A Crane Picks Up Scrap Iron and Drops It by Flicking a Switch
Learn how the Earth behaves as a giant magnet, make an electromagnet and list what sets its strength, compare permanent with temporary magnets, and know the uses, demagnetisation methods and storage precautions.
How can a crane hold tonnes of iron and then let go instantly?
A scrapyard crane lifts a heap of iron, swings it across the yard, and drops it exactly where it is wanted. No hooks, no chains, no grabbing.
A permanent magnet could never do that. It would pick the iron up perfectly well and then refuse to put it down, because its magnetism cannot be switched off.
The crane uses an electromagnet — a coil of insulated wire wound round a soft iron core. Send a current through the coil and the core becomes a powerful magnet. Break the circuit and the magnetism vanishes, and the load falls.
Two things make that work. The current creates the magnetism, so a switch controls it. And the soft iron core loses its magnetism the instant the current stops, because soft iron keeps almost nothing of what it was given.
Use a steel core instead and the crane would jam on its first lift — steel holds on to magnetism, which is exactly why steel is used for permanent magnets and never for an electromagnet.
So the choice of material is the whole design. This page covers the second part of the ICSE Class 9 Physics chapter on magnetism — the Earth's magnetic field, making an electromagnet, permanent against temporary magnets, and the uses, demagnetisation and care of magnets.
A permanent magnet could never do that. It would pick the iron up perfectly well and then refuse to put it down, because its magnetism cannot be switched off.
The crane uses an electromagnet — a coil of insulated wire wound round a soft iron core. Send a current through the coil and the core becomes a powerful magnet. Break the circuit and the magnetism vanishes, and the load falls.
Two things make that work. The current creates the magnetism, so a switch controls it. And the soft iron core loses its magnetism the instant the current stops, because soft iron keeps almost nothing of what it was given.
Use a steel core instead and the crane would jam on its first lift — steel holds on to magnetism, which is exactly why steel is used for permanent magnets and never for an electromagnet.
So the choice of material is the whole design. This page covers the second part of the ICSE Class 9 Physics chapter on magnetism — the Earth's magnetic field, making an electromagnet, permanent against temporary magnets, and the uses, demagnetisation and care of magnets.
How does the Earth behave as a magnet?
The Earth behaves as though an enormous bar magnet were buried along its axis, with its magnetic SOUTH pole near the geographic NORTH.
That arrangement is forced by what a compass does. A compass needle's north pole swings towards geographic north, and a north pole is attracted only by a south pole. So whatever lies up there must be magnetically south.
The pattern of the field lines. Outside the Earth the lines run from the Earth's magnetic north pole — near the geographic south — round to its magnetic south pole near the geographic north. So over the surface they point broadly from geographic south towards geographic north, which is the direction a compass reports.
Over a small region the field is uniform. On a laboratory bench, or across a sheet of paper, the Earth's field lines are effectively parallel, equally spaced straight lines running from geographic south to geographic north. That is the uniform-field pattern of the previous part of this chapter, and it is why plotting a bar magnet's field on paper produces neutral points at all — the magnet's curved field is competing with a straight, even one.
Two angles describe the field at any place.
- The magnetic meridian is the vertical plane containing the Earth's field at that place; the geographic meridian is the vertical plane through the geographic poles. The angle between them is the angle of declination
- The Earth's field is not horizontal except near the equator. The angle it makes with the horizontal is the angle of dip, which is near zero at the magnetic equator and near ninety degrees at the magnetic poles
Only the horizontal part turns a compass. A flat compass needle is free to swing in a horizontal plane, so it responds to the horizontal component of the Earth's field and not to the whole of it. That is why a compass still works where the dip is large, though the needle tries to tip and has to be balanced.
Magnetic north and geographic north are not the same direction. The angle between them — the declination — differs from place to place, so a navigator using a compass has to correct for it. A compass points to magnetic north, and a map is drawn to geographic north, and confusing the two is a real navigational error rather than a technicality.
That arrangement is forced by what a compass does. A compass needle's north pole swings towards geographic north, and a north pole is attracted only by a south pole. So whatever lies up there must be magnetically south.
