A Magnet Attracts a Nail by Turning It Into a Magnet First
Learn the properties of a magnet and why repulsion alone proves magnetism, understand induced magnetism, define magnetic field lines and their rules, and plot a bar magnet's field to find its neutral points.
Why is attraction not proof that something is a magnet?
Bring an iron nail near a bar magnet and the nail is pulled in. Bring a second bar magnet near the first and it may be pulled in too, or it may be pushed away.
That difference is the whole test.
A magnet attracts any piece of iron, magnetised or not — because the iron becomes a magnet while it sits in the field, with the pole facing the magnet turning out to be the opposite one. Attraction follows automatically.
So attraction tells you only that the object is made of a magnetic material. It cannot tell you whether it was already a magnet.
Repulsion is different. Only a magnet can be repelled, because repulsion needs two like poles facing each other, and an unmagnetised piece of iron has no poles of its own to offer. Repulsion is therefore the sure test of magnetism, and it is the one experiment that settles the question.
That single point — that a magnet creates magnetism in what it touches — is what the whole of this page is built on. It explains the chain of pins hanging from a magnet, the need for keepers in storage, and the field that surrounds every magnet whether or not anything is in it.
This page covers the first part of the ICSE Class 9 Physics chapter on magnetism — the properties of a magnet, induced magnetism, magnetic field lines, and plotting a field to find its neutral points.
That difference is the whole test.
A magnet attracts any piece of iron, magnetised or not — because the iron becomes a magnet while it sits in the field, with the pole facing the magnet turning out to be the opposite one. Attraction follows automatically.
So attraction tells you only that the object is made of a magnetic material. It cannot tell you whether it was already a magnet.
Repulsion is different. Only a magnet can be repelled, because repulsion needs two like poles facing each other, and an unmagnetised piece of iron has no poles of its own to offer. Repulsion is therefore the sure test of magnetism, and it is the one experiment that settles the question.
That single point — that a magnet creates magnetism in what it touches — is what the whole of this page is built on. It explains the chain of pins hanging from a magnet, the need for keepers in storage, and the field that surrounds every magnet whether or not anything is in it.
This page covers the first part of the ICSE Class 9 Physics chapter on magnetism — the properties of a magnet, induced magnetism, magnetic field lines, and plotting a field to find its neutral points.
What are the properties of a magnet?
Five properties, and each one can be demonstrated on a bench in a minute.
The attractive property. A magnet attracts pieces of magnetic material. The attraction is strongest at the poles and falls to nothing at the centre — dip a bar magnet in iron filings and they cling in thick tufts at the two ends and hardly at all in the middle. That middle region is the neutral region of the magnet itself.
The directive property. A magnet suspended freely by a thread, or floated on cork, always comes to rest pointing roughly north-south. The end that turns towards the north is the north pole and the other the south pole — which is where the names came from, and why the same object is called a magnet and a compass.
Like poles repel, unlike poles attract. North against north pushes apart; north against south pulls together. And this is the only reliable test for a magnet, as the opening section argued.
Poles exist in pairs and cannot be separated. Cut a bar magnet in half and you do not get one north piece and one south piece — you get two complete magnets, each with its own north and south. Cut those and you get four. A single isolated pole has never been obtained.
Worked count. A bar magnet cut into equal pieces gives
and cut into pieces it gives magnets and poles. The number of poles is always even, which is the arithmetic form of the same statement.
Magnetism can be destroyed. Heating a magnet strongly, hammering it, or dropping it repeatedly makes it lose its magnetism, because the rough treatment disturbs the ordered arrangement inside it.
Magnetic and non-magnetic substances.
- Magnetic — attracted by a magnet: iron, cobalt, nickel, steel, and alloys such as alnico. Magnetite, a naturally magnetic ore, is the original lodestone
- Non-magnetic — not attracted: wood, glass, plastic, rubber, paper, cloth, water, and the metals copper, aluminium, brass, zinc and silver
Not every metal is magnetic, and this surprises most people. Copper and aluminium are excellent conductors of electricity and are not attracted by a magnet at all. So "metal" and "magnetic" are two different properties, and a coin that a magnet ignores is not therefore a fake — it may simply be made of an alloy with no iron in it.
