9th Class Physics · Punjab Board · Complete learning guide
Chapter 8Magnetism
Every topic, every MCQ and every exercise question, explained step by step in simple language. Written for the Punjab Board new-syllabus Physics book, English medium.
The force that a magnet exerts at a distance on magnetic materials and on other magnets.
Definition
Magnetic materials
Materials that are attracted by a magnet, such as iron, nickel, cobalt and steel.
Definition
Non-magnetic materials
Materials that are not attracted by a magnet, such as copper, brass, aluminium, wood, glass and plastic.
Magnetic
Non-magnetic
Iron, steel
Copper, brass
Nickel
Aluminium
Cobalt
Wood, glass, plastic, paper
Remember
All metals are not magnetic: a magnet does not pick up a copper or an aluminium coin.
Topic 2
Properties of magnets
Definition
Magnetic poles
The ends of a magnet, where its magnetism is strongest. A freely suspended bar magnet comes to rest pointing north-south; the end pointing north is its north pole (N), the other its south pole (S).
Attraction and repulsion
Like poles repel; unlike poles attract.
Like poles repel; unlike poles attract. Two north poles push each other away, and so do two south poles; a north pole and a south pole pull together.
Identifying a magnet
Bring one end of the object near one pole of a suspended magnet. Attraction means it is either an opposite pole or plain magnetic material.
Bring the same end near the other pole of the suspended magnet.
If it is attracted again, the object is only a magnetic material. If it is repelled, the object is a magnet.
Remember
Repulsion is the real test of a magnet: a magnet attracts plain iron too, but it can only repel another magnet.
No isolated poles
Breaking a magnet gives two complete magnets, each with an N and an S pole.
However many times a magnet is broken, each piece has both an N-pole and an S-pole. A single isolated pole does not exist.
Topic 3
Induced magnetism, temporary and permanent magnets
Definition
Induced magnetism
The magnetism produced in a magnetic material, such as iron or steel, when it is placed near or touching a magnet. Also called magnetisation.
The S-pole of a magnet induces an N-pole at the near end of an iron nail, which then attracts a chain of pins.
The near end of the iron always becomes the opposite pole to the pole of the magnet, which is why the iron is attracted. The induced magnetism of soft iron disappears when the magnet is taken away.
Definition
Temporary magnets
Magnets that act only while they are in the field of another magnet (or while a current flows) and lose their magnetism when it is removed. Soft iron, paper clips, iron nails and electromagnets are examples.
Definition
Permanent magnets
Magnets that keep their magnetism for a long time. They are found in nature (lodestone) or made by putting steel or special alloys (alnico, ferrite, neodymium) in a strong field for long enough.
Temporary
Permanent
Soft iron
Steel, alnico, ferrite, neodymium
Easily magnetised, easily lost
Hard to magnetise, keeps it
Electromagnets, relays, cranes, transformer cores
Motors, generators, loudspeakers, compasses, door catchers
Topic 4
Magnetic field and lines of force
Definition
Magnetic field
The region around a magnet in which another magnet or a magnetic material experiences a magnetic force.
Definition
Magnetic lines of force
Imaginary lines drawn to show a magnetic field. They follow the pattern of iron filings round a magnet and show the direction and the strength of the field.
Field of a bar magnet: lines leave N and enter S; small compasses point along them.
Definition
Direction of the field
The direction in which the N-pole of a small compass needle points when placed at that point.
Definition
Strength of the field
Proportional to the number of lines of force passing through unit area held at right angles to them: strong where the lines are close together, weak where they are far apart.
Properties of lines of force
Outside a magnet they go from the N-pole to the S-pole.
They never cross each other.
They are crowded near the poles, where the field is strongest.
Field patterns
(a) Unlike poles facing; (b) like poles facing, with a neutral point X; (c) a horseshoe magnet.
Definition
Neutral point
A point where the field of one magnet exactly cancels the field of another, so the net field is zero. It forms between like poles facing each other (point X in figure b).
Between the poles of a horseshoe magnet the field is almost uniform (straight, evenly spaced lines), except near the edges.
Topic 5
Uses of permanent magnets
Device
How the permanent magnet is used
D.C. motor
A coil carrying current turns in the field of a permanent magnet
A.C. and D.C. generators
A coil rotated between the poles of a magnet has a current induced in it
Moving-coil loudspeaker
A coil carrying a varying current moves in the field of a permanent magnet
Door catchers
A magnet holds the steel plate on a cupboard or fridge door
Separating iron
Magnets pull iron objects out of waste and mixtures
A.C. generator
When a coil is rotated between the poles of a permanent magnet, the magnetic field through the coil keeps changing and an e.m.f. is induced between its ends. Connected to a circuit, it drives an alternating current. This is the reverse of the magnetic effect of a current: a changing magnetic field produces a current (electromagnetic induction).
Moving-coil loudspeaker
Moving-coil loudspeaker: the voice coil on the cone sits over a pole of a permanent magnet.
A voice coil fixed to a paper cone fits over one pole of a strong permanent magnet.
The alternating current from the amplifier flows through the coil.
