Chapter 10: Magnetism
In the year 1600, a London physician named William Gilbert published a book on magnets. For centuries sailors had trusted the compass without knowing why it worked. Some believed that a huge mountain of lodestone stood somewhere in the far north and pulled the needle towards itself. Others thought the Pole star was doing the pulling.
Gilbert decided to test the idea rather than argue about it. He had a lump of lodestone cut and ground into a smooth ball, which he called a ‘terrella’ — a little earth. Then he took a tiny compass needle and moved it, point by point, across the surface of that stone ball.
Near the middle of the sphere the needle lay flat. As he carried it towards either end, the needle tilted more and more steeply, until close to the ends it stood almost upright.
Now, sailors had been reporting exactly this behaviour for years. A needle free to swing in a vertical plane lies level near the equator, and dips more and more steeply as a ship sails far to the north. Gilbert’s little stone ball was behaving just like the Earth.
So he drew a bold conclusion. Nothing distant is pulling on the compass at all. The Earth is itself a great magnet, and the needle simply turns to line up with the Earth’s own magnetic field.
Diagram
Fig. 10.0-1 — Gilbert’s Terrella A smooth sphere of lodestone resting on a small wooden stand, with a tiny compass needle drawn at five positions around its surface: lying flat and parallel to the surface at the middle of the sphere, tilted at a small angle a little above and below the middle, tilted steeply nearer the top and bottom, and standing almost vertical at the very top and bottom. Label the sphere ‘terrella (ball of lodestone)’, label the flat position ‘needle lies level here’, label the vertical position ‘needle stands upright here’, and draw a dotted line round the middle of the sphere.
10.1 Magnets, Poles And Their Behaviour
10.1.1 Lodestone And Natural Magnets
The first magnets anyone knew about were not made in a workshop. They were lumps of a dark, heavy rock dug out of the ground. This rock is called lodestone — a naturally occurring ore of iron oxide, \(\\mathrm{Fe\_3O\_4}\), which was found in large quantities in Magnesia, a district in Asia Minor.
Lodestone was noticed because it did two odd things that ordinary stones do not do. First, it attracted small pieces of iron lying near it. Second, when a piece of it was hung up so that it could turn freely, it always settled along one particular direction and stayed there.
That second property is the useful one. It was known in ancient China that a piece of lodestone free to turn would point out a fixed direction, and direction-finding devices based on it were in use there long before they reached Europe. By about a thousand years ago, Chinese sailors were using such devices to hold a course at sea.
Because these pieces were dug out of the earth just as they were, they are called natural magnets. Our word ‘magnet’ comes from Magnesia, the place the first pieces came from.
10.1.2 Artificial Magnets And Their Shapes
Natural magnets are of little practical use. They come in irregular, odd shapes that cannot be fitted into an instrument, and they are simply not strong enough magnetically for most jobs.
So magnets are instead manufactured from iron and steel in whatever shape the work demands. A magnet made in this way is called an artificial magnet. The common shapes are drawn in Fig. 10.1-1.
Diagram
Fig. 10.1-1 — Common Shapes Of Artificial Magnets Four separate labelled sketches in a row. (a) A straight rectangular bar with N marked at one end and S at the other, labelled ‘bar magnet’. (b) A U-shaped magnet with N on one arm-end and S on the other, labelled ‘horse shoe magnet’. (c) A slim diamond-shaped needle balanced on a sharp vertical pivot, one half shaded and marked N, the other marked S, labelled ‘magnetic needle’. (d) A small circular box with a glass top, a pivoted needle inside, and the letters N, E, S, W printed on a dial, labelled ‘magnetic compass’.
A bar magnet is a straight rectangular bar, and a horse shoe magnet is bent into a U so that both its poles face the same way. A magnetic needle is a slender magnet balanced on a sharp pivot so that it can swing freely in a horizontal plane. Mount that needle in a small box over a printed dial and you have a magnetic compass.
10.1.3 The Directive Property Of A Freely Suspended Magnet
Tie a bar magnet at its middle with a silk thread and hang it so that it is free to turn in a horizontal plane. Give it a push. It swings to and fro, slows down, and finally comes to rest — and it always comes to rest pointing along the geographic north–south direction, as shown in Fig. 10.1-2.
Diagram
Fig. 10.1-2 — A Freely Suspended Magnet Sets Itself North–South A bar magnet hung horizontally at its mid-point from a silk thread tied to a fixed rigid support drawn at the top; label the thread ‘silk thread’. Mark the two ends of the magnet N and S. Beside the magnet draw a horizontal reference arrow labelled at its two ends ‘geographic north’ and ‘geographic south’, drawn parallel to the length of the magnet to show that the magnet has set itself along this line.
Silk thread is chosen deliberately, because it is untwisted and offers almost no resistance to turning. The magnet is therefore free to take up whatever direction it prefers, and it prefers north–south. This behaviour of a freely suspended magnet is called its directive property.
The two ends of a magnet are called its poles, and this is how they get their names. The end that turns towards the geographic north is the north-seeking pole, shortened in use to the north pole of the magnet. The other end is the south pole.
10.1.4 Attraction And Repulsion Between Poles
Now bring a second magnet slowly up to a freely suspended one and watch what its ends do. The behaviour depends entirely on which pair of poles you bring together, and it is shown in Fig. 10.1-3.
Diagram
Fig. 10.1-3 — Like Poles Repel, Unlike Poles Attract Three labelled pairs of bar magnets drawn one below the other, each pair placed end to end with a small gap. (a) N of the left magnet facing N of the right magnet, with two arrows pointing away from the gap, labelled ‘repulsion’. (b) S facing S, with two arrows pointing away from the gap, labelled ‘repulsion’. (c) N facing S, with two arrows pointing towards the gap, labelled ‘attraction’.
Bring two north poles together and they push each other apart. Bring two south poles together and they also push each other apart. But bring a north pole up to a south pole and they pull towards each other.
This law is what makes a compass needle useful. Its north pole is pulled one way and its south pole the other, so the needle as a whole turns until it lies along a definite direction and then stays there.
10.2 Induced Magnetism
10.2.1 Magnetic Induction And Induced Magnetism
Take an unmagnetised bar \(AB\) made of a magnetic material — a material such as soft iron or steel, which a magnet can attract and which can itself be turned into a magnet. On its own the bar does nothing. Sprinkle iron filings near its ends and none of them cling to it.
Now place a magnet close to the bar, or in contact with it, as in Fig. 10.2-1(a), and repeat the test. This time the filings cling to both ends of the bar. The bar has become a magnet.