The pattern of the field lines. Outside the Earth the lines run from the Earth's magnetic north pole — near the geographic south — round to its magnetic south pole near the geographic north. So over the surface they point broadly from geographic south towards geographic north, which is the direction a compass reports.
Over a small region the field is uniform. On a laboratory bench, or across a sheet of paper, the Earth's field lines are effectively parallel, equally spaced straight lines running from geographic south to geographic north. That is the uniform-field pattern of the previous part of this chapter, and it is why plotting a bar magnet's field on paper produces neutral points at all — the magnet's curved field is competing with a straight, even one.
Two angles describe the field at any place.
- The magnetic meridian is the vertical plane containing the Earth's field at that place; the geographic meridian is the vertical plane through the geographic poles. The angle between them is the angle of declination
- The Earth's field is not horizontal except near the equator. The angle it makes with the horizontal is the angle of dip, which is near zero at the magnetic equator and near ninety degrees at the magnetic poles
Only the horizontal part turns a compass. A flat compass needle is free to swing in a horizontal plane, so it responds to the horizontal component of the Earth's field and not to the whole of it. That is why a compass still works where the dip is large, though the needle tries to tip and has to be balanced.
Magnetic north and geographic north are not the same direction. The angle between them — the declination — differs from place to place, so a navigator using a compass has to correct for it. A compass points to magnetic north, and a map is drawn to geographic north, and confusing the two is a real navigational error rather than a technicality.
Formula
How do you make an electromagnet, and what sets its strength?
Wind insulated copper wire round a soft iron core and pass a current through it. The core is magnetic only while the current flows.
The construction.
- Take a soft iron nail or rod as the core
- Wind insulated copper wire round it in many closely packed turns, all in the same direction
- Connect the two ends to a cell through a key and, if the current is to be varied, a rheostat
- Close the key: the nail becomes a magnet and picks up pins. Open the key: the pins drop
The insulation matters. Bare wire would let the current short across from turn to turn instead of going round the coil, so the magnetising effect would collapse. The wire must be insulated and the core must not be, which is why enamelled copper wire is used.
The factors that decide its strength.
- The number of turns — more turns give a stronger magnet
- The current — a larger current gives a stronger magnet
- The core material — soft iron gives a strong magnet that demagnetises completely; steel would give a weaker one that would not switch off
- The shape of the core — a U-shaped (horseshoe) electromagnet is stronger than an I-shaped (straight) one of the same coil, because both its poles act on the same piece of iron at once
- The length and thickness of the core — a shorter, thicker core is more effective than a long thin one
The magnetising effect depends on the PRODUCT of turns and current, measured in ampere-turns:
Worked example 1. A coil of turns carries A:
Worked example 2. A coil of turns carries A:
The same magnetising effect from both — half the turns with twice the current gives an identical result. So turns and current are interchangeable in this product, which is why a strong electromagnet can be built either by winding many turns or by driving a large current.
Worked example 3. A coil of turns at A gives ampere-turns — twice the effect of worked example 1.
Worked example 4 — reaching a target. How many turns are needed to give ampere-turns at a current of A?
There is a practical limit on the current and none on the turns. A large current heats the wire, and too large a current melts the insulation. Adding turns costs nothing but wire, which is why real electromagnets have many thousands of turns and modest currents rather than the reverse.
The core is not what carries the current. The current flows entirely in the coil; the core's job is to concentrate the magnetic effect, and a coil with no core is still a magnet, just a much weaker one. Removing the core does not break the circuit — it only weakens the magnet, which is the clearest evidence that the coil and the core are doing two separate jobs.
The construction.
- Take a soft iron nail or rod as the core
- Wind insulated copper wire round it in many closely packed turns, all in the same direction
- Connect the two ends to a cell through a key and, if the current is to be varied, a rheostat
- Close the key: the nail becomes a magnet and picks up pins. Open the key: the pins drop
The insulation matters. Bare wire would let the current short across from turn to turn instead of going round the coil, so the magnetising effect would collapse. The wire must be insulated and the core must not be, which is why enamelled copper wire is used.
The factors that decide its strength.