The attractive property. A magnet attracts pieces of magnetic material. The attraction is strongest at the poles and falls to nothing at the centre — dip a bar magnet in iron filings and they cling in thick tufts at the two ends and hardly at all in the middle. That middle region is the neutral region of the magnet itself.
The directive property. A magnet suspended freely by a thread, or floated on cork, always comes to rest pointing roughly north-south. The end that turns towards the north is the north pole and the other the south pole — which is where the names came from, and why the same object is called a magnet and a compass.
Like poles repel, unlike poles attract. North against north pushes apart; north against south pulls together. And this is the only reliable test for a magnet, as the opening section argued.
Poles exist in pairs and cannot be separated. Cut a bar magnet in half and you do not get one north piece and one south piece — you get two complete magnets, each with its own north and south. Cut those and you get four. A single isolated pole has never been obtained.
Worked count. A bar magnet cut into equal pieces gives
and cut into pieces it gives magnets and poles. The number of poles is always even, which is the arithmetic form of the same statement.
Magnetism can be destroyed. Heating a magnet strongly, hammering it, or dropping it repeatedly makes it lose its magnetism, because the rough treatment disturbs the ordered arrangement inside it.
Magnetic and non-magnetic substances.
- Magnetic — attracted by a magnet: iron, cobalt, nickel, steel, and alloys such as alnico. Magnetite, a naturally magnetic ore, is the original lodestone
- Non-magnetic — not attracted: wood, glass, plastic, rubber, paper, cloth, water, and the metals copper, aluminium, brass, zinc and silver
Not every metal is magnetic, and this surprises most people. Copper and aluminium are excellent conductors of electricity and are not attracted by a magnet at all. So "metal" and "magnetic" are two different properties, and a coin that a magnet ignores is not therefore a fake — it may simply be made of an alloy with no iron in it.
What is induced magnetism, and how does it happen?
Induced magnetism is the temporary magnetism a piece of magnetic material acquires when it is placed in a magnetic field, without needing to touch the magnet at all.
How it works. Bring the north pole of a magnet near one end of an iron nail. The nail's own internal arrangement lines up, and the near end of the nail develops a south pole while its far end develops a north pole.
Since the near end is now a south pole facing a north pole, the two attract — and the nail is pulled in. Attraction is not the first thing that happens; induction is, and attraction is its consequence.
Worked observation — the chain of pins. Hold a bar magnet and touch a steel pin to one pole. The pin sticks. Now touch a second pin to the free end of the first, and it sticks too, and a third to that.
Each pin has become an induced magnet, with a pole at each end, and each holds the next by the same attraction. Take the bar magnet away and the whole chain falls apart at once, because the induced magnetism goes with the field that produced it.
Induction happens without contact. Slide a sheet of paper or a thin plate of glass between the magnet and the nail and the nail is still attracted, a little more weakly. The magnetic field passes straight through non-magnetic material, so contact is never required — which is why a magnet works through the wall of a plastic box.
Induction is why attraction fails as a test. An unmagnetised nail has no poles of its own, so it can never be repelled. Put it in a field and it acquires precisely the pole that guarantees attraction. So every magnetic object is attracted and only a magnet can be repelled, which is the reasoning behind the repulsion test.
Soft iron and steel behave differently after the field is removed. Soft iron loses its induced magnetism almost completely and at once; steel keeps a good deal of it. That single difference decides which material is used for an electromagnet and which for a permanent magnet, and it is the subject of the next part of this chapter.
How it works. Bring the north pole of a magnet near one end of an iron nail. The nail's own internal arrangement lines up, and the near end of the nail develops a south pole while its far end develops a north pole.
Since the near end is now a south pole facing a north pole, the two attract — and the nail is pulled in. Attraction is not the first thing that happens; induction is, and attraction is its consequence.