Diagram
Fig. 10.2-1 — A Bar Becomes A Magnet While A Magnet Is Near It Two labelled sketches, one below the other. (a) A bar magnet drawn horizontally on the left with N at its left end and S at its right end, labelled ‘magnet’; immediately to its right, in line with it and just touching, a plain rectangular bar labelled ‘iron or steel bar’ with A at its left end and B at its right end; small induced letters n at end A and s at end B; a bushy cluster of iron filings drawn clinging to end A and another cluster clinging to end B, labelled ‘iron filings’; caption line inside the figure ‘iron filings cling to the bar in the presence of the magnet’. (b) The same plain bar AB alone with the magnet removed, and the iron filings drawn as a loose heap lying on the table below the bar; caption line inside the figure ‘the filings fall off when the magnet is taken away’.
Take the magnet away and test the bar once more. Nearly all the filings that were clinging to it now fall off, as in Fig. 10.2-1(b). The bar has lost its magnetism again.
So a bar of a magnetic material behaves like a magnet only so long as it is kept near, or in contact with, a magnet. The magnetism it gains in this way is called induced magnetism, and the process that produces it is called magnetic induction.
Notice that nothing has to touch for this to happen. The bar becomes a magnet even when the magnet is merely brought near it, without contact.
10.2.2 Polarity Developed On The Induced Magnet
The bar \(AB\) is now a magnet, so it must have a north pole and a south pole. Which end is which? Bring a compass needle up to each end in turn and read off the polarity from the way the needle responds, as shown in Fig. 10.2-2.
Diagram
Fig. 10.2-2 — Testing The Induced Polarity With A Compass Needle A bar magnet on the left with N at its left end and S at its right end, and immediately to its right a plain bar with A at its near (left) end and B at its far (right) end. A small pivoted compass needle is drawn just beyond end A and a second one just beyond end B. The needle near A is drawn with its own S end turned towards A; the needle near B is drawn with its own N end turned towards B. Mark the induced polarity as n at A and s at B, and label the whole ‘the near end becomes the opposite pole, the far end the same pole’.
In Fig. 10.2-2 the south pole of the magnet faces end \(A\) of the bar. The test shows that end \(A\) becomes a north pole — the opposite of the pole facing it. End \(B\), at the far side, becomes a south pole — the same as the pole facing \(A\).
The rule works whichever pole of the magnet is presented. Turn the magnet round so that its north pole faces \(A\), and \(A\) becomes a south pole while \(B\) becomes a north pole.
10.2.3 Induction Precedes Attraction
We can now answer a question that has been waiting since the start of the chapter: why is an ordinary, unmagnetised piece of iron attracted to a magnet at all? An unmagnetised piece has no poles, and there is nothing for the magnet to pull on.
The answer is that it does not stay unmagnetised. As the magnet comes near, the nearer end of the iron acquires the opposite polarity by magnetic induction. That end and the pole of the magnet facing it are now unlike poles.
Unlike poles attract, so the iron is drawn towards the magnet. The iron first becomes a magnet by induction, and only then is it attracted. In short, induction precedes attraction.
Diagram
Fig. 10.2-3 — Iron Needles Hanging From The Poles Of A Horse Shoe Magnet A U-shaped horse shoe magnet drawn with its two arm-ends pointing downwards, N marked on the left arm-end and S on the right arm-end. A thin straight iron needle hangs from each arm-end, touching it at the needle’s upper end. On the left needle mark s at the top and n at the bottom; on the right needle mark n at the top and s at the bottom. Draw the two needles tilted so that their lower tips lean in towards each other, with a small arrow on each tip pointing inwards.
10.2.4 Induced Magnetism Is Temporary
Hold one pole of a bar magnet just above a heap of small iron nails. A nail jumps up and clings to the pole — and then a second nail clings to the first, a third to the second, and so on. A hanging chain of nails forms, as shown in Fig. 10.2-4.
Diagram
Fig. 10.2-4 — A Chain Of Iron Nails Hanging From One Pole Of A Bar Magnet A bar magnet drawn horizontally with S at its left end and N at its right end. Hanging vertically from the N end, a chain of four or five iron nails drawn one below the other, each nail’s head touching the tip of the nail above it. On every nail mark the induced polarity: s at its upper end and n at its lower end. Label the chain ‘each nail is magnetised by the nail above it’.
The chain forms by induction, one link at a time. The magnet magnetises the first nail, which is therefore attracted and clings to it. That first nail is now a magnet, so it magnetises the second nail and attracts it, and the second magnetises the third.
The chain does not grow for ever. It stops when the force with which the magnet attracts the first nail is just enough to balance the total weight of all the nails hanging below it. Add one more nail and the chain cannot hold it.
Now hold the topmost nail steady between your fingers and slide the magnet away. Every nail in the chain falls. The topmost nail loses its magnetism the moment the magnet is removed, so the nail below it loses its magnetism too, and so on down the chain. With nothing left to hold them together, they separate and fall under gravity.
This is the clearest evidence that magnetism acquired by induction is purely temporary. It lasts only as long as the magnet causing the induction stays in the neighbourhood.
10.3 Lines Of Magnetic Field
10.3.1 The Magnetic Field Of A Magnet
Place a magnetic compass by itself on a table, well away from anything magnetic. Its needle settles along the geographic north–south direction. It does this because the earth itself has a magnetic field, which is taken up later in this chapter.
Now bring a magnet near the compass. The needle swings round and comes to rest pointing some other way altogether. Move the compass to a different place around the magnet and the needle takes up yet another direction, as Fig. 10.3-1 shows.
Diagram
Fig. 10.3-1 — A Compass Needle At Different Points Around A Bar Magnet A bar magnet drawn horizontally in the centre with N at its right end and S at its left end. Six small pivoted compass needles drawn at different points around it — one beyond the N end, one beyond the S end, two above the magnet and two below it — each needle drawn tilted at a different angle, and in every case with its own N end turned towards the nearer pole of the bar magnet. Draw a dotted line to one side labelled ‘geographic north–south direction’ for comparison, and label the whole ‘the needle rests in a different direction at each point’.
Wherever the compass is put, the needle turns so that one of its ends points towards the nearer pole of the magnet. The needle behaves this way because the magnet is influencing the space around it. The region in which this influence is felt is called the magnetic field of the magnet.
Carry the compass further and further away and the effect weakens. At a large enough distance the needle swings back to the geographic north–south direction, because the magnet’s influence there is too feeble to matter. The magnetic field of a magnet grows weaker as the distance from the magnet increases.
10.3.2 Magnetic Field As A Vector Quantity
Magnetic field is a vector quantity, which means it has both a magnitude and a direction, and both have to be stated before the field at a point is fully described.
Its magnitude is judged by the force felt by a magnetic pole of unit strength placed at that point. A stronger field means a bigger force on the same pole, so the force is a fair measure of how strong the field is.
Its direction at a point is simply the direction in which the north pole of a compass needle rests when the needle is placed at that point.