- The number of turns — more turns give a stronger magnet
- The current — a larger current gives a stronger magnet
- The core material — soft iron gives a strong magnet that demagnetises completely; steel would give a weaker one that would not switch off
- The shape of the core — a U-shaped (horseshoe) electromagnet is stronger than an I-shaped (straight) one of the same coil, because both its poles act on the same piece of iron at once
- The length and thickness of the core — a shorter, thicker core is more effective than a long thin one
The magnetising effect depends on the PRODUCT of turns and current, measured in ampere-turns:
Worked example 1. A coil of turns carries A:
Worked example 2. A coil of turns carries A:
The same magnetising effect from both — half the turns with twice the current gives an identical result. So turns and current are interchangeable in this product, which is why a strong electromagnet can be built either by winding many turns or by driving a large current.
Worked example 3. A coil of turns at A gives ampere-turns — twice the effect of worked example 1.
Worked example 4 — reaching a target. How many turns are needed to give ampere-turns at a current of A?
There is a practical limit on the current and none on the turns. A large current heats the wire, and too large a current melts the insulation. Adding turns costs nothing but wire, which is why real electromagnets have many thousands of turns and modest currents rather than the reverse.
The core is not what carries the current. The current flows entirely in the coil; the core's job is to concentrate the magnetic effect, and a coil with no core is still a magnet, just a much weaker one. Removing the core does not break the circuit — it only weakens the magnet, which is the clearest evidence that the coil and the core are doing two separate jobs.
What is the difference between a permanent and a temporary magnet?
A permanent magnet keeps its magnetism indefinitely; a temporary magnet has it only while something maintains it. The difference comes down to the material.
A permanent magnet.
- Made of steel, or an alloy such as alnico
- Retains its magnetism for a very long time
- Its magnetism cannot be switched off or varied
- Harder to magnetise in the first place
- Generally weaker than an electromagnet of comparable size
- Examples: a bar magnet, a compass needle, a fridge magnet, the magnet in a loudspeaker
A temporary magnet, of which the electromagnet is the standard example.
- Made with a soft iron core
- Loses its magnetism as soon as the current stops
- Its magnetism can be switched on and off, and its strength varied by changing the current
- Easier to magnetise
- Can be made extremely strong
- Examples: an electromagnet, an electric bell's core, a transformer core
The property behind all of it is retentivity. Soft iron has low retentivity — it takes magnetism readily and gives it up readily. Steel has high retentivity — it resists being magnetised and then resists being demagnetised.
Worked comparison — why each material suits its job. A scrapyard crane needs magnetism it can cancel, so soft iron. A compass needs magnetism that survives being carried about for years, so steel. Swap the materials and both devices fail: a steel-cored crane would not release its load, and a soft-iron compass needle would lose its poles almost at once.
An electromagnet is not a weak magnet. It can be made far stronger than any permanent magnet of the same size, simply by adding turns or current. So "temporary" describes how long the magnetism lasts, not how strong it is — and that distinction is often assumed the wrong way round.
Soft iron is not soft to the touch. The word describes its magnetic behaviour, not its hardness — soft iron is a perfectly hard metal that happens to be magnetically easy-going. Steel is an alloy of iron with carbon, and the carbon is what makes it magnetically stubborn, so the two materials are chemically close and magnetically opposite.
A permanent magnet.
- Made of steel, or an alloy such as alnico
- Retains its magnetism for a very long time
- Its magnetism cannot be switched off or varied
- Harder to magnetise in the first place
- Generally weaker than an electromagnet of comparable size
- Examples: a bar magnet, a compass needle, a fridge magnet, the magnet in a loudspeaker
A temporary magnet, of which the electromagnet is the standard example.
- Made with a soft iron core
- Loses its magnetism as soon as the current stops
- Its magnetism can be switched on and off, and its strength varied by changing the current
- Easier to magnetise
- Can be made extremely strong
- Examples: an electromagnet, an electric bell's core, a transformer core
The property behind all of it is retentivity. Soft iron has low retentivity — it takes magnetism readily and gives it up readily. Steel has high retentivity — it resists being magnetised and then resists being demagnetised.