The current and the magnet's field give a force that keeps reversing, so the coil and cone vibrate in and out.
The vibrating cone pushes the air and produces sound.
Topic 6
Electromagnets
Definition
Electromagnet
A soft-iron core, such as a nail or rod, that becomes a magnet when an electric current flows through a coil of wire wound round it.
An iron nail with a coil connected to a cell becomes a magnet and picks up pins.
It is a temporary magnet: it is a magnet only while the current flows. Reversing the current swaps its poles.
Making an electromagnet stronger
Increase the current (more cells).
Increase the number of turns of the coil.
Use a soft-iron core.
Definition
Solenoid
A long coil of wire. When a current flows through it, it produces a magnetic field like that of a bar magnet.
Definition
Right-hand grip rule
Grip the solenoid with the right hand so that the fingers curl in the direction of the current in the turns; the thumb then points to the N-pole.
Right-hand grip rule: fingers along the current, thumb towards the N-pole.
Use
Why an electromagnet
Electric bell, telephone receiver
Magnetism switched on and off by the current
Relay and circuit breaker
Current controls a switch
Crane in a scrapyard
Lifts iron and drops it when switched off
Maglev train
Lifts and drives the train
Topic 7
Magnetic relay, circuit breaker and maglev train
Magnetic relay
Definition
Magnetic relay
A switch worked by an electromagnet. A small, safe current in the input circuit switches on a second circuit that carries a large current.
Relay: a small current in the coil pulls the iron armature, which closes the contacts of the high-current circuit.
Closing switch S lets a small current flow through the coil.
The coil becomes an electromagnet and pulls the iron armature.
The armature pushes the contacts together and the high-current circuit (a motor, say) is switched on.
Opening S releases the armature and a spring opens the contacts.
Circuit breaker
Definition
Circuit breaker
A safety switch that lets up to a set maximum current pass and switches the circuit OFF if the current becomes too large, protecting the wiring and appliances.
Circuit breaker: copper strip, iron armature, coil round an iron core, pivot and spring.
The current flows along a copper strip, through the iron armature and through the coil of an electromagnet.
Normally the electromagnet is too weak to move the armature.
If the current becomes too large, the electromagnet pulls the armature down; it swings about the pivot and the spring pulls the contacts apart.
The circuit is broken. It is reset by hand after the fault is cleared.
Maglev train
A magnetically levitated train has no wheels. Levitation electromagnets lift it a little above the guideway, and propulsion electromagnets pull and push it along. With no friction from the track it can reach about 400 km h⁻¹ quietly and smoothly.
Topic 8
Domain theory of magnetism
Electrons spinning and moving in their orbits produce tiny magnetic fields. How these add up in an atom decides the kind of material.
Definition
Magnetic dipole
An atom whose electron fields add up to a resultant field, so it behaves like a tiny magnet.
Type
In the atom
Examples
Diamagnetic
Fields of the orbital and spin motions cancel: no resultant field
Copper, bismuth, water
Paramagnetic
Fields support each other: each atom is a tiny magnet, but neighbours point at random
Aluminium, lithium
Ferromagnetic
Spins of large groups of neighbouring atoms (about 10¹⁶) line up parallel by themselves
Iron, steel, nickel, cobalt
Definition
Magnetic domain
A region about 0.1 mm across in a ferromagnetic material in which the atoms' fields all point the same way, so it is strongly magnetised and acts as a small magnet with its own N and S poles.
(a) Unmagnetised: domains point at random and cancel. (b) Magnetised: domains lined up with the external field.
Magnetising, in domain terms
In unmagnetised iron the domains point in random directions and their fields cancel.
In an external field, domains pointing along the field grow at the expense of the others, and the others turn towards the field.
With the domains lined up, the iron is a magnet with its own N and S poles.
Remember
Soft iron: domains line up easily and fall back to random when the field is removed (temporary magnet). Steel: hard to line up, but stays lined up (permanent magnet).
Topic 9
Making magnets
Stroking
(a) Single touch: one pole stroked along the bar again and again. (b) Double touch: unlike poles of two magnets stroked outwards from the centre.
Single touch: the bar lies on a table and is stroked from one end to the other, many times, always in the same direction with the same pole (say N), lifting the magnet high on the way back.
Double touch: starting at the centre, the unlike poles of two magnets are stroked outwards together, towards opposite ends. It is quicker and more effective.
Remember
The end of the bar where a stroke finishes gets the opposite pole to the stroking pole.
Using a solenoid (electrical method)
A steel bar inside a solenoid connected to a D.C. battery; the right-hand grip rule gives end A as the N-pole.
The steel bar is placed inside a solenoid of several hundred turns of insulated copper wire, and a direct current is switched on for a short time. The bar becomes a strong permanent magnet; its poles are given by the right-hand grip rule.
Topic 10
Demagnetisation and keeping magnets
Three ways to demagnetise
Heating: strong heating makes the atoms vibrate so violently that the domains lose their order.
Hammering: beating or dropping a magnet jolts its domains out of line.
Alternating current: the magnet is placed in a solenoid carrying A.C. and pulled out slowly. The field reverses again and again, getting weaker as the magnet moves away, and leaves the domains at random.