10.3.3 The Pattern Made By Iron Filings
The compass tells us the field direction at one point at a time, which is slow work. Iron filings show the whole pattern at once.
Lay a bar magnet on a table and place a sheet of stiff paper or a glass plate over it. Sprinkle iron filings uniformly over the plate, then tap the plate gently. The filings shift and settle themselves along a set of curved lines, as in Fig. 10.3-2.
Diagram
Fig. 10.3-2 — Iron Filings Settling Into A Pattern Over A Bar Magnet A rectangular bar magnet seen from above, drawn in outline in the centre with N at its right end and S at its left end, lying beneath a plate. All around it, hundreds of short dashes representing iron filings, arranged end to end along smooth curves that leave the N end, arc outwards above and below the magnet, and return to the S end. Draw the dashes packed densely close to the two ends and sparsely near the middle of the magnet and far from it. Label ‘iron filings sprinkled on the glass plate’.
The reason the filings line up is magnetic induction, which we met in the previous section. Each tiny filing becomes a little magnet in the field of the bar magnet, and each little magnet then experiences a force that turns it. Tapping the plate frees the filings for a moment from friction so that they can swing round and take up their preferred direction.
The curved lines the filings pick out are called magnetic field lines.
10.3.4 Magnetic Field Lines And Their Direction
Place a compass needle at any point in a magnetic field and it rests along the field line passing through that point. The needle is, in effect, a short piece of the line made visible.
This gives us a way of putting a direction on the line. An arrow is marked on the field line running from the south pole of the needle towards its north pole. That arrow gives the direction of the magnetic field at that point, and it is drawn in Fig. 10.3-3.
Diagram
Fig. 10.3-3 — Reading The Direction Of A Field Line A single smooth curve drawn across the figure, labelled ‘magnetic field line’. A small compass needle drawn lying along the curve at a point P, with its S end and N end marked, and its length lying along the curve. An arrowhead drawn on the curve at P pointing from the needle’s S end towards its N end, labelled ‘direction of the magnetic field at P’. A straight dashed line drawn touching the curve at P and nowhere else, labelled ‘tangent at P’.
The line itself is curved, but at any one point it has a definite direction — the direction of the tangent drawn to it at that point. This is what lets a curved line carry direction information.
10.4 Properties Of Magnetic Field Lines
Field lines are not drawn at random. They follow a set of rules, and once you know the rules you can read a field diagram — or catch a wrong one. The magnetic field lines have the following seven properties.
How The Lines Run
- They are closed and continuous curves.
- Outside the magnet, they are directed from the north pole towards the south pole.
- The tangent at any point on a field line gives the direction of the magnetic field at that point.
Outside the magnet a line leaves the north pole, sweeps round and arrives at the south pole. It does not stop there. Inside the magnet the same line continues from the south pole back to the north pole, so it closes on itself and forms a complete loop with no free ends. This is what property 1 means, and it is drawn in Fig. 10.4-1.
Diagram
Fig. 10.4-1 — Field Lines Of A Bar Magnet Form Closed Curves A bar magnet drawn in outline in the centre, seen from above, with N at its right end and S at its left end. Several smooth closed loops drawn around it: each loop leaves the N end, arcs outwards, curves round and enters the S end, and is then continued as a dashed line straight through the inside of the magnet from S back to N. Arrowheads on every loop point away from N outside the magnet and from S towards N inside it. Draw the loops packed close together at the two ends and spread widely apart opposite the middle of the magnet. Label one point close to the N end as X and one point well out to the side of the middle of the magnet as Y.
Property 3 we have already met. Because the line is curved, its direction at a point is the direction of the tangent drawn to the curve at that point, and that is the direction of the field there.
Field Lines Never Cross
- They never intersect one another.
Suppose two field lines \(PQ\) and \(PR\) did cross at a point \(P\), as drawn in Fig. 10.4-2. A compass needle placed at \(P\) would have to rest along \(PQ\), because a needle always lies along a field line. It would also have to rest along \(PR\), for the same reason.
Diagram
Fig. 10.4-2 — Two Field Lines Cannot Intersect Two smooth curves drawn crossing each other at a single point, which is labelled P. The upper curve carries an arrowhead at its far end, which is labelled Q; the lower curve carries an arrowhead at its far end, which is labelled R. A small compass needle drawn at P, sketched twice in faint outline — once lying along PQ and once lying along PR — with a question mark beside it and the label ‘the needle cannot point two ways at once’.
The needle cannot point in two directions at the same time. That would mean the magnetic field at \(P\) had two directions at once, which is impossible. So two field lines never intersect.
Spacing Shows Strength
- They are crowded near the poles of the magnet, where the magnetic field is strong, and are far apart near the middle of the magnet and far from the magnet, where the magnetic field is weak.
- Parallel and equidistant field lines represent a uniform magnetic field. The earth’s magnetic field in a limited space is uniform.
Look again at Fig. 10.4-1. The lines are packed tightly at the two ends and spread out opposite the middle. Wherever they are crowded, the field is strong; wherever they are spread out, it is weak. So the spacing of a field diagram is not decoration — it is a reading of the field’s strength.
A uniform magnetic field is one that has the same magnitude and the same direction at every point in the region concerned. Since the direction is everywhere the same, the lines must be parallel, and since the strength is everywhere the same, they must be equally spaced. Fig. 10.4-3 shows what such a field looks like.
Diagram
Fig. 10.4-3 — Field Lines Of A Uniform Magnetic Field Five straight vertical lines drawn parallel to one another and exactly equally spaced, each carrying an arrowhead pointing upwards. Label the top of the figure ‘geographic north’ and the bottom ‘geographic south’. Add the label ‘parallel and equidistant lines — a uniform field’ below the figure.
Field Lines Under Tension
- They behave like stretched elastic rubber strings.
A stretched rubber string does two things: it tries to shorten along its own length, and it pushes sideways against any string lying alongside it. Field lines behave in just this way, and that single picture accounts for a great deal of what magnets do.
When a north pole faces a south pole, field lines run across from one to the other. Those lines try to shorten, and so the two poles are pulled together. When two like poles face each other, the lines from each are forced side by side, they push each other apart, and so the two poles are driven apart.
10.5 Magnetic Field Of The Earth
10.5.1 Evidences Of The Earth’s Magnetic Field
The earth itself has a magnetic field, and behaves like a magnet. Nobody can dig down and look, so the claim has to be built up from evidence at the surface. Four pieces of evidence point to it.
A Freely Suspended Needle Rests North–South
When a magnetic needle, or a magnet, is suspended so that it is free to turn in a horizontal plane, it always comes to rest indicating the geographic north–south direction.
Now think about what that requires. The north pole of a magnet will turn towards the geographic north only if there is a magnetic south pole up there attracting it, since unlike poles attract. In the same way, its south pole will turn towards the geographic south only if there is a magnetic north pole down there.