Worked comparison — why each material suits its job. A scrapyard crane needs magnetism it can cancel, so soft iron. A compass needs magnetism that survives being carried about for years, so steel. Swap the materials and both devices fail: a steel-cored crane would not release its load, and a soft-iron compass needle would lose its poles almost at once.
An electromagnet is not a weak magnet. It can be made far stronger than any permanent magnet of the same size, simply by adding turns or current. So "temporary" describes how long the magnetism lasts, not how strong it is — and that distinction is often assumed the wrong way round.
Soft iron is not soft to the touch. The word describes its magnetic behaviour, not its hardness — soft iron is a perfectly hard metal that happens to be magnetically easy-going. Steel is an alloy of iron with carbon, and the carbon is what makes it magnetically stubborn, so the two materials are chemically close and magnetically opposite.
What are electromagnets used for, and how is a magnet cared for?
Electromagnets are used wherever magnetism has to be switched, varied or made very strong.
Uses of electromagnets.
- Lifting and moving heavy iron — a scrapyard crane picks up a load and releases it by breaking the circuit
- Separating iron from a mixture of scrap or from other ores on a moving belt
- The electric bell and buzzer — the electromagnet pulls an armature that strikes the gong and breaks its own circuit, so the process repeats
- Loudspeakers, headphones and microphones — a varying current in a coil makes a diaphragm vibrate, or the reverse
- Electric motors and generators, where the magnetic field is produced by coils
- Relays and circuit breakers, where a small current switches a large one, or an overload trips the supply
- Telephone earpieces and telegraph receivers
- In medicine — removing iron splinters from a patient's eye, and providing the very strong field of a magnetic resonance scanner
Methods of demagnetisation.
- Heating the magnet strongly and allowing it to cool while lying east-west, so that the Earth's field cannot re-magnetise it
- Hammering it roughly, or dropping it repeatedly, again lying east-west
- Passing alternating current through a coil wound around it — the most effective method, since the rapidly reversing field scrambles the internal alignment
- Rough handling and repeated jarring over time
Why east-west matters in the first two. A magnet cooling or being hammered while lying north-south sits along the Earth's field, which tends to magnetise it again. Lying east-west it lies across the field, so nothing re-aligns it — the orientation is part of the method and not an afterthought.
Care and storage of magnets.
- Store bar magnets in pairs, laid side by side with unlike poles adjacent, and place soft iron keepers across the two ends
- Store a horseshoe magnet with a single soft iron keeper across its poles
- Do not heat, hammer or drop a magnet
- Do not bring a like pole of another magnet near it
- Keep magnets away from electrical machinery and from devices that could be damaged by a field
Why keepers work. Left alone, a magnet's own field lines have to pass out of one pole, through the air, and back into the other — and the free poles slowly demagnetise themselves. A soft iron keeper becomes an induced magnet at once, with a pole of the opposite kind against each end of the bar. The field lines then run through iron instead of air, completing a closed magnetic circuit, and the induced poles hold the magnet's own poles in place.
That is induced magnetism from the previous part of this chapter, put to work. The keeper is not merely a spacer or a protective bar — it is a magnet that the magnet makes, arranged so as to oppose the magnet's own decay. And it must be soft iron rather than steel, because it has to take up its induced poles instantly and give them up when the magnet is removed.
Uses of electromagnets.
- Lifting and moving heavy iron — a scrapyard crane picks up a load and releases it by breaking the circuit
- Separating iron from a mixture of scrap or from other ores on a moving belt
- The electric bell and buzzer — the electromagnet pulls an armature that strikes the gong and breaks its own circuit, so the process repeats
- Loudspeakers, headphones and microphones — a varying current in a coil makes a diaphragm vibrate, or the reverse
- Electric motors and generators, where the magnetic field is produced by coils
- Relays and circuit breakers, where a small current switches a large one, or an overload trips the supply
- Telephone earpieces and telegraph receivers
- In medicine — removing iron splinters from a patient's eye, and providing the very strong field of a magnetic resonance scanner
Methods of demagnetisation.