Worked observation — the chain of pins. Hold a bar magnet and touch a steel pin to one pole. The pin sticks. Now touch a second pin to the free end of the first, and it sticks too, and a third to that.
Each pin has become an induced magnet, with a pole at each end, and each holds the next by the same attraction. Take the bar magnet away and the whole chain falls apart at once, because the induced magnetism goes with the field that produced it.
Induction happens without contact. Slide a sheet of paper or a thin plate of glass between the magnet and the nail and the nail is still attracted, a little more weakly. The magnetic field passes straight through non-magnetic material, so contact is never required — which is why a magnet works through the wall of a plastic box.
Induction is why attraction fails as a test. An unmagnetised nail has no poles of its own, so it can never be repelled. Put it in a field and it acquires precisely the pole that guarantees attraction. So every magnetic object is attracted and only a magnet can be repelled, which is the reasoning behind the repulsion test.
Soft iron and steel behave differently after the field is removed. Soft iron loses its induced magnetism almost completely and at once; steel keeps a good deal of it. That single difference decides which material is used for an electromagnet and which for a permanent magnet, and it is the subject of the next part of this chapter.
Formula
What are magnetic field lines, and what rules do they obey?
A magnetic field is the region around a magnet in which its magnetic force can be detected, and a magnetic line of force is the path along which a free north pole would move in that field.
Equivalently, a field line is a line drawn so that the tangent at any point gives the direction of the field there — which is the direction a compass needle would point.
The properties of field lines.
- Outside the magnet they run from the north pole to the south pole; inside the magnet they run from south to north. So every field line is a closed loop
- They never intersect. If two lines crossed, a compass at that point would have to point in two directions at once, which is impossible
- They are crowded where the field is strong — near the poles — and widely spaced where it is weak
- They behave as though they are in tension along their length and repel one another sideways. That is why the lines between two unlike poles pull the magnets together, and why the lines between two like poles bulge apart
- A uniform field is represented by parallel, equally spaced straight lines
Worked deduction from the crowding rule. Iron filings sprinkled around a bar magnet gather thickly at the two ends and thinly in the middle. Since the line spacing measures the field strength, the crowding at the poles is the statement that the field is strongest there — the same fact the attractive property showed in the previous section, now drawn as a picture.
Worked example — a compass in two fields at once. Place a compass where a magnet's field is perpendicular to the Earth's horizontal field and happens to be equal in strength to it. The needle settles along the resultant of the two, which for two equal perpendicular fields is at
to each. Make the magnet's field times the Earth's, and the needle turns to
since . A compass needle is a device that finds the direction of the total field, and it never reports one field alone.
The lines are a drawing, not an object. There are no actual threads in the space round a magnet, and the field exists continuously everywhere rather than only along the lines drawn. The number of lines on a diagram is a choice made by whoever drew it — what carries meaning is their direction and their relative spacing, and a diagram with twice as many lines everywhere describes the same field.
Equivalently, a field line is a line drawn so that the tangent at any point gives the direction of the field there — which is the direction a compass needle would point.
The properties of field lines.
- Outside the magnet they run from the north pole to the south pole; inside the magnet they run from south to north. So every field line is a closed loop
- They never intersect. If two lines crossed, a compass at that point would have to point in two directions at once, which is impossible
- They are crowded where the field is strong — near the poles — and widely spaced where it is weak
- They behave as though they are in tension along their length and repel one another sideways. That is why the lines between two unlike poles pull the magnets together, and why the lines between two like poles bulge apart
- A uniform field is represented by parallel, equally spaced straight lines
Worked deduction from the crowding rule. Iron filings sprinkled around a bar magnet gather thickly at the two ends and thinly in the middle. Since the line spacing measures the field strength, the crowding at the poles is the statement that the field is strongest there — the same fact the attractive property showed in the previous section, now drawn as a picture.