So we are forced to assume that there is a magnet inside the earth, with its south pole in the geographic north and its north pole in the geographic south.
An Iron Rod Buried North–South Becomes A Magnet
If an iron rod is buried a few metres inside the earth and kept along the north–south direction, it is found after some days that the rod has become a weak magnet.
Nothing has touched it and no magnet has been brought near it, so the magnetism must have been induced by something already present in that space. This is possible only if the earth itself behaves like a magnet.
Neutral Points Appear When Field Lines Are Plotted
Place a bar magnet on a horizontal plane with its north pole facing the geographic north and plot the field lines around it. Two points are found, one on either side of the magnet on its broad side-on position, at which the resultant magnetic field is zero. A compass needle placed at such a point rests in any direction at all. These are called neutral points — points at which the net magnetic field is zero.
Place the magnet the other way round, with its north pole facing the geographic south, and again two such points appear, one on either side of the magnet, this time on its end-on position.
Now, the magnet certainly has a field of its own at those points. For the resultant to be zero, some other field must be present there, equal in magnitude and opposite in direction, cancelling it exactly. That other field is the horizontal component of the earth’s magnetic field. Neutral points are taken up in full in Section 10.8.
A Needle Rests At Different Angles In Different Places
Suspend a magnetic needle so that it is free to turn in a vertical plane, and carry it around the earth passing over the geographic poles. Its geometric axis makes a different angle with the horizontal at different places, as Fig. 10.5-1 shows.
Diagram
Fig. 10.5-1 — Inclination Of A Magnetic Needle At Different Places On The Earth’s Surface A circle representing the earth, with the outlines of the continents drawn lightly inside it; where India is shown, draw it to the full extent of its territory as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Mark ‘geographic N pole’ at the top and ‘geographic S pole’ at the bottom, and draw the geographic equator as a horizontal line across the middle, labelled. Draw a tilted line through the centre of the circle, inclined at a small angle to the vertical, with ‘magnetic S pole’ marked where it meets the surface in the north and ‘magnetic N pole’ where it meets the surface in the south; label the line ‘magnetic axis’. Draw a tilted circle at right angles to this axis and label it ‘magnetic equator’. Around the outside of the earth, place about ten small magnetic needles drawn as short arrows: standing exactly perpendicular to the surface at the two magnetic poles, lying exactly along the surface at two points on the magnetic equator, and inclined at gradually increasing angles to the surface at the points between.
At two places the needle becomes normal to the earth’s surface — it stands vertical. At two other places it becomes parallel to the earth’s surface — it lies horizontal. Everywhere else it rests at some angle in between. A needle could only behave in this orderly way if the earth had a magnetic field of its own.
10.5.2 Magnetic Poles And The Magnetic Equator
The two special sets of places found in the last experiment are given names.
10.5.3 The Earth As A Huge Bar Magnet
Putting the four pieces of evidence together, we can draw a conclusion. The earth behaves as if a huge bar magnet were placed at its centre, with its magnetic south pole in the geographic north and its magnetic north pole in the geographic south. This picture is drawn in Fig. 10.5-2.
Diagram
Fig. 10.5-2 — The Earth Behaves Like A Huge Bar Magnet At Its Centre A circle representing the earth with ‘geographic N pole’ at the top and ‘geographic S pole’ at the bottom, and a vertical dashed line through the centre labelled ‘axis of rotation’. Inside the circle, a short bar magnet drawn at the centre and tilted from the vertical, with its S end towards the geographic north and its N end towards the geographic south. Extend the length of this magnet as a dashed line to the surface at both ends and label it ‘magnetic axis’; mark the angle between the magnetic axis and the axis of rotation as 17°. Draw the field lines of the bar magnet looping outside the earth, with arrowheads showing them running from the magnetic north pole region in the south, round the outside, to the magnetic south pole region in the north.
The two sets of poles do not coincide. The geographic poles are fixed by the earth’s spin, the magnetic poles by its magnetism, and the two are somewhat displaced from each other. The magnetic axis of the earth — the line joining its magnetic poles — makes an angle of \(17^\\circ\) with the axis of rotation.
The magnetic south pole of the earth is in Canada, roughly two thousand kilometres away from the geographic north pole, at \(70.75^\\circ\) north latitude and \(96^\\circ\) west longitude. The earth’s magnetic north pole lies a comparable distance from the geographic south pole, at \(73^\\circ\) south latitude and \(155^\\circ\) east longitude.
These positions are not fixed. It has been observed experimentally that the magnetic poles do not stay still, but gradually change their positions over long periods of time.
10.5.4 Magnetic Field Lines Of The Earth
Over the whole planet the earth’s field lines curve, as Fig. 10.5-2 shows. But over a small region — a table top, a laboratory bench — the curvature is far too slight to notice. Within such a limited space the field lines of the earth are parallel to one another and equidistant, exactly the pattern drawn in Fig. 10.4-3. The earth’s magnetic field in a limited space is therefore uniform.
These lines are always directed from the geographic south towards the geographic north. That is the direction in which a freely suspended needle sets itself, and it is the reason the north pole of a compass needle points north.
Their angle to the ground changes with place. They are horizontal at the magnetic equator and vertical at the magnetic poles, and at every other point they are inclined to the horizontal.
10.6 Plotting The Uniform Magnetic Field Lines Of The Earth
We have said that the earth’s magnetic field is uniform in a limited space. That is a claim about the world, and it can be checked on a table top with nothing more than a compass needle and a pencil. The method below traces the earth’s field lines one at a time.
The Experiment
Fix a sheet of paper on a drawing board, or on a table top, using brass pins. Brass is chosen deliberately, because it is not a magnetic material and so cannot disturb the field being plotted. For the same reason the board must be well away from magnets, iron objects and electrical fittings.
Place a small compass needle on the paper at position 1, as in Fig. 10.6-1(a). The compass must be small, so that the field does not change appreciably from one end of the needle to the other.
Diagram
Fig. 10.6-1 — Tracing A Field Line Of The Earth With A Compass Needle Two labelled parts side by side. (a) A sheet of paper pinned at its corners, with a small circular compass drawn three times, one above the other, marked position 1, position 2 and position 3, each with its needle marked N at the upper end and S at the lower end; a pencil dot marked just beyond each end of the needle at every position, with the dot in front of the N end of position 1 coinciding exactly with the S end of the needle at position 2, and the dot in front of the N end of position 2 coinciding with the S end at position 3. (b) The same dots joined by a single straight line carrying an arrowhead pointing from the S end towards the N end, labelled ‘one magnetic field line of the earth’. Label the pins ‘brass pins’.
Look down from directly above the needle and mark two pencil dots on the paper, exactly in front of the two ends of the needle. Now move the compass to position 2 in such a way that one end of the needle sits on the second pencil dot. Mark the position of its other end with a fresh dot.
