there is little or no attraction, B is the magnet, because the middle of a magnet has almost no pull and A is unmagnetised.
Common mistakeMany try ends against ends, but two ends attract whichever bar is the magnet; the middle of a magnet is the key.
43Multiple choice
End X of a solenoid is a north pole and a compass is near it. The current in the solenoid is reversed. What happens?
1. AX stays N but becomes weaker
2. BThe solenoid stops being a magnet
3. CX becomes S and the needle turns round
4. DBoth ends of the solenoid become north poles
Show answer
Answer: C. X becomes S and the needle turns round
The direction of the field depends on the direction of the current, so reversing the current swaps the poles and the needle turns round.
Common mistake'Both ends become N' is impossible: a solenoid, like a bar magnet, always has one N and one S end.
44Short answer
Describe how a steel bar is magnetised by the double-touch (divided-touch) method.
Show answer
Model answer: Place the steel bar on the bench. Put the N pole of one magnet and the S pole of another on the middle of the bar. Move them apart together to opposite ends, lift them clear, return them to the middle and repeat many times. The end stroked by the N pole becomes an S pole, and the end stroked by the S pole becomes an N pole.
Common mistakeLearners often use two like poles; the two stroking poles must be OPPOSITE, or their effects cancel.
45Short answer · ★ Challenge
Three materials are tested. While touching a magnet, P holds 20 paper clips, Q holds 25 and R holds none. After the magnet is removed, P holds 18 clips, Q holds 2 and R holds none. Choose a material for (a) a fridge-door magnet, (b) the core of a relay's electromagnet and (c) the case of a compass. Give reasons.
Show answer
Model answer: (a) P: it keeps most of its magnetism (a hard magnetic material), so it holds the door shut for years. (b) Q: it is magnetised strongly while current flows but loses almost all its magnetism when the current stops (soft), so the relay switches off. (c) R: it is non-magnetic, so it does not disturb the compass needle.
Common mistakeChoosing Q for the fridge magnet looks only at the 'while touching' column; a permanent magnet must keep its magnetism afterwards.
46Short answer
Give one use of a permanent magnet and one use of an electromagnet, and explain why each type suits its use.
Show answer
Model answer: Permanent magnet: a compass needle, loudspeaker or fridge-door seal, where a steady field is needed all the time without a power supply. Electromagnet: a scrap-metal crane, electric bell or relay, where the magnetism must be switched on and off (and can be made very strong).
Common mistakeLearners give uses without the reason; the key difference is that an electromagnet can be switched off.
47Short answer · ★ Challenge
A girl finds that a magnet picks up a steel bottle top but not an aluminium cooking-pot lid. She concludes: 'Aluminium is not a metal.' Evaluate her conclusion.
Show answer
Model answer: Her observation is right but her conclusion is wrong. Aluminium is a metal (shiny, conducts heat and electricity), but it is a non-magnetic material. Being attracted by a magnet is a property of only a few materials (iron, nickel, cobalt, steel), not a test for being a metal.
Common mistakeThe error is to treat 'attracted by a magnet' as the test for a metal; most metals are not magnetic.
48Multiple choice
A plotting compass is placed on the axis of a bar magnet, just beyond its N pole. Which way does the N end of the compass needle point?
1. AAway from the magnet
2. BTowards the magnet
3. CAt right angles to the magnet
4. DIn no fixed direction
Show answer
Answer: A. Away from the magnet
Field lines leave the N pole outwards, and a compass N end points along the field, i.e. away from the N pole (like poles repel).
Common mistake'Towards the magnet' forgets that the needle's N end is repelled by the magnet's N pole.
49Short answer · ★ Challenge
Design a fair test to find how the number of turns of wire affects the strength of an electromagnet.
Show answer
Model answer: Wind 10 turns on a soft-iron nail, connect it in series with a cell, a rheostat, an ammeter and a switch. Set a fixed current (e.g. 1.0 A) and count how many paper clips it lifts. Repeat with 20, 30, 40 and 50 turns, adjusting the rheostat to keep the same current, and using the same nail and the same clips. Repeat each reading and average; plot clips against number of turns.
Common mistakeForgetting to keep the current the same is the usual flaw: longer wire has more resistance, so the current would fall.
50Multiple choice
Which material is best for the needle of a magnetic compass?
1. ASoft iron
2. BSteel
3. CCopper
4. DAluminium
Show answer
Answer: B. Steel
The needle must stay magnetised for years, so a magnetically hard material such as steel is used.
Common mistakeSoft iron is attracted strongly, which makes it tempting, but it loses its magnetism easily.