Worked example — a compass in two fields at once. Place a compass where a magnet's field is perpendicular to the Earth's horizontal field and happens to be equal in strength to it. The needle settles along the resultant of the two, which for two equal perpendicular fields is at
to each. Make the magnet's field times the Earth's, and the needle turns to
since . A compass needle is a device that finds the direction of the total field, and it never reports one field alone.
The lines are a drawing, not an object. There are no actual threads in the space round a magnet, and the field exists continuously everywhere rather than only along the lines drawn. The number of lines on a diagram is a choice made by whoever drew it — what carries meaning is their direction and their relative spacing, and a diagram with twice as many lines everywhere describes the same field.
How do you plot a field and find the neutral points?
Trace the direction a small compass points, step by step, and join the dots into a smooth curve.
The procedure.
- Fix a sheet of white paper on a board and place the bar magnet in the middle. Draw round its outline and mark its poles
- Place a small plotting compass near one pole and mark a dot at each end of its needle with a sharp pencil
- Move the compass so that its tail now sits exactly on the dot the head previously made, and mark the new head position
- Repeat until the compass reaches the other pole or leaves the paper
- Join the dots freehand into a smooth curve and put an arrowhead on it, pointing from north to south
- Start again from several different points around the magnet, so the whole pattern is built up
Neutral points. A neutral point is a place where the magnet's field and the Earth's horizontal field are equal in magnitude and opposite in direction, so the resultant is zero.
A compass placed exactly at a neutral point has no definite direction to point in, and iron filings there show a blank gap in the pattern.
Where they are depends on which way the magnet lies. The Earth's horizontal field points from geographic south to north everywhere on the paper.
Case 1 — the magnet's north pole pointing north. Along the magnet's axis beyond the north pole, the magnet's field also points northward, so the two add and can never cancel. Along the perpendicular bisector, the magnet's field points southward, opposing the Earth's field. So the neutral points lie on the equatorial line — one to the east and one to the west of the magnet's centre. Two neutral points.
Case 2 — the magnet's north pole pointing south. Now the magnet's axial field beyond its north pole points southward, against the Earth's field. So the neutral points lie on the axial line — one beyond each pole, to the north and to the south. Two neutral points.
The two cases put the neutral points at right angles to each other, and that is the whole content of the question: turning the magnet round by moves the blank spots from the sides to the ends.
A neutral point is not a place with no field. Two fields are present there and perfectly opposed, so their sum is zero — like two people pushing a box equally from opposite sides. Remove the magnet and the Earth's field at that point is exactly what it always was, which is why a neutral point exists only for a particular magnet in a particular orientation.
And a neutral point is not the magnet's neutral region. The neutral region is the middle of the magnet itself, where it picks up no filings; a neutral point is a spot in the surrounding space where two fields cancel. Two different things with confusingly similar names, and a question naming one is not asking about the other.
The procedure.
- Fix a sheet of white paper on a board and place the bar magnet in the middle. Draw round its outline and mark its poles
- Place a small plotting compass near one pole and mark a dot at each end of its needle with a sharp pencil
- Move the compass so that its tail now sits exactly on the dot the head previously made, and mark the new head position
- Repeat until the compass reaches the other pole or leaves the paper
- Join the dots freehand into a smooth curve and put an arrowhead on it, pointing from north to south
- Start again from several different points around the magnet, so the whole pattern is built up
Neutral points. A neutral point is a place where the magnet's field and the Earth's horizontal field are equal in magnitude and opposite in direction, so the resultant is zero.
A compass placed exactly at a neutral point has no definite direction to point in, and iron filings there show a blank gap in the pattern.
Where they are depends on which way the magnet lies. The Earth's horizontal field points from geographic south to north everywhere on the paper.
Case 1 — the magnet's north pole pointing north. Along the magnet's axis beyond the north pole, the magnet's field also points northward, so the two add and can never cancel. Along the perpendicular bisector, the magnet's field points southward, opposing the Earth's field. So the neutral points lie on the equatorial line — one to the east and one to the west of the magnet's centre. Two neutral points.