Repeat this, moving the compass to positions 3, 4 and so on, until several dots have been marked across the sheet. Join the dots. You get a straight line, and that line is one magnetic field line of the earth.
Flowchart
FC 10.1 — Plotting One Field Line Of The Earth Pin the paper down with brass pins → place the small compass at position 1 → mark a pencil dot in front of each end of the needle → move the compass so that one end of the needle rests on the second dot → mark a dot in front of the other end → repeat for positions 3, 4, 5 … across the sheet → join all the dots → mark an arrow on the line from the S end of the needle towards its N end
Now start again from a different point on the paper and trace out a second line in the same way. Draw several lines in this manner, each begun at a different starting point.
Finally, label each line with an arrow drawn from the south pole of the needle towards its north pole. The arrow shows the direction of the magnetic field along that line.
What The Plotted Lines Show
The set of lines obtained is drawn in Fig. 10.6-2, and three things are noticed about it.
Diagram
Fig. 10.6-2 — Field Lines Of The Earth Obtained In A Limited Space Five straight lines drawn on a pinned sheet of paper, all vertical, all exactly parallel and equally spaced, each carrying an arrowhead pointing upwards. Mark ‘geographic north’ at the top of the sheet and ‘geographic south’ at the bottom. Label the figure ‘the lines are straight, parallel, equidistant and do not intersect’.
First, the lines do not intersect each other, which is what property 4 of field lines requires. Second, they are parallel and equidistant — and by property 6 that means the field they represent is uniform.
Third, every arrow points the same way, from the geographic south towards the geographic north. This is exactly the direction in which a magnetic needle suspended freely in a horizontal plane comes to rest.
10.7 Plotting The Non-Uniform Field Of A Strong Bar Magnet
The earth’s field over a table top was uniform, and plotting it gave straight, parallel, equally spaced lines. The field around a bar magnet is nothing like that. It is non-uniform.
In a non-uniform magnetic field the lines are neither equispaced nor parallel. They are curved, either converging towards a place or diverging away from one, and their spacing changes from point to point. Where they are closely spaced the field at that place is strong; where they are widely spaced it is weak. Four such patterns are drawn in Fig. 10.7-1.
Diagram
Fig. 10.7-1 — Non-Uniform Magnetic Field Lines Four labelled patterns arranged in two rows. (a) A single bar magnet with N at the right and S at the left, with curved closed lines leaving N, arcing out above and below, and entering S; arrowheads on every line pointing from N to S outside the magnet; caption ‘field lines due to a bar magnet’. (b) A U-shaped horse shoe magnet with N on the left arm-end and S on the right arm-end, with curved lines crossing the gap between the two arm-ends and further lines arcing round the outside; caption ‘field lines due to a horse shoe magnet’. (c) Two bar magnets placed end to end in a line with a gap between them, the left one with S at its left and N at its right, the right one with S at its left and N at its right, so that a north pole faces a south pole across the gap; draw the lines running straight across the gap from the N to the S and bulging outwards on either side; caption ‘field lines due to two unlike poles facing each other’. (d) Two bar magnets placed end to end with a gap, both with N facing the gap; draw the lines from each N curving away sharply from the gap and turning outwards, leaving a clear space in the middle with a small cross marked at the mid-point of the gap; caption ‘field lines due to two like poles facing each other’. Draw the lines closely packed near every pole and widely spaced far from the magnets.
To plot the field of a bar magnet, the magnet is laid along the magnetic meridian — the direction in which a freely suspended magnet comes to rest, which is the geographic north–south direction. Two arrangements are used.
- The magnet is placed with its north pole pointing towards the north.
- The magnet is placed with its south pole pointing towards the north.
10.7.1 Magnet Placed With Its North Pole Pointing North
Tracing The Lines
Fix a sheet of white paper on a drawing board with brass pins. Place a compass needle in the middle of the paper, let it settle, and mark the north–south direction it gives. Draw a line along this direction.
Lay the bar magnet on the paper along that line, with its north pole pointing towards the north, and mark its outline with a fine pencil. The outline matters, because after the magnet is lifted away you must still know exactly where it stood.
Place the compass needle close to the north pole of the magnet. Looking from directly above, mark two pencil dots exactly in front of the two ends of the needle. Move the needle so that one of its ends coincides with the second dot, and mark the position of the other end with a fresh dot.
Repeat this step by step until the other end of the bar magnet is reached. Join the dots, and this time they give a continuous smooth curve rather than a straight line. That curve is one magnetic field line.
Now repeat the whole process from the same pole of the magnet but starting at a different point, and trace out another line. In this manner draw several field lines, each starting from a different point near the same pole. Label every line with an arrow running from the north pole towards the south pole of the magnet — which is the same as running from the south pole of the compass needle to its north pole.
Reading The Pattern
The lines obtained are shown in Fig. 10.7-2. It is important to be clear about what they represent. They are due to the combined effect of two fields: the magnetic field of the magnet, and the magnetic field of the earth. There is no way to switch the earth off.
Diagram
Fig. 10.7-2 — Field Lines Of A Bar Magnet With Its North Pole N Facing Geographic North A sheet of paper marked ‘north’ at the top and ‘south’ at the bottom, with a small compass rose drawn to one side showing N up, S down, W left and E right. A bar magnet drawn vertically in the centre with N at its upper end and S at its lower end. Around it, several smooth curved field lines with arrowheads, leaving the N end, sweeping outwards to the left and right, and returning to the S end. Well away from the magnet at the top and bottom of the sheet, draw the lines becoming nearly straight, parallel and pointing from south to north. On the horizontal line through the centre of the magnet, mark a cross to the east of the magnet and another cross to the west of it, both at the same distance from the centre, and label them ‘neutral points’.
Two features stand out. First, close to the magnet the lines are strongly curved. In that region the magnet’s own field is much stronger than the earth’s, so it is the magnet that decides the pattern.
As the distance from the magnet increases, the magnet’s field weakens, and at distant points it becomes weaker than the earth’s field. So far from the magnet the lines straighten out and become parallel to each other. There the pattern is due mainly to the earth’s magnetic field.
Second, there are two points equidistant from the centre of the magnet, marked with a cross in Fig. 10.7-2, lying in the east and west directions. At these two points the field of the magnet and the horizontal component of the earth’s field are equal in magnitude but opposite in direction, so they neutralise each other exactly.
These are neutral points. A compass needle placed at one of them is left unaffected and rests in any direction you leave it in. Notice where they lie — out to the side of the magnet, level with its middle, in what is called the broad side-on position.
10.7.2 Magnet Placed With Its South Pole Pointing North
Now turn the magnet end for end. It still lies along the magnetic meridian, but this time its south pole points towards the north. The field lines are traced by exactly the same method as before, and the result is shown in Fig. 10.7-3.