A magnet pulled slowly out of a solenoid carrying alternating current is demagnetised.
Storing magnets
Two bar magnets stored side by side with unlike poles together; soft-iron keepers P and Q close the loop.
Bar magnets slowly weaken if left alone. They are stored in pairs with unlike poles side by side and two pieces of soft iron, called keepers, across the ends. The field lines then run in a closed loop through the magnets and keepers, which keeps the domains lined up.
Topic 11
Magnetic recording and shielding
Definition
Magnetic recording
Storing sound, video or data by magnetising a magnetic material (tapes and disks coated with iron oxide) with an electromagnet.
Tape: sound is turned into a varying current; in the recording head (an electromagnet) it magnetises the oxide coating of the moving tape in a matching pattern.
Hard disk: a write head magnetises tiny spots on a spinning disk in one direction or the other, storing the binary digits 1 and 0; a read head detects the direction.
Advantage of a disk: it can reach any piece of information almost instantly, while finding a place on a tape can take minutes.
Definition
Magnetic permeability
The ability of a material to let magnetic lines of force pass through it.
A soft-iron ring draws the field lines into itself, so the space inside it is almost free of the field.
Strong magnetic fields can damage sensitive electronic devices. These are protected by enclosing them in soft iron, which has a very high permeability: the field lines pass through the casing rather than through the space inside. This is magnetic shielding.
The wording follows published solved exercises and every answer was re-checked by hand, so the wording or the order of options may differ slightly from your print.
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Set A: Textbook exercise 8 MCQs
The multiple-choice questions at the end of Chapter 8 in your book, in the book's option order.
Q8.1Magnetic Materials and Properties of Magnets
Which one of the following is not a magnetic material?
Answer(c) aluminium
Step by step
Magnetic materials: iron, nickel, cobalt (and steel).
Aluminium is not pulled by an ordinary magnet.
Why the others are wrong
(a) cobalt is a magnetic material.
(b) iron is the most common magnetic material.
(d) nickel is a magnetic material.
Remember
The magnetic three: iron, nickel, cobalt.
Q8.2Magnetic Field
Magnetic lines of force:
Answer(d) enter into the south pole
Step by step
Lines leave N and curve round to S.
They never cross, because the field has only one direction at a point.
Why the others are wrong
(a) they are curved round a bar magnet.
(b) lines of force never cross.
(c) they leave the north pole.
Remember
Out of North, into South.
Q8.3Magnetic Materials and Properties of Magnets
Permanent magnets cannot be made by:
Answer(a) soft iron
Step by step
Soft iron: domains line up easily and fall back easily.
Steel, alnico and neodymium keep their alignment: permanent magnets.
Why the others are wrong
(b) steel is used for permanent magnets.
(c) neodymium alloy makes very strong permanent magnets.
(d) alnico (aluminium-nickel-cobalt) is a permanent-magnet alloy.
Remember
Soft iron = temporary.
Q8.4Uses of Magnets and Electromagnets
Permanent magnets are used in:
Answer(b) loudspeaker
Step by step
Circuit breaker, crane, recording head: need magnetism that can be switched, so electromagnets.
Loudspeaker: the voice coil moves in the field of a permanent magnet.
Why the others are wrong
(a) a circuit breaker uses an electromagnet.
(c) a crane must drop its load, so it uses an electromagnet.
(d) the recording head is an electromagnet.
Remember
Switched on and off = electromagnet; always on = permanent.
Q8.5Magnetisation and Demagnetisation
A common method used to magnetise a material is:
Answer(a) stroking
Step by step
Magnetising: stroking, or a solenoid carrying D.C.
Demagnetising: heating, hammering, or a solenoid carrying A.C.
Why the others are wrong
(b) hitting (hammering) demagnetises.
(c) heating demagnetises.
(d) A.C. reverses the field again and again, so it demagnetises.
Remember
D.C. makes a magnet; A.C. unmakes it.
Q8.6Magnetic Field
A magnetic compass is placed at four points around a bar magnet whose N-pole is on the left and S-pole on the right: just left of N, just right of S, above the middle and below the middle. Which set of needle directions (left of N, right of S, above, below) is correct?
Answer(c) left, left, right, right
Step by step
Left of the N-pole: the field points away from N, so left.
Right of the S-pole: the field points into S, so left again.
Above and below the middle: lines run from N to S, so right in both places.
Why the others are wrong
(a) right of the S-pole the field goes into S (left), and below the magnet it runs N to S (right).
(b) above the magnet the field runs from N to S, which is to the right.
(d) the needle right of S must point into S, and both sides must point from N to S.
Remember
At both ends the needle points the same way; along the sides it points the other way.
Q8.7Magnetisation and Demagnetisation
A steel bar AB is magnetised by the double-touch method: starting at the centre, the S-pole of one magnet is stroked towards end A and the N-pole of another towards end B. What will be the polarity of the bar?
Answer(a) A becomes N and B becomes S
Step by step
Rule: the end where a stroke finishes gets the opposite pole to the stroking pole.
S strokes towards A, so A is N.
N strokes towards B, so B is S.