Case 2 — the magnet's north pole pointing south. Now the magnet's axial field beyond its north pole points southward, against the Earth's field. So the neutral points lie on the axial line — one beyond each pole, to the north and to the south. Two neutral points.
The two cases put the neutral points at right angles to each other, and that is the whole content of the question: turning the magnet round by moves the blank spots from the sides to the ends.
A neutral point is not a place with no field. Two fields are present there and perfectly opposed, so their sum is zero — like two people pushing a box equally from opposite sides. Remove the magnet and the Earth's field at that point is exactly what it always was, which is why a neutral point exists only for a particular magnet in a particular orientation.
And a neutral point is not the magnet's neutral region. The neutral region is the middle of the magnet itself, where it picks up no filings; a neutral point is a spot in the surrounding space where two fields cancel. Two different things with confusingly similar names, and a question naming one is not asking about the other.
Exam tip
Exam tip: quote repulsion as the test and name the induced pole
Repulsion is the sure test of magnetism, because an unmagnetised piece of iron can be attracted but never repelled. Give the reason, not just the word.
In induced magnetism the near end takes the OPPOSITE pole to the inducing pole. State which pole where.
Say that induction comes first and attraction follows — that is the full explanation of why a magnet picks up a nail.
**Cutting a magnet into pieces gives magnets and poles — the count is always even.
Field lines go N to S outside and S to N inside, forming closed loops. Draw the arrowheads.
They never cross, because a compass cannot point two ways at once. Give that reason.
Crowded lines mean a strong field — that is why the lines bunch at the poles.
A uniform field is parallel, equally spaced straight lines.
For plotting, describe the step-by-step compass method — tail on the previous head dot — and say you repeat from several starting points.
Neutral points: N pointing north gives them on the EQUATORIAL line; N pointing south gives them on the AXIAL line. Two in each case.
A neutral point has two fields cancelling, not no field — and it is not the magnet's neutral region.
And remember copper, aluminium and brass are metals and NOT magnetic** — metal and magnetic are different properties.
In induced magnetism the near end takes the OPPOSITE pole to the inducing pole. State which pole where.
Say that induction comes first and attraction follows — that is the full explanation of why a magnet picks up a nail.
**Cutting a magnet into pieces gives magnets and poles — the count is always even.
Field lines go N to S outside and S to N inside, forming closed loops. Draw the arrowheads.
They never cross, because a compass cannot point two ways at once. Give that reason.
Crowded lines mean a strong field — that is why the lines bunch at the poles.
A uniform field is parallel, equally spaced straight lines.
For plotting, describe the step-by-step compass method — tail on the previous head dot — and say you repeat from several starting points.
Neutral points: N pointing north gives them on the EQUATORIAL line; N pointing south gives them on the AXIAL line. Two in each case.
A neutral point has two fields cancelling, not no field — and it is not the magnet's neutral region.
And remember copper, aluminium and brass are metals and NOT magnetic** — metal and magnetic are different properties.
Did you know
Why you can never get a magnet with only one pole
Cut a bar magnet in half, hoping to separate the north from the south. You get two shorter magnets, each with a full north and a full south.
Cut those in half and you get four. Keep going and you keep getting complete magnets, each with two poles, however small the pieces become.
The reason is that a magnet's magnetism is not a pair of substances stored at its ends. It comes from the alignment of enormous numbers of tiny magnetic regions inside the material, all pointing the same way. Each of those regions is itself a two-poled magnet.
So when you cut the bar, you are not dividing north from south — you are dividing a queue of aligned small magnets into two shorter queues. Each queue still has a free north at one end and a free south at the other, because that is what a line of aligned magnets looks like from outside.
That also explains what happens when you hammer a magnet or heat it strongly. The rough treatment jostles those internal regions out of alignment, and a disordered collection of small magnets pointing every way has no net pole at either end. The magnetism has not leaked out anywhere; it has been scrambled.