Diagram
Fig. 10.7-3 — Field Lines Of A Bar Magnet With Its South Pole Facing Geographic North A sheet of paper marked ‘north’ at the top and ‘south’ at the bottom, with a compass rose to one side showing N up, S down, W left and E right. A bar magnet drawn vertically in the centre with S at its upper end and N at its lower end. Around it, several smooth curved field lines with arrowheads leaving the N end at the bottom, sweeping widely outwards to left and right, and entering the S end at the top. Far from the magnet at the left and right edges of the sheet, draw the lines as nearly straight parallel lines running from south to north. On the vertical line through the magnet, mark a cross directly above the magnet and another directly below it, both at the same distance from the centre, and label them ‘neutral points’.
The first feature is the same as before. Near the magnet the lines are curved, because there the magnet’s field is much stronger than the earth’s. At distant points the magnet’s field has fallen below the earth’s, and the lines become nearly parallel straight lines running from south to north, set by the earth alone.
The second feature has moved. There are again two points equidistant from the centre of the magnet at which the magnet’s field and the horizontal component of the earth’s field are equal and opposite, so that the two fields cancel. But this time they lie in the north and south directions, beyond the two ends of the magnet, in what is called the end-on position. A compass needle placed at either point again comes to rest pointing in any direction.
10.8 Neutral Points
10.8.1 What A Neutral Point Is
We have met neutral points twice already, once as evidence for the earth’s magnetism and once on the plotting sheet. It is time to state exactly what they are.
A neutral point is not a place where there is no magnetic field being produced. It is a place where two magnetic fields arrive together, equal in magnitude but opposite in direction, so that they cancel and leave nothing behind.
The needle rests in any direction because there is nothing left to turn it. A compass needle is turned by the field it sits in, and at a neutral point the resultant field is zero, so the needle simply stays where it is put.
The Two Fields That Cancel
The two fields that meet at a neutral point can be produced in either of two ways. They may come from two separate magnets, or one may come from a magnet and the other from the earth.
For the field of a magnet, two facts settle where it points and how strong it is. Outside the magnet it is directed from the north pole towards the south pole, and it decreases as the distance from the magnet increases.
The earth’s magnetic field is resolved into two parts. Its horizontal component is directed from the geographic south to the geographic north. Its vertical component, in the northern hemisphere, points vertically downwards.
It is almost always the horizontal component that does the cancelling on a table top, because a magnet lying flat on the paper has a field that is itself horizontal there.
10.8.2 Positions Of Neutral Points In Different Cases
Where the neutral points appear depends entirely on how the magnet is placed. Four cases cover the arrangements you will meet.
- For two magnets at a separation in one line with their like poles facing each other, the neutral point is at some point between the two magnets. This case is drawn in Fig. 10.8-1.
- For a magnet standing vertical with its north pole up, the neutral point is at some point on the vertical line away from its north pole, as in Fig. 10.8-2.
- If the north pole of the magnet faces the geographic north, the neutral points are situated symmetrically on either side of the magnet, at equal distances from its centre, in the east–west direction. This is the case already plotted in Fig. 10.7-2.
- If the south pole of the magnet faces the geographic north, the neutral points are on either side of the magnet at equal distances from its centre, in the north–south directions, as plotted in Fig. 10.7-3.
Diagram
Fig. 10.8-1 — Neutral Point Between Two Magnets With Like Poles Facing Two bar magnets drawn horizontally in one straight line with a gap between them. The left magnet has S at its left end and N at its right end; the right magnet has N at its left end and S at its right end, so that two north poles face each other across the gap. Draw a short arrow in the gap pointing rightwards, starting from the left magnet’s N pole and labelled ‘field of the left magnet’; draw a second short arrow in the gap pointing leftwards, starting from the right magnet’s N pole and labelled ‘field of the right magnet’. Mark a cross at the point in the gap where the two arrows meet and label it ‘neutral point’.
Diagram
Fig. 10.8-2 — Neutral Point Above A Vertical Magnet A bar magnet drawn standing upright on a table with N at its top end and S at its bottom end. A vertical dashed line drawn upwards from the N pole. On this line, well above the magnet, mark a cross labelled ‘neutral point’. Draw a short arrow at the cross pointing vertically upwards labelled ‘field of the magnet’, and a second short arrow at the same point pointing vertically downwards labelled ‘vertical component of the earth’s field’, the two arrows drawn equal in length.
In every one of these cases the test is the same. Look for the place where the two fields present are equal in size and exactly opposed in direction, and that is where the compass needle will lose its bearings.
10.9 Summary
Lodestone is a naturally occurring ore of iron oxide, \(\\mathrm{Fe\_3O\_4}\), which attracts small pieces of iron and sets itself along a definite direction when suspended freely. Pieces of lodestone are called natural magnets.
Natural magnets are of odd shapes and are not magnetically strong enough for use, so artificial magnets are made from iron and steel in convenient shapes — the bar magnet, the horse shoe magnet, the magnetic needle and the magnetic compass.
A magnet suspended by a silk thread so that it is free to turn in a horizontal plane always comes to rest along the geographic north–south direction. The end that turns towards the geographic north is the north pole of the magnet, and the other end is its south pole.
Two like poles repel each other, while two unlike poles attract each other.
A magnetic material, such as soft iron or steel, is one that a magnet can attract and that can itself be magnetised.
The temporary magnetism acquired by a magnetic material when it is kept near, or in contact with, a magnet is called induced magnetism. The process by which it acquires these properties is called magnetic induction.
A magnetic pole induces an opposite polarity on the near end of an iron bar and a similar polarity on its farther end.
Induction precedes attraction. A piece of iron first becomes a magnet by induction, its near end acquiring the opposite polarity, and only then is it attracted, because unlike poles attract.
Magnetism acquired by induction is purely temporary. It lasts only as long as the magnet causing the induction remains in the neighbourhood, which is why a chain of iron nails hanging from a magnet collapses the moment the magnet is taken away.
The space around a magnet in which the needle of a compass rests in a direction other than the geographic north–south direction is called the magnetic field of the magnet. Its effect decreases as the distance from the magnet increases.
Magnetic field is a vector quantity. Its magnitude at a point is measured by the force experienced by a magnetic pole of unit strength placed there, and its direction is the direction in which the north pole of a compass needle rests when placed at that point.
Iron filings sprinkled on a plate over a magnet and gently tapped arrange themselves along curved lines, because each filing is magnetised by induction and then experiences a force. These curved lines are the magnetic field lines.
A magnetic field line is a continuous curve in a magnetic field such that the tangent at any point of the curve gives the direction of the magnetic field at that point. An arrow is marked on it from the south pole of the compass needle towards its north pole.
The magnetic field lines have the following properties.
- They are closed and continuous curves.