Why the others are wrong
(b) that gives each end the same polarity as its stroking pole.
(c) unlike poles stroke the two halves, so the bar becomes one simple magnet.
(d) the same mistake the other way round.
Remember
Stroke with S, get N at the end.
Q8.8Uses of Magnets and Electromagnets
The best material to protect a device from an external magnetic field is:
Answer(d) soft iron
Step by step
Wood and plastic let the field straight through.
Soft iron draws the field lines into itself, leaving the inside nearly field-free.
Why the others are wrong
(a) wood is non-magnetic; the field passes straight through.
(b) plastic is non-magnetic.
(c) steel is magnetic but its permeability is lower, and it can become a magnet itself.
Remember
Magnetic shield = soft iron.
Set B: More MCQs from this chapter 16 MCQs
Extra questions on the same ideas, of the kind set in board papers and class tests.
QB1Magnetic Materials and Properties of Magnets
A freely suspended bar magnet comes to rest pointing:
Answer(b) north-south
Step by step
The Earth behaves like a huge magnet.
A free magnet turns until it lies north-south.
Why the others are wrong
(a) it comes to rest north-south.
(c) a horizontally hung magnet stays horizontal.
(d) it always settles in the same direction.
Remember
This is how a compass works.
QB2Magnetic Materials and Properties of Magnets
The real test to identify a magnet is:
Answer(b) repulsion
Step by step
Attraction: could be a magnet or just a magnetic material.
Repulsion: only between like poles of two magnets.
Why the others are wrong
(a) plain iron is attracted too.
(c) weight tells nothing about magnetism.
(d) colour tells nothing.
Remember
Only a magnet can be repelled.
QB3Magnetic Materials and Properties of Magnets
A bar magnet is broken into two pieces. Each piece will:
Answer(c) be a complete magnet with an N-pole and an S-pole
Step by step
Each tiny domain is a complete magnet.
So any piece, however small, has both poles.
Why the others are wrong
(a) a single N-pole cannot exist.
(b) a single S-pole cannot exist.
(d) the pieces stay magnetised.
Remember
No magnet has only one pole.
QB4Magnetic Field
The direction of the magnetic field at a point is the direction in which:
Answer(b) the N-pole of a compass needle points
Step by step
Place a small compass at the point.
Its N-pole points along the field.
Why the others are wrong
(a) the S-pole points opposite to the field.
(c) filings show the pattern, not the direction.
(d) moving the magnet is not the definition.
Remember
Follow the N of the compass.
QB5Magnetic Field
Around a bar magnet, the magnetic field is strongest:
Answer(b) near the poles
Step by step
Field strength ∝ number of lines through unit area.
Lines are closest together at the poles.
Why the others are wrong
(a) the field is weakest beside the middle.
(c) far away the lines are spread out.
(d) it varies from place to place.
Remember
Crowded lines = strong field.
QB6Magnetic Field
A neutral point between two magnets is a point where:
Answer(b) the fields of the two magnets cancel each other
Step by step
Put like poles facing each other.
Midway, the two fields are equal and opposite: the net field is zero.
Why the others are wrong
(a) it is where the field is zero.
(c) lines of force never cross.
(d) there is no pole there.
Remember
Neutral = cancelled.
QB7Uses of Magnets and Electromagnets
The core of an electromagnet is made of soft iron because soft iron:
Answer(b) is magnetised strongly when the current flows and loses it when the current stops
Step by step
An electromagnet must be switchable.
Soft iron gains and loses magnetism easily.
Why the others are wrong
(a) a permanent core would not switch off.
(c) soft iron is strongly magnetic.
(d) cost is not the reason.
Remember
Switch on, magnet; switch off, no magnet.
QB8Uses of Magnets and Electromagnets
The strength of an electromagnet can be increased by:
Answer(b) increasing the number of turns of the coil
Step by step
Each turn adds its field.
So more turns or more current give a stronger magnet.
Why the others are wrong
(a) less current makes it weaker.
(c) wood is non-magnetic; it would weaken it.
(d) reversing only swaps the poles.
Remember
More turns, more current.
QB9Magnetisation and Demagnetisation
By the right-hand grip rule for a solenoid, when the fingers curl in the direction of the current, the thumb points towards the:
Answer(b) N-pole
Step by step
Curl the right-hand fingers in the direction of the current in the turns.
Magnets that are magnets only while they are in the field of another magnet (or while a current flows), and lose their magnetism when it is removed. Example: soft iron, an electromagnet.
Definition
Permanent magnets
Magnets that keep their magnetism for a long time. They are found in nature or made from steel and special alloys (alnico, ferrite, neodymium) placed in a strong field.
Short Q 8.2
Define magnetic field of a magnet.
Answer
Definition
Magnetic field
The region around a magnet in which another magnet or a magnetic material experiences a magnetic force.
Short Q 8.3
What are magnetic lines of force?
Answer
Definition
Magnetic lines of force
Imaginary lines drawn to represent a magnetic field. They follow the pattern of iron filings round a magnet, go from N to S outside the magnet, never cross, and are closest together where the field is strongest.
Lines of force round a bar magnet.