And it explains induction. An unmagnetised nail has all those internal regions pointing randomly, so it shows no poles. Put it in a field and they swing into line, and the nail promptly has two poles of its own — the near one opposite to whatever is doing the aligning.
Compare this with electric charge, which behaves quite differently. A positive charge can be had entirely on its own, in a way no magnetic north pole ever has been. That single asymmetry between electricity and magnetism is one of the genuinely strange facts of physics, and it is why every magnetic field line has to be a closed loop with no beginning and no end.
Cut those in half and you get four. Keep going and you keep getting complete magnets, each with two poles, however small the pieces become.
The reason is that a magnet's magnetism is not a pair of substances stored at its ends. It comes from the alignment of enormous numbers of tiny magnetic regions inside the material, all pointing the same way. Each of those regions is itself a two-poled magnet.
So when you cut the bar, you are not dividing north from south — you are dividing a queue of aligned small magnets into two shorter queues. Each queue still has a free north at one end and a free south at the other, because that is what a line of aligned magnets looks like from outside.
That also explains what happens when you hammer a magnet or heat it strongly. The rough treatment jostles those internal regions out of alignment, and a disordered collection of small magnets pointing every way has no net pole at either end. The magnetism has not leaked out anywhere; it has been scrambled.
And it explains induction. An unmagnetised nail has all those internal regions pointing randomly, so it shows no poles. Put it in a field and they swing into line, and the nail promptly has two poles of its own — the near one opposite to whatever is doing the aligning.
Compare this with electric charge, which behaves quite differently. A positive charge can be had entirely on its own, in a way no magnetic north pole ever has been. That single asymmetry between electricity and magnetism is one of the genuinely strange facts of physics, and it is why every magnetic field line has to be a closed loop with no beginning and no end.
Exam relevance
How does magnetism feed into JEE Main and NEET?
Because field lines become the language of every later field, and the neutral-point condition is the first case of adding two fields as vectors.
This is the foundation for Class 12 Physics Magnetism and Matter and Moving Charges and Magnetism, examined in JEE Main and NEET. The field lines drawn qualitatively here are given a magnitude there, and the bar magnet is treated as a magnetic dipole with a dipole moment. Its field on the axial line and on the equatorial line get separate formulas, and the two neutral-point cases on this page are exactly the two situations those formulas describe. **The condition becomes an equation to solve for a distance, and that is a standard JEE Main numerical.
The tangent law is the formal version of the compass calculation.** Class 12 gives
for a needle in two perpendicular fields, which is the and working on this page. The tangent galvanometer is built on it, and questions about a deflected needle use it directly.
The closed-loop property becomes a law. Class 12 states that the net magnetic flux through any closed surface is zero — Gauss's law for magnetism — and the physical content is exactly what the previous section observed: magnetic poles never come singly, so there is no source from which lines begin. Assertion-reason questions contrasting this with electric charge appear in both papers.
Induced magnetism becomes magnetic materials. Class 12 classifies substances as diamagnetic, paramagnetic and ferromagnetic, and explains induction through the alignment of domains — the internal regions described in the previous section. Soft iron against steel becomes the hysteresis loop, and which material suits an electromagnet and which a permanent magnet is asked directly.
The Earth's field is examined in its own right, with the angle of declination, the angle of dip and the horizontal component, and the relation between them.
For NEET Physics, expect field-line diagrams, the tangent law and magnetic-material questions; the magnetic-field concepts also support the magnetic resonance imaging material in applied biology.
What the questions look like. For board work, expect state the properties of a magnet, explain why repulsion is the test, explain induced magnetism with the pole named, state the properties of field lines, describe the plotting method, and locate the neutral points for both orientations with a diagram. Labelled diagrams with arrowheads carry much of the credit. For JEE Main and NEET, expect axial and equatorial field numericals, neutral-point distances, the tangent law, and magnetic-material classification.
How board and competitive emphasis differ. A board paper rewards the reason attached to each property — why lines cannot cross, why attraction fails as a test. A competitive paper assumes all of it and tests whether two fields can be added correctly as vectors.