- Outside the magnet they are directed from the north pole towards the south pole, and inside it from the south pole to the north pole.
- The tangent at any point on a field line gives the direction of the magnetic field at that point.
- They never intersect one another, since the field cannot have two directions at one point.
- They are crowded near the poles where the field is strong, and far apart near the middle of the magnet and far from it where the field is weak.
- Parallel and equidistant field lines represent a uniform magnetic field, and the earth’s magnetic field in a limited space is uniform.
- They behave like stretched elastic rubber strings.
- The existence of the earth’s magnetic field is inferred from four facts.
- A freely suspended magnetic needle always rests in the geographic north–south direction.
- An iron rod buried inside the earth along the north–south direction becomes a weak magnet.
- Neutral points are obtained on plotting the field lines of a magnet, where the net magnetic field is zero.
- A magnetic needle rests making different angles with the horizontal when suspended at different places on the earth.
The two places where a magnetic needle free to turn in a vertical plane becomes vertical are called the magnetic poles of the earth. The line joining the places where such a needle becomes horizontal is called the magnetic equator.
The earth behaves as if a huge bar magnet were placed at its centre, with its magnetic south pole in the geographic north and its magnetic north pole in the geographic south. The magnetic axis makes an angle of \(17^\\circ\) with the axis of rotation, and the positions of the magnetic poles change gradually over long periods of time.
The magnetic field lines of the earth are normal to the earth’s surface near the magnetic poles and parallel to the earth’s surface near the magnetic equator. In a limited space they are parallel and equidistant, and they are always directed from the geographic south to the geographic north.
The uniform field lines of the earth are plotted with a small compass needle on paper held by brass pins, marking successive pairs of dots in front of the ends of the needle and joining them. The lines obtained are straight, parallel, equidistant and non-intersecting.
The magnetic field around a bar magnet or a horse shoe magnet is non-uniform. Its field lines are not equispaced and parallel but curved, either converging or diverging; closely spaced lines mean a strong field and widely spaced lines a weak field.
To plot the field of a bar magnet, the magnet is laid along the magnetic meridian, either with its north pole pointing north or with its south pole pointing north. The pattern obtained is always due to the combined effect of the magnet’s field and the earth’s field.
Near the magnet the plotted lines are curved, because the magnet’s field is the stronger there. At distant points they become nearly parallel straight lines running from south to north, because the earth’s field is the stronger there.
Neutral points are the points at which two magnetic fields are equal in magnitude but opposite in direction. The net magnetic field at a neutral point is zero, and a compass needle placed there rests in any direction.
The earth’s magnetic field has a horizontal component directed from the geographic south to the geographic north, and a vertical component directed vertically downwards in the northern hemisphere.
The positions of the neutral points depend on how the magnet is placed.
- For two magnets in one line with their like poles facing each other, the neutral point lies between the two magnets.
- For a magnet standing vertical with its north pole up, the neutral point lies on the vertical line away from its north pole.
- If the north pole of the magnet faces the geographic north, the neutral points lie on either side of it at equal distances from its centre, in the east–west direction.
- If the south pole of the magnet faces the geographic north, the neutral points lie on either side of it at equal distances from its centre, in the north–south direction.
Flowchart
FC 10.2 — Why A Magnet Attracts An Unmagnetised Piece Of Iron Magnet brought near the iron → near end of the iron acquires the opposite polarity by magnetic induction → the near end and the facing pole of the magnet are now unlike poles → unlike poles attract → the iron is drawn towards the magnet → magnet taken away → the induced magnetism is lost and the attraction ceases
10.10 Key Terms
Artificial magnet — a magnet manufactured from iron or steel in a convenient shape and size, made because natural magnets are irregular and too weak for use.
Bar magnet — an artificial magnet in the form of a straight rectangular bar, with a north pole at one end and a south pole at the other.
Horse shoe magnet — an artificial magnet bent into a U shape so that its two poles face the same way, close to each other.
Induced magnetism — the temporary magnetism acquired by a magnetic material when it is kept near, or in contact with, a magnet.
Lodestone — a naturally occurring ore of iron oxide, \(\\mathrm{Fe\_3O\_4}\), which attracts small pieces of iron and sets itself along a definite direction when freely suspended.
Magnetic axis of the earth — the line joining the two magnetic poles of the earth, which makes an angle of \(17^\\circ\) with the earth’s axis of rotation.
Magnetic compass — a magnetic needle pivoted inside a small box over a marked dial, used to find direction.
Magnetic equator — the line joining the places on the earth’s surface where a magnetic needle free to turn in a vertical plane rests horizontal.
Magnetic field — the space around a magnet in which a compass needle rests in a direction other than the geographic north–south direction; a vector quantity, measured in tesla (T).
Magnetic field line — a continuous curve in a magnetic field such that the tangent at any point of the curve gives the direction of the magnetic field at that point.
Magnetic induction — the process by which a piece of magnetic material acquires magnetic properties temporarily in the presence of another magnet near it.
Magnetic material — a material such as soft iron or steel which a magnet can attract and which can itself be magnetised.
Magnetic meridian — the direction in which a freely suspended magnet comes to rest, that is, the geographic north–south direction.
Magnetic needle — a slender magnet balanced on a sharp pivot so that it is free to swing in a horizontal plane.
Magnetic poles of the earth — the two places on the earth’s surface where a magnetic needle free to turn in a vertical plane rests vertical.
Natural magnet — a piece of lodestone found in the earth, magnetic as it is dug out, without being manufactured.
Neutral point — a point at which two magnetic fields are equal in magnitude but opposite in direction, so that the net magnetic field there is zero.
North pole of a magnet — the end of a freely suspended magnet that turns towards the geographic north.
South pole of a magnet — the end of a freely suspended magnet that turns towards the geographic south.
Uniform magnetic field — a field having the same magnitude and the same direction at every point in the region, represented by parallel and equidistant field lines.
10.11 Points To Ponder
Iron filings tapped on a plate settle into a pattern, and we say the pattern reveals the field lines. But the filings are objects and the lines are drawings. In what sense, then, does a magnetic field exist at a point where nothing at all has been placed?
A soft iron bar loses its induced magnetism the instant the magnet is taken away, while a steel bar magnet keeps its magnetism for years with nothing near it. What might be different inside the two materials to account for such different behaviour?
The magnetic poles of the earth are known to shift gradually over long periods. If the earth’s magnetic poles were one day to exchange places, what would change for a ship’s compass, for a plotted field diagram, and for a migrating bird — and what would not change at all?
At a neutral point the resultant magnetic field is zero, so a compass needle placed there is left undisturbed. Would a small iron nail placed at that same point also stay where it is put? Think carefully about the fact that a nail has length, and that the field a short distance away is not zero.