Short Q 8.4
Name some uses of permanent magnets and electromagnets.
Answer
Permanent magnets
Electromagnets
D.C. motors
Electric bells
A.C. and D.C. generators
Telephone receivers
Moving-coil loudspeakers
Magnetic relays and circuit breakers
Door catchers, compasses
Cranes, maglev trains
Short Q 8.5
What are magnetic domains?
Answer
Definition
Magnetic domain
A tiny region, about 0.1 mm across, in a ferromagnetic material in which the magnetic fields of about 10¹⁶ atoms point the same way. Each domain is strongly magnetised and acts as a small magnet with its own N and S poles.
Short Q 8.6
Which type of magnetic field is formed by a current-carrying long coil?
Answer
A long current-carrying coil (a solenoid) produces a field like that of a bar magnet: one end acts as an N-pole and the other as an S-pole, and the field inside is strong and nearly uniform. The N end is found by the right-hand grip rule.
Right-hand grip rule for a solenoid.
Short Q 8.7
Differentiate between paramagnetic and diamagnetic materials.
Answer
Paramagnetic
Diamagnetic
The fields of the electrons' orbital and spin motions support each other
These test whether you really understand. Give a reason every time.
CRQ 8.1
Two bar magnets are stored in a wooden box. Label the poles of the magnets and identify the objects P and Q.
Answer
Unlike poles side by side; P and Q are keepers.
The magnets are stored side by side with unlike poles together: if the top of the left magnet is N, the top of the right magnet is S, and the bottom ends are S and N.
P and Q are keepers: pieces of soft iron placed across the ends.
With keepers, the lines of force go round a closed loop (N → keeper → S through each magnet), so the domains stay lined up and the magnets do not lose their strength.
CRQ 8.2
A steel bar has to be magnetised by placing it inside a solenoid such that end A of the bar becomes the N-pole and end B the S-pole. Draw the circuit diagram of the solenoid showing the steel bar inside it.
Answer
Steel bar AB inside a solenoid connected to a D.C. battery.
Wind the solenoid round the bar and connect it to a D.C. battery through a switch.
Choose the current direction with the right-hand grip rule: grip the coil with the fingers along the current; the thumb must point to end A.
Viewed from end A, the current in the turns must flow anticlockwise.
Switch on for a short time: A becomes the N-pole and B the S-pole.
Exam tip
If the battery is reversed, the poles are reversed too. A.C. would not work: it would demagnetise the bar.
CRQ 8.3
Two bar magnets are lying as shown in the figure. A compass is placed at the middle of the gap. Its needle settles in the north-south direction. Label the N and S poles of the magnets. Justify your answer by drawing field lines.
Answer
Figure (b): like poles facing each other, with a neutral point X between them.
The needle settles north-south, the way it would with no magnets near it. So the magnets' fields cancel at that point: it is a neutral point.
A neutral point midway between two magnets forms only when like poles face each other: N facing N (or S facing S). The field of each points away from its own N-pole, so midway they are equal and opposite and cancel, leaving only the Earth's field to set the compass.
CRQ 8.4
Electric current or the motion of electrons produces a magnetic field. Is the reverse process true, that is, does a magnetic field give rise to electric current? If yes, give an example and describe it briefly.
Answer
Yes, but only a changing magnetic field produces a current. This is electromagnetic induction.
Example: an A.C. generator. A coil is rotated between the poles of a permanent magnet. The field through the coil keeps changing, so an e.m.f. is induced and an alternating current flows in the circuit connected to it. Pushing a bar magnet in and out of a coil connected to a galvanometer shows the same effect.
CRQ 8.5
Four similar solenoids are placed in a circle as shown in the figure, with the same current in each. Show by diagram the direction of current in each solenoid such that when the current in any one solenoid is switched OFF, the net magnetic field at the centre O is directed towards that solenoid. Explain your answer.
Answer
Each solenoid has its N-pole facing O, so its field at O points away from it.
Pass the current so that the end of every solenoid facing O is an N-pole (viewed from O, the current in that end flows anticlockwise). Each solenoid's field at O then points away from it.
With all four ON, opposite solenoids (A and C, B and D) give equal and opposite fields at O, so the net field is zero.
Switch A OFF: B and D still cancel, but C's field at O is left, pointing away from C, which is towards A.
The same happens for whichever solenoid is switched off.
Remember
The field of the switched-off solenoid's partner is what is left, and it points away from that partner, towards the switched-off one.
Set A: Textbook comprehensive questions 6 questions
The long questions at the end of the chapter in your book.
Long Q 8.1
How can you identify whether an object is a magnet or a magnetic material?
Answer
Magnet and magnetic material
A magnet attracts both another magnet (opposite pole) and plain magnetic material such as iron, so attraction alone cannot tell them apart.
Method
Suspend a bar magnet freely so that it can turn.
Bring one end of the object near the N-pole of the suspended magnet. Suppose it is attracted: the end is either an S-pole or plain iron.
Now bring the same end near the S-pole of the suspended magnet.
If it is attracted again, the object is only a magnetic material: it is attracted to both poles because magnetism is induced in it.