- Heating the magnet strongly and allowing it to cool while lying east-west, so that the Earth's field cannot re-magnetise it
- Hammering it roughly, or dropping it repeatedly, again lying east-west
- Passing alternating current through a coil wound around it — the most effective method, since the rapidly reversing field scrambles the internal alignment
- Rough handling and repeated jarring over time
Why east-west matters in the first two. A magnet cooling or being hammered while lying north-south sits along the Earth's field, which tends to magnetise it again. Lying east-west it lies across the field, so nothing re-aligns it — the orientation is part of the method and not an afterthought.
Care and storage of magnets.
- Store bar magnets in pairs, laid side by side with unlike poles adjacent, and place soft iron keepers across the two ends
- Store a horseshoe magnet with a single soft iron keeper across its poles
- Do not heat, hammer or drop a magnet
- Do not bring a like pole of another magnet near it
- Keep magnets away from electrical machinery and from devices that could be damaged by a field
Why keepers work. Left alone, a magnet's own field lines have to pass out of one pole, through the air, and back into the other — and the free poles slowly demagnetise themselves. A soft iron keeper becomes an induced magnet at once, with a pole of the opposite kind against each end of the bar. The field lines then run through iron instead of air, completing a closed magnetic circuit, and the induced poles hold the magnet's own poles in place.
That is induced magnetism from the previous part of this chapter, put to work. The keeper is not merely a spacer or a protective bar — it is a magnet that the magnet makes, arranged so as to oppose the magnet's own decay. And it must be soft iron rather than steel, because it has to take up its induced poles instantly and give them up when the magnet is removed.
Exam tip
Exam tip: say soft iron for electromagnets and steel for permanent
Soft iron for an electromagnet, steel for a permanent magnet — and give retentivity as the reason, not just the material.
The Earth's magnetic SOUTH pole is near the geographic NORTH. State it that way round, and justify it by what a compass needle does.
Over a small region the Earth's field is uniform — draw parallel, equally spaced lines from geographic south to north.
Only the HORIZONTAL component turns a compass.
List the electromagnet factors: number of turns, current, core material, shape of the core (U beats I), and core dimensions.
**Ampere-turns **, so turns at A equals turns at A.
Say the wire must be insulated and the current would otherwise short between turns.
"Temporary" describes duration, not strength — an electromagnet can be far stronger than a permanent magnet.
For demagnetisation, name the orientation: heat or hammer while lying east-west, so the Earth's field cannot re-magnetise it. Alternating current is the most effective method.
Store bar magnets in pairs with UNLIKE poles adjacent and soft iron keepers across the ends.
And explain keepers by induced magnetism — they complete the magnetic circuit so the lines run through iron rather than air.
The Earth's magnetic SOUTH pole is near the geographic NORTH. State it that way round, and justify it by what a compass needle does.
Over a small region the Earth's field is uniform — draw parallel, equally spaced lines from geographic south to north.
Only the HORIZONTAL component turns a compass.
List the electromagnet factors: number of turns, current, core material, shape of the core (U beats I), and core dimensions.
**Ampere-turns **, so turns at A equals turns at A.
Say the wire must be insulated and the current would otherwise short between turns.
"Temporary" describes duration, not strength — an electromagnet can be far stronger than a permanent magnet.
For demagnetisation, name the orientation: heat or hammer while lying east-west, so the Earth's field cannot re-magnetise it. Alternating current is the most effective method.
Store bar magnets in pairs with UNLIKE poles adjacent and soft iron keepers across the ends.
And explain keepers by induced magnetism — they complete the magnetic circuit so the lines run through iron rather than air.
Did you know
Why an electric bell breaks its own circuit to keep ringing
An electric bell needs to strike its gong many times a second while the button is held down. A simple electromagnet cannot do that — press the button and the armature is pulled in once, and there it stays.
The trick is that the bell is wired to interrupt itself.
The circuit runs from the battery, through the push button, through a light springy contact, and then through the electromagnet's coil. The armature that the electromagnet attracts is part of that same springy strip.
So the sequence goes like this. Current flows, the electromagnet pulls the armature across, and the hammer on the armature strikes the gong. But moving the armature also pulls the contact apart, which breaks the circuit. With no current, the electromagnet releases at once — and here the soft iron core is essential, because steel would hold on. The spring pulls the armature back, the contact closes again, and the whole cycle repeats.