The single trap that costs the most marks. Putting the neutral points in the wrong place. With the magnet's north pointing north they lie on the equatorial line, to east and west; with its north pointing south they lie on the axial line, beyond the poles. The defence is to draw the Earth's field as an arrow pointing north on the diagram first, then mark the magnet's field direction at a trial point and check whether the two oppose — the arrows settle it before any reasoning is needed.
This is the foundation for Class 12 Physics Magnetism and Matter and Moving Charges and Magnetism, examined in JEE Main and NEET. The field lines drawn qualitatively here are given a magnitude there, and the bar magnet is treated as a magnetic dipole with a dipole moment. Its field on the axial line and on the equatorial line get separate formulas, and the two neutral-point cases on this page are exactly the two situations those formulas describe. **The condition becomes an equation to solve for a distance, and that is a standard JEE Main numerical.
The tangent law is the formal version of the compass calculation.** Class 12 gives
for a needle in two perpendicular fields, which is the and working on this page. The tangent galvanometer is built on it, and questions about a deflected needle use it directly.
The closed-loop property becomes a law. Class 12 states that the net magnetic flux through any closed surface is zero — Gauss's law for magnetism — and the physical content is exactly what the previous section observed: magnetic poles never come singly, so there is no source from which lines begin. Assertion-reason questions contrasting this with electric charge appear in both papers.
Induced magnetism becomes magnetic materials. Class 12 classifies substances as diamagnetic, paramagnetic and ferromagnetic, and explains induction through the alignment of domains — the internal regions described in the previous section. Soft iron against steel becomes the hysteresis loop, and which material suits an electromagnet and which a permanent magnet is asked directly.
The Earth's field is examined in its own right, with the angle of declination, the angle of dip and the horizontal component, and the relation between them.
For NEET Physics, expect field-line diagrams, the tangent law and magnetic-material questions; the magnetic-field concepts also support the magnetic resonance imaging material in applied biology.
What the questions look like. For board work, expect state the properties of a magnet, explain why repulsion is the test, explain induced magnetism with the pole named, state the properties of field lines, describe the plotting method, and locate the neutral points for both orientations with a diagram. Labelled diagrams with arrowheads carry much of the credit. For JEE Main and NEET, expect axial and equatorial field numericals, neutral-point distances, the tangent law, and magnetic-material classification.
How board and competitive emphasis differ. A board paper rewards the reason attached to each property — why lines cannot cross, why attraction fails as a test. A competitive paper assumes all of it and tests whether two fields can be added correctly as vectors.
The single trap that costs the most marks. Putting the neutral points in the wrong place. With the magnet's north pointing north they lie on the equatorial line, to east and west; with its north pointing south they lie on the axial line, beyond the poles. The defence is to draw the Earth's field as an arrow pointing north on the diagram first, then mark the magnet's field direction at a trial point and check whether the two oppose — the arrows settle it before any reasoning is needed.
Key takeaways
Magnets, induction and magnetic field lines: quick revision
- Attractive property — a magnet attracts magnetic materials, strongest at the poles and zero at the centre (the neutral region).
- Directive property — a freely suspended magnet rests pointing north-south, which is where the pole names come from.
- Like poles repel, unlike poles attract, and repulsion is the sure test of magnetism — an unmagnetised iron piece can be attracted but never repelled.
- Poles exist only in pairs. Cutting into pieces gives magnets and poles, so the count is always even; pieces give poles.
- Magnetism is destroyed by strong heating, hammering or repeated dropping.
- Magnetic: iron, cobalt, nickel, steel, alnico, magnetite. Non-magnetic: wood, glass, plastic, rubber, paper, water, and the metals copper, aluminium, brass, zinc, silver.
- Metal and magnetic are different properties — copper conducts superbly and is not attracted at all.
- Induced magnetism is temporary magnetism acquired in a field, without contact.