If it is repelled, the object is a magnet: its end is an S-pole.
Only like poles repel.
Conclusion
Repulsion is the real test to identify a magnet.
Long Q 8.2
Describe the strength of a magnetic field in terms of magnetic lines of force. Explain it by drawing a few diagrams for the fields as examples.
Answer
Strength of a magnetic field
The strength of a magnetic field is proportional to the number of lines of force passing through unit area held at right angles to the lines. Where the lines are close together the field is strong; where they spread out it is weak.
Bar magnet
Lines are crowded at the poles: the field is strongest there.
Combinations of magnets
Three field patterns.
(a) Unlike poles facing: the lines run straight across the gap from N to S and are crowded: a strong field between them, and the magnets attract.
(b) Like poles facing: the lines push away from each other; at point X they cancel. X is a neutral point, where the field is zero.
(c) Horseshoe magnet: between its poles the lines are straight, parallel and evenly spaced: a strong, nearly uniform field (except near the edges).
Conclusion
Close lines: strong field. Far apart: weak field. No lines (neutral point): zero field.
Long Q 8.3
What is a circuit breaker? Describe its working with the help of a diagram.
Answer
Definition
Definition
Circuit breaker
A safety switch that passes up to a certain maximum current safely and switches OFF the circuit automatically if the current becomes too large.
Diagram
Copper strip, switch contacts, iron armature, pivot, spring and coil on an iron core.
Construction
A copper strip carries the current to the switch contacts.
An iron armature, pivoted, holds the contacts closed against a spring.
A coil round an iron core (an electromagnet) is in series with the circuit.
Working
The current flows along the copper strip, through the armature and the coil of the electromagnet.
With a normal current, the electromagnet is too weak to move the armature.
If the current becomes excessive (a short circuit or overload), the electromagnet becomes strong enough to pull the armature.
The armature turns about the pivot, the spring pulls the contacts apart and the circuit is broken.
After the fault is repaired, the breaker is reset by hand.
Advantage
Unlike a fuse, a circuit breaker does not need replacing: it is simply reset.
Long Q 8.4
A magnet attracts only a magnet. Explain the statement.
Answer
The statement is not correct
A magnet attracts both other magnets (their opposite poles) and magnetic materials such as iron, nickel and cobalt.
Why a magnet attracts iron
The magnet induces an opposite pole in the near end of the iron.
The field of the magnet passes into the unmagnetised iron and induces magnetism in it.
The near end of the iron becomes the opposite pole: the S-pole of the magnet induces an N-pole at the near end and an S-pole at the far end.
Opposite poles attract, so the iron is pulled towards the magnet.
The induced magnetism of soft iron disappears when the magnet is removed.
Conclusion
A magnet attracts magnets and magnetic materials; it repels only magnets.
Long Q 8.5
Differentiate between paramagnetic, diamagnetic and ferromagnetic materials with reference to the domain theory.
Answer
Origin of magnetism
Every electron spins and moves in an orbit, and each of these motions produces a tiny magnetic field. The way they combine decides the type of material.
Diamagnetic materials
The fields of the orbital and spin motions add up to zero, so the atom has no resultant field. Examples: copper, bismuth, water.
Paramagnetic materials
The fields support each other, so each atom is a tiny magnet (a magnetic dipole). The atoms point at random, so the material is only weakly attracted. Examples: aluminium, lithium.
Ferromagnetic materials
In iron, steel, nickel and cobalt, the spins of large groups of neighbouring atoms (about 10¹⁶) line up parallel by themselves, forming domains about 0.1 mm across, each a strong small magnet.
Random domains (unmagnetised) and aligned domains (magnetised).
Unmagnetised: the domains point at random and cancel, so the material shows no magnetism.
In an external field: domains along the field grow at the expense of the others, and the others rotate towards the field.
Result: the material becomes a magnet with N and S poles.
Soft iron's domains return to random when the field is removed (temporary); steel's stay aligned (permanent).
Type
Atom
Domains
Examples
Diamagnetic
no resultant field
none
Cu, Bi, water
Paramagnetic
tiny magnet
none
Al, Li
Ferromagnetic
tiny magnet
yes, aligned groups
Fe, Ni, Co, steel
Long Q 8.6
Why are ferromagnetic materials suitable for making magnets?
Answer
Domains
In ferromagnetic materials (iron, steel, nickel, cobalt) large groups of neighbouring atoms have their electron spins lined up parallel by nature. These groups form domains, each about 0.1 mm across and each a strong little magnet.
Easy to magnetise strongly
Because the atoms in each domain are already aligned, an external field only has to turn and grow the domains, not line up each atom one by one. So a moderate field produces a very strong magnet. Para- and diamagnetic materials have no domains and can hardly be magnetised.
Aligning the domains makes the material a magnet.
Temporary or permanent, as needed
Soft iron: domains align easily and return to random easily, which is ideal for electromagnets, relays and transformer cores.
Steel and alloys: domains are hard to align but then stay aligned, which is ideal for permanent magnets.
Conclusion
Only ferromagnetic materials have domains, so only they can be made into useful magnets.