The bell therefore rings because it is failing many times a second, and failing in a way that fixes itself.
That arrangement has a name in engineering — a make-and-break contact — and it turns up wherever a steady supply has to produce a repeating action. A car's indicator flasher works the same way, using a bimetallic strip from the heat chapter rather than an armature: it heats, bends, breaks its own circuit, cools, straightens and reconnects.
And the reason the bell needs soft iron rather than steel is exactly the reason the crane does. A device that has to let go quickly cannot be built out of a material that remembers, and the difference between iron and steel is nothing more than how long each one remembers for.
The trick is that the bell is wired to interrupt itself.
The circuit runs from the battery, through the push button, through a light springy contact, and then through the electromagnet's coil. The armature that the electromagnet attracts is part of that same springy strip.
So the sequence goes like this. Current flows, the electromagnet pulls the armature across, and the hammer on the armature strikes the gong. But moving the armature also pulls the contact apart, which breaks the circuit. With no current, the electromagnet releases at once — and here the soft iron core is essential, because steel would hold on. The spring pulls the armature back, the contact closes again, and the whole cycle repeats.
The bell therefore rings because it is failing many times a second, and failing in a way that fixes itself.
That arrangement has a name in engineering — a make-and-break contact — and it turns up wherever a steady supply has to produce a repeating action. A car's indicator flasher works the same way, using a bimetallic strip from the heat chapter rather than an armature: it heats, bends, breaks its own circuit, cools, straightens and reconnects.
And the reason the bell needs soft iron rather than steel is exactly the reason the crane does. A device that has to let go quickly cannot be built out of a material that remembers, and the difference between iron and steel is nothing more than how long each one remembers for.
Exam relevance
How do electromagnets feed into JEE Main and NEET?
Because the coil's field becomes a formula, and the soft-iron-against-steel distinction becomes the hysteresis loop.
This is the foundation for Class 12 Physics Moving Charges and Magnetism and Magnetism and Matter, examined in JEE Main and NEET. The qualitative factors listed on this page become an exact expression for a long coil, or solenoid:
where is the number of turns per unit length. The ampere-turns product used here is that formula without the constants, which is why turns and current are interchangeable, and numericals on a solenoid's field are recurring JEE Main material.
The core's effect is explained by magnetic materials. Class 12 introduces relative permeability, and a soft iron core multiplies the field of the bare coil enormously — which is why the core matters at all. Diamagnetic, paramagnetic and ferromagnetic classification is examined directly, and soft iron and steel are both ferromagnetic with very different loops.
Retentivity and coercivity become the hysteresis curve. Soft iron has a narrow loop with low retentivity and low coercivity, so it magnetises and demagnetises easily and wastes little energy per cycle. Steel has a broad loop. That is precisely why a transformer core is soft iron and a permanent magnet is steel, and match-the-column questions pairing a material with an application rest on it. The energy lost per cycle is the loop's area, which explains why an alternating-current device needs a narrow-loop core.
Alternating-current demagnetisation is explained there as driving the material repeatedly round a shrinking loop until it returns to the origin — the formal version of this page's "scrambles the alignment".
The Earth's field is examined in its own right, with the horizontal component, the angle of dip and the angle of declination related by
and the point made here — that only the horizontal component turns a compass — is what that equation says.
Electromagnetic induction in Class 12 reverses the whole arrangement: instead of a current producing magnetism, a changing magnetism produces a current. Motors, generators, relays and loudspeakers, all listed as uses here, are treated quantitatively there, and for NEET the magnetic resonance scanner appears as an application.
What the questions look like. For board work, expect describe the Earth's field with the pole positions, describe how to make an electromagnet, list the strength factors, compare permanent with temporary magnets in a table, state uses, give demagnetisation methods with the east-west orientation, and explain keepers. These are recall and reasoning questions and the wording carries the marks. For JEE Main and NEET, expect solenoid-field numericals, hysteresis and material questions, and the Earth's-field components.
How board and competitive emphasis differ. A board paper rewards the material named with its reason and the complete list of factors. A competitive paper assumes all of it and asks for the field of a coil as a number, or which material suits a stated device.