- The near end takes the OPPOSITE pole to the inducing pole, so attraction follows — induction first, attraction second.
- A chain of pins hangs from a magnet because each becomes an induced magnet; remove the magnet and the chain collapses.
- The field passes through non-magnetic material, so paper or glass between does not stop it.
- Soft iron loses induced magnetism at once; steel keeps it — which decides electromagnet against permanent magnet.
- A magnetic field is the region where the magnetic force can be detected; a line of force is the path a free north pole would take, and its tangent gives the field direction.
- Field lines: N to S outside, S to N inside, so closed loops; they never cross; crowded means strong; they are in tension lengthwise and repel sideways; a uniform field is parallel, equally spaced straight lines.
- Iron filings crowd at the poles, which is the crowding rule drawn as a picture.
- A compass in two equal perpendicular fields settles at ****; if the magnet's field is times the Earth's, at **.
- The lines are a drawing — the field is continuous everywhere, and only their direction and relative spacing carry meaning.
- To plot: outline the magnet, mark both ends of a compass needle, move the compass so its tail sits on the previous head dot, repeat, join into a smooth curve with arrowheads, and start again from several points.
- A neutral point is where the magnet's field and the Earth's horizontal field are equal and opposite, so the resultant is zero and a compass has no direction.
- North pointing north gives neutral points on the equatorial line, east and west. North pointing south gives on the axial line, beyond the poles. Two in each case.
- A neutral point has two fields cancelling, not no field, and it is not** the magnet's neutral region.
Suspend a bar magnet on a thread and note which way it settles, then bring another magnet near it end-on and find the orientation that pushes rather than pulls.
- Directive property — a freely suspended magnet rests pointing north-south, which is where the pole names come from.
- Like poles repel, unlike poles attract, and repulsion is the sure test of magnetism — an unmagnetised iron piece can be attracted but never repelled.
- Poles exist only in pairs. Cutting into pieces gives magnets and poles, so the count is always even; pieces give poles.
- Magnetism is destroyed by strong heating, hammering or repeated dropping.
- Magnetic: iron, cobalt, nickel, steel, alnico, magnetite. Non-magnetic: wood, glass, plastic, rubber, paper, water, and the metals copper, aluminium, brass, zinc, silver.
- Metal and magnetic are different properties — copper conducts superbly and is not attracted at all.
- Induced magnetism is temporary magnetism acquired in a field, without contact.
- The near end takes the OPPOSITE pole to the inducing pole, so attraction follows — induction first, attraction second.
- A chain of pins hangs from a magnet because each becomes an induced magnet; remove the magnet and the chain collapses.
- The field passes through non-magnetic material, so paper or glass between does not stop it.
- Soft iron loses induced magnetism at once; steel keeps it — which decides electromagnet against permanent magnet.
- A magnetic field is the region where the magnetic force can be detected; a line of force is the path a free north pole would take, and its tangent gives the field direction.
- Field lines: N to S outside, S to N inside, so closed loops; they never cross; crowded means strong; they are in tension lengthwise and repel sideways; a uniform field is parallel, equally spaced straight lines.
- Iron filings crowd at the poles, which is the crowding rule drawn as a picture.
- A compass in two equal perpendicular fields settles at ****; if the magnet's field is times the Earth's, at **.
- The lines are a drawing — the field is continuous everywhere, and only their direction and relative spacing carry meaning.
- To plot: outline the magnet, mark both ends of a compass needle, move the compass so its tail sits on the previous head dot, repeat, join into a smooth curve with arrowheads, and start again from several points.
- A neutral point is where the magnet's field and the Earth's horizontal field are equal and opposite, so the resultant is zero and a compass has no direction.
- North pointing north gives neutral points on the equatorial line, east and west. North pointing south gives on the axial line, beyond the poles. Two in each case.
- A neutral point has two fields cancelling, not no field, and it is not** the magnet's neutral region.
Suspend a bar magnet on a thread and note which way it settles, then bring another magnet near it end-on and find the orientation that pushes rather than pulls.