Set B: More important long questions 6 questions
Long questions often set in board papers from this chapter, answered in the same full style.
Imp. LQ 1
What is an electromagnet? How can its strength be increased? Give its uses.
Answer
Definition
Definition
Electromagnet
A soft-iron core that becomes a magnet when a current flows through a coil of wire wound round it.
Diagram
A coil on an iron nail, connected to a cell, picks up pins.
Working
While the current flows, the coil's field magnetises the soft-iron core strongly. When the current stops, the core loses its magnetism, so an electromagnet is a temporary magnet that can be switched on and off. Its poles follow the right-hand grip rule; reversing the current swaps them.
Increasing its strength
Increase the current (use more cells).
Increase the number of turns of the coil.
Use a soft-iron core.
Uses
Electric bells and telephone receivers.
Magnetic relays and circuit breakers.
Cranes that lift scrap iron.
Maglev trains.
Recording heads of tapes and hard disks.
Imp. LQ 2
Describe the methods of making a magnet. State the right-hand grip rule.
Answer
1. Single-touch method
A steel bar is laid on a table and stroked from one end to the other, many times, always in the same direction with the same pole of a permanent magnet, lifting it high above the bar on the way back.
2. Double-touch method
Unlike poles of two magnets are placed together at the centre of the bar and stroked outwards towards opposite ends at the same time, many times. It is more effective than single touch.
Single touch and double touch.
Remember
In both, the end where the stroke finishes gets the pole opposite to the stroking pole.
3. Electrical method
Steel bar in a solenoid connected to D.C.
The steel bar is placed inside a solenoid of several hundred turns and a direct current is passed for a short time. This is the strongest and quickest method.
Right-hand grip rule
Definition
Right-hand grip rule
Grip the solenoid with the right hand so that the fingers curl in the direction of the current; the thumb then points towards the N-pole.
Thumb points to the N-pole.
Imp. LQ 3
What is demagnetisation? Describe three methods of demagnetising a magnet. How are magnets stored?
Answer
Demagnetisation
Destroying the magnetism of a magnet by putting its domains back into random directions.
1. Heating
Strong heating makes the atoms vibrate violently, which breaks up the order of the domains; the magnet loses its magnetism quickly.
2. Hammering
Beating a magnet with a hammer (or dropping it repeatedly) jolts the domains out of alignment.
3. Alternating current
Magnet pulled out slowly from a solenoid carrying A.C.
The magnet is placed inside a solenoid carrying A.C. and pulled out slowly. The field keeps reversing and grows weaker as the magnet leaves, so the domains end up at random. This is the best method.
Storing magnets
Magnets stored with keepers.
Store them in pairs with unlike poles side by side and soft-iron keepers across the ends, so the field forms a closed loop. Do not drop or heat them.
Imp. LQ 4
Describe the construction and working of a magnetic relay. How does a maglev train use electromagnets?
Answer
Magnetic relay
Definition
Relay
A switch worked by an electromagnet, which lets a small current switch on a circuit carrying a large current.
Input circuit with coil; output circuit with contacts.
Construction
An input circuit: a cell, a switch and a coil on a soft-iron core.
A pivoted iron armature near the core, held back by a spring.
Contacts in a second (output) circuit carrying a large current.
Working
Closing the input switch sends a small current through the coil, which becomes an electromagnet.
It pulls the armature, which closes the contacts of the output circuit.
The high-current device (a motor or a car's starter) starts. Opening the switch lets the spring open the contacts.
The person works only the safe low-current circuit, which is the purpose of a relay.
Maglev train
It has no wheels. Levitation electromagnets lift it slightly above the guideway, and propulsion electromagnets attract and repel it forward. With no friction from the track it moves at about 400 km h⁻¹, smoothly and quietly.
Imp. LQ 5
Describe the uses of permanent magnets in a moving-coil loudspeaker and an A.C. generator.
Answer
Moving-coil loudspeaker
Voice coil, cone and permanent magnet.
A voice coil fixed to a paper cone is slipped over one pole of a strong permanent magnet.
The alternating current from the amplifier flows through the coil.
The current in the magnet's field feels a force that keeps reversing, so the coil and the cone vibrate.
The vibrating cone produces sound waves in the air, matching the current.
A.C. generator
A coil is rotated between the poles of a permanent magnet.
The magnetic field through the coil keeps changing as it turns.
An e.m.f. is induced between the ends of the coil (electromagnetic induction).
Connected to a circuit, it drives an alternating current, which reverses every half turn.
Other uses
D.C. motors
Door catchers
Compasses
Separating iron from scrap
Imp. LQ 6
Explain magnetic recording and magnetic shielding.
Answer
Magnetic recording
Sound, video and data are stored by magnetising a material coated with iron oxide, using an electromagnet called a head.
Tape: sound is changed into a varying current; in the recording head it magnetises the moving tape in a matching pattern. On playback the pattern produces the current again.
Hard disk: the write head magnetises tiny spots on a spinning disk in one of two directions, storing 1s and 0s; the read head detects the direction. A disk finds any data almost instantly, while a tape may take minutes.
Magnetic shielding
Definition
Permeability
The ability of a material to let lines of force pass through it.