The single trap that costs the most marks. Saying a temporary magnet is weaker than a permanent one. "Temporary" is about how long the magnetism lasts, and an electromagnet with enough turns and current is far stronger than any permanent magnet of the same size. The defence is to answer the two questions separately — how long does it last and how strong is it — since the material decides the first and the ampere-turns decide the second.
This is the foundation for Class 12 Physics Moving Charges and Magnetism and Magnetism and Matter, examined in JEE Main and NEET. The qualitative factors listed on this page become an exact expression for a long coil, or solenoid:
where is the number of turns per unit length. The ampere-turns product used here is that formula without the constants, which is why turns and current are interchangeable, and numericals on a solenoid's field are recurring JEE Main material.
The core's effect is explained by magnetic materials. Class 12 introduces relative permeability, and a soft iron core multiplies the field of the bare coil enormously — which is why the core matters at all. Diamagnetic, paramagnetic and ferromagnetic classification is examined directly, and soft iron and steel are both ferromagnetic with very different loops.
Retentivity and coercivity become the hysteresis curve. Soft iron has a narrow loop with low retentivity and low coercivity, so it magnetises and demagnetises easily and wastes little energy per cycle. Steel has a broad loop. That is precisely why a transformer core is soft iron and a permanent magnet is steel, and match-the-column questions pairing a material with an application rest on it. The energy lost per cycle is the loop's area, which explains why an alternating-current device needs a narrow-loop core.
Alternating-current demagnetisation is explained there as driving the material repeatedly round a shrinking loop until it returns to the origin — the formal version of this page's "scrambles the alignment".
The Earth's field is examined in its own right, with the horizontal component, the angle of dip and the angle of declination related by
and the point made here — that only the horizontal component turns a compass — is what that equation says.
Electromagnetic induction in Class 12 reverses the whole arrangement: instead of a current producing magnetism, a changing magnetism produces a current. Motors, generators, relays and loudspeakers, all listed as uses here, are treated quantitatively there, and for NEET the magnetic resonance scanner appears as an application.
What the questions look like. For board work, expect describe the Earth's field with the pole positions, describe how to make an electromagnet, list the strength factors, compare permanent with temporary magnets in a table, state uses, give demagnetisation methods with the east-west orientation, and explain keepers. These are recall and reasoning questions and the wording carries the marks. For JEE Main and NEET, expect solenoid-field numericals, hysteresis and material questions, and the Earth's-field components.
How board and competitive emphasis differ. A board paper rewards the material named with its reason and the complete list of factors. A competitive paper assumes all of it and asks for the field of a coil as a number, or which material suits a stated device.
The single trap that costs the most marks. Saying a temporary magnet is weaker than a permanent one. "Temporary" is about how long the magnetism lasts, and an electromagnet with enough turns and current is far stronger than any permanent magnet of the same size. The defence is to answer the two questions separately — how long does it last and how strong is it — since the material decides the first and the ampere-turns decide the second.
Key takeaways
The Earth's field, electromagnets and the care of magnets: quick revision
- The Earth behaves as a giant bar magnet with its magnetic SOUTH pole near the geographic NORTH — which is why a compass needle's north end points north.
- Outside the Earth the field lines run from geographic south towards geographic north, and over a small region they are parallel, equally spaced straight lines — a uniform field.
- Angle of declination is between the magnetic and geographic meridians; angle of dip is between the field and the horizontal, near zero at the magnetic equator and near ninety degrees at the poles.
- Only the horizontal component turns a compass, and magnetic north is not geographic north.
- To make an electromagnet: wind insulated copper wire in many turns round a soft iron core and pass a current through a key.
- The wire must be insulated, or the current would short from turn to turn.
- Strength factors: number of turns, the current, the core material, the core shape (a U-shaped core beats an I-shaped one), and the core's length and thickness.
- **Ampere-turns **: turns at A gives , and so does turns at A — turns and current are interchangeable.
- turns at A gives ampere-turns; reaching at A needs turns.
- Adding turns is cheaper than adding current, since a large current overheats the wire.
- The core does not carry the current — a coil with no core is still a magnet, only weaker.