Field lines pass through the soft-iron ring, not the space inside.
Sensitive devices are enclosed in a box of soft iron. Its permeability is very high, so the lines of force crowd into the iron and pass round the inside, which stays almost free of the field.
Dia: Cu, Bi, water · Para: Al, Li · Ferro: Fe, Ni, Co
Demagnetise by
Heating, hammering, A.C.
Key definitions 21 terms
Magnetism
The force that a magnet exerts at a distance on magnetic materials and on other magnets.
Magnetic materials
Materials that are attracted by a magnet, such as iron, nickel, cobalt and steel.
Non-magnetic materials
Materials that are not attracted by a magnet, such as copper, brass, aluminium, wood, glass and plastic.
Magnetic poles
The ends of a magnet, where its magnetism is strongest. A freely suspended bar magnet comes to rest pointing north-south; the end pointing north is its north pole (N), the other its south pole (S).
Induced magnetism
The magnetism produced in a magnetic material, such as iron or steel, when it is placed near or touching a magnet. Also called magnetisation.
Temporary magnets
Magnets that act only while they are in the field of another magnet (or while a current flows) and lose their magnetism when it is removed. Soft iron, paper clips, iron nails and electromagnets are examples.
Permanent magnets
Magnets that keep their magnetism for a long time. They are found in nature (lodestone) or made by putting steel or special alloys (alnico, ferrite, neodymium) in a strong field for long enough.
Magnetic field
The region around a magnet in which another magnet or a magnetic material experiences a magnetic force.
Magnetic lines of force
Imaginary lines drawn to show a magnetic field. They follow the pattern of iron filings round a magnet and show the direction and the strength of the field.
Direction of the field
The direction in which the N-pole of a small compass needle points when placed at that point.
Strength of the field
Proportional to the number of lines of force passing through unit area held at right angles to them: strong where the lines are close together, weak where they are far apart.
Neutral point
A point where the field of one magnet exactly cancels the field of another, so the net field is zero. It forms between like poles facing each other (point X in figure b).
Electromagnet
A soft-iron core, such as a nail or rod, that becomes a magnet when an electric current flows through a coil of wire wound round it.
Solenoid
A long coil of wire. When a current flows through it, it produces a magnetic field like that of a bar magnet.
Right-hand grip rule
Grip the solenoid with the right hand so that the fingers curl in the direction of the current in the turns; the thumb then points to the N-pole.
Magnetic relay
A switch worked by an electromagnet. A small, safe current in the input circuit switches on a second circuit that carries a large current.
Circuit breaker
A safety switch that lets up to a set maximum current pass and switches the circuit OFF if the current becomes too large, protecting the wiring and appliances.
Magnetic dipole
An atom whose electron fields add up to a resultant field, so it behaves like a tiny magnet.
Magnetic domain
A region about 0.1 mm across in a ferromagnetic material in which the atoms' fields all point the same way, so it is strongly magnetised and acts as a small magnet with its own N and S poles.
Magnetic recording
Storing sound, video or data by magnetising a magnetic material (tapes and disks coated with iron oxide) with an electromagnet.
Magnetic permeability
The ability of a material to let magnetic lines of force pass through it.
Common mistakes to avoid
Saying attraction proves an object is a magnet: only repulsion does.
Drawing lines of force that cross, or that enter the N-pole.
Saying soft iron makes good permanent magnets: it makes temporary ones.
Magnetising with A.C.: A.C. demagnetises; magnetise with D.C.
In stroking, giving the end the same pole as the stroking pole: it gets the opposite pole.
Saying a magnetic field always produces a current: only a changing field does.
Calling aluminium a magnetic material.
Thinking a broken magnet gives a separate N and S pole.
Extra practice tap a question to see its answer
Name three magnetic materials.Show answer
Iron, nickel, cobalt
Which end of a freely suspended magnet points north?Show answer
Its north pole
An object is attracted by both poles of a magnet. What is it?Show answer
A magnetic material, not a magnet
What happens if a bar magnet is cut into three pieces?Show answer
Three magnets, each with N and S poles
Where is the field of a bar magnet strongest?Show answer
Near the poles
What are P and Q placed across stored magnets called?Show answer
Keepers (soft iron)
Give two ways to make an electromagnet stronger.Show answer
More turns; more current
Which device lets a small current switch a large one?Show answer
A magnetic relay
Why does a circuit breaker not need replacing like a fuse?Show answer
It is simply reset after it trips
Give an example of a diamagnetic material.Show answer
Copper (or bismuth, water)
Why is steel used for permanent magnets?Show answer
Its domains stay aligned once aligned
Which current is used to magnetise a bar in a solenoid?Show answer
Direct current (D.C.)
Name the best method of demagnetisation.Show answer
Pulling the magnet slowly out of a solenoid carrying A.C.
Which material protects a watch from magnetic fields?Show answer
Soft iron (high permeability)
Final self-test
Final self-test: all 24 MCQs, no answers
Give yourself 25 minutes. Pick one option for each question, then submit to see your score and which questions to revise. Any question you get wrong tells you which topic to read again.