- Permanent magnet: steel or alnico, retains magnetism, cannot be switched off, harder to magnetise, generally weaker. Examples: bar magnet, compass needle, loudspeaker magnet.
- Temporary magnet: soft iron core, loses magnetism at once, can be switched and varied, easier to magnetise, can be made very strong. Examples: electromagnet, bell core, transformer core.
- Retentivity is the property: soft iron is easy in, easy out; steel is hard in, hard out.
- "Temporary" means short-lived, not weak — an electromagnet can far exceed any permanent magnet of its size.
- Soft iron is not physically soft — the word describes its magnetic behaviour; steel is iron with carbon, which makes it magnetically stubborn.
- Uses: scrapyard cranes, separating iron from scrap, the electric bell and buzzer, loudspeakers and microphones, motors and generators, relays and circuit breakers, telephone earpieces, removing iron splinters from an eye, magnetic resonance scanners.
- Demagnetisation: strong heating, rough hammering or repeated dropping — both while lying east-west so the Earth's field cannot re-magnetise it — and passing alternating current through a surrounding coil, which is the most effective.
- Storage: bar magnets in pairs with unlike poles adjacent and soft iron keepers across the ends; a horseshoe magnet with one keeper across its poles. Avoid heat, hammering, dropping and nearby like poles.
- Keepers work by induced magnetism — they take opposite poles against the bar's ends and complete a closed magnetic circuit through iron instead of air, and they must be soft iron so they release when removed.
Open an old doorbell or buzzer if you have one, find the coil and the springy contact, and trace how moving the armature breaks the very circuit that moved it.
- Outside the Earth the field lines run from geographic south towards geographic north, and over a small region they are parallel, equally spaced straight lines — a uniform field.
- Angle of declination is between the magnetic and geographic meridians; angle of dip is between the field and the horizontal, near zero at the magnetic equator and near ninety degrees at the poles.
- Only the horizontal component turns a compass, and magnetic north is not geographic north.
- To make an electromagnet: wind insulated copper wire in many turns round a soft iron core and pass a current through a key.
- The wire must be insulated, or the current would short from turn to turn.
- Strength factors: number of turns, the current, the core material, the core shape (a U-shaped core beats an I-shaped one), and the core's length and thickness.
- **Ampere-turns **: turns at A gives , and so does turns at A — turns and current are interchangeable.
- turns at A gives ampere-turns; reaching at A needs turns.
- Adding turns is cheaper than adding current, since a large current overheats the wire.
- The core does not carry the current — a coil with no core is still a magnet, only weaker.
- Permanent magnet: steel or alnico, retains magnetism, cannot be switched off, harder to magnetise, generally weaker. Examples: bar magnet, compass needle, loudspeaker magnet.
- Temporary magnet: soft iron core, loses magnetism at once, can be switched and varied, easier to magnetise, can be made very strong. Examples: electromagnet, bell core, transformer core.
- Retentivity is the property: soft iron is easy in, easy out; steel is hard in, hard out.
- "Temporary" means short-lived, not weak — an electromagnet can far exceed any permanent magnet of its size.
- Soft iron is not physically soft — the word describes its magnetic behaviour; steel is iron with carbon, which makes it magnetically stubborn.
- Uses: scrapyard cranes, separating iron from scrap, the electric bell and buzzer, loudspeakers and microphones, motors and generators, relays and circuit breakers, telephone earpieces, removing iron splinters from an eye, magnetic resonance scanners.
- Demagnetisation: strong heating, rough hammering or repeated dropping — both while lying east-west so the Earth's field cannot re-magnetise it — and passing alternating current through a surrounding coil, which is the most effective.
- Storage: bar magnets in pairs with unlike poles adjacent and soft iron keepers across the ends; a horseshoe magnet with one keeper across its poles. Avoid heat, hammering, dropping and nearby like poles.
- Keepers work by induced magnetism — they take opposite poles against the bar's ends and complete a closed magnetic circuit through iron instead of air, and they must be soft iron so they release when removed.
Open an old doorbell or buzzer if you have one, find the coil and the springy contact, and trace how moving the armature breaks the very circuit that moved it.