The north pole of a compass needle is attracted towards the geographic north, which means the earth has a magnetic south pole up there. Was the naming of the poles a mistake, or does it make sense once you know how the names were chosen? What would have to be renamed if the convention were reversed today?
Field lines are said to behave like stretched elastic rubber strings, and this picture explains neatly why unlike poles are drawn together and like poles pushed apart. Where else does the picture serve you well, and is there anything about a magnetic field that a rubber string could never capture?
10.12 Objective Questions
- A magnetic pole induces on the near end of an iron bar a polarity which is: (1 mark)
- similar to its own
- opposite to its own
- always north
- always south
- Inside a magnet, the magnetic field lines are directed: (1 mark)
- from the north pole to the south pole
- from the south pole to the north pole
- from both poles towards the middle
- nowhere, since no lines exist inside a magnet
- Magnetic field lines which are parallel and equidistant represent: (1 mark)
- a uniform magnetic field
- a converging magnetic field
- a diverging magnetic field
- a region of zero magnetic field
- The magnetic axis of the earth makes with its axis of rotation an angle of about: (1 mark)
- \(0^\\circ\)
- \(17^\\circ\)
- \(45^\\circ\)
- \(90^\\circ\)
- A bar magnet is placed along the magnetic meridian with its south pole pointing towards the geographic north. Its neutral points lie: (1 mark)
- in the east and west directions
- in the north and south directions
- at the two poles of the magnet
- nowhere, since none are formed
- A magnetic needle free to turn in a vertical plane rests parallel to the earth’s surface at the: (1 mark)
- magnetic poles
- magnetic equator
- geographic poles
- neutral points
- Which of the following is a magnetic material? (1 mark)
- copper
- brass
- soft iron
- aluminium
- While plotting magnetic field lines, the sheet of paper is fixed with brass pins because brass: (1 mark)
- is a good conductor of heat
- is not a magnetic material
- is lighter than iron
- does not rust
For each of the following, choose the correct option from the codes given below. (1 mark each) a. Both A and R are true, and R is the correct explanation of A b. Both A and R are true, but R is not the correct explanation of A c. A is false but R is true d. A is true but R is false
Assertion (A): A compass needle placed at a neutral point rests in any direction. Reason (R): The resultant magnetic field at a neutral point is zero.
Assertion (A): Iron filings sprinkled over a plate covering a bar magnet arrange themselves along curved lines when the plate is tapped. Reason (R): Each filing becomes a magnet by induction and then experiences a force due to the magnet.
Assertion (A): The magnetism induced in a soft iron bar by a nearby magnet is permanent. Reason (R): Induced magnetism lasts only so long as the magnet causing the induction stays in the vicinity.
Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Lodestone | i. Its net magnetic field is zero |
| b. Magnetic equator | ii. A naturally occurring ore of iron oxide |
| c. Neutral point | iii. Parallel and equidistant field lines |
| d. Uniform field | iv. The direction in which a freely suspended magnet rests |
| e. Magnetic meridian | v. The needle of a dip circle rests horizontal here |
Complete the following sentences. (4 marks)
- Outside a magnet, magnetic field lines are directed from the ………. pole towards the ………. pole.
- The magnetism acquired by a magnetic material kept near a magnet is called ………. magnetism.
- The magnetic field lines of the earth are ………. to the earth’s surface near the magnetic poles.
- The horizontal component of the earth’s magnetic field is directed from the geographic ………. to the geographic ……….
State whether each statement is true or false. (3 marks)
- Two magnetic field lines may cross each other where the field is very strong.
- Magnetic field lines are crowded near the poles of a magnet.
- The magnetic poles of the earth stay at fixed positions for all time.
Name the following. (3 marks)
- The ore whose pieces were the first known magnets.
- The points near a bar magnet at which a compass needle rests in any direction.
- The shape of magnet whose two poles lie close together and face the same way.
Give one word or phrase for each of the following. (2 marks)
- The process by which a piece of soft iron acquires magnetic properties temporarily in the presence of a magnet.
- A field having the same magnitude and direction at every point of a region.
State the SI unit of magnetic field. (1 mark)
Pick the odd one out and give a reason for your choice: bar magnet, horse shoe magnet, lodestone, magnetic needle. (1 mark)
10.13 Check Your Concepts
Define induced magnetism. (2 marks)
State the law of magnetic poles. (2 marks)
What is a lodestone? State the two properties by which it was recognised. (2 marks)
Why are artificial magnets preferred to natural magnets? Give two reasons. (2 marks)
Give a reason for the following: a compass needle placed at a neutral point does not settle in any particular direction. (2 marks)
Differentiate between induced magnetism and permanent magnetism, stating the basis of difference. (3 marks)
State any three properties of magnetic field lines. (3 marks)
Why does the spacing between magnetic field lines change from place to place around a bar magnet? What does close spacing indicate? (3 marks)
Explain the statement ‘induction precedes attraction’ with reference to a piece of soft iron brought near one pole of a magnet. (3 marks)
Two magnetic field lines can never intersect each other. Give the reason, and state what would follow if they did. (3 marks)
State any four evidences for the existence of the earth’s magnetic field. (4 marks)
Several soft iron nails hang one below the other in a chain from one pole of a bar magnet. Explain how the chain forms, why it stops growing at a certain length, and what happens when the magnet is withdrawn. (4 marks)
What is meant by a neutral point? State the positions of the neutral points when a bar magnet is placed in the magnetic meridian with (i) its north pole pointing towards the geographic north, and (ii) its south pole pointing towards the geographic north. (4 marks)
Describe how the uniform magnetic field lines of the earth are plotted in a limited space using a small compass needle. State two features of the pattern obtained, and explain why brass pins are used to fix the paper. (5 marks)
What would happen if the earth suddenly lost its magnetic field? Discuss the effect on a freely suspended magnet, on a plotted field pattern of a bar magnet, and on the neutral points near that magnet. (5 marks)
10.14 Diagrams, Graphs & Constructions
Draw a neat labelled diagram of a bar magnet suspended freely by a silk thread from a rigid support. Mark the polarity of its two ends and indicate the geographic north–south direction. (3 marks)
A soft iron bar \(PQ\) is placed in line with a bar magnet, with the north pole of the magnet facing end \(P\). Draw the arrangement, mark the polarity induced at \(P\) and at \(Q\), and state which of the two ends will attract the magnet. (3 marks)
Fig. 10.7-2 shows the field lines of a bar magnet placed along the magnetic meridian with its north pole facing the geographic north. Redraw the pattern, mark the two neutral points with crosses, and state the direction in which they lie from the centre of the magnet. (4 marks)
Sketch four magnetic field lines as obtained on a horizontal plane in a limited space in the earth’s magnetic field alone. Put an arrowhead on each line, and state what the spacing of your lines tells you about the field. (3 marks)
Two bar magnets are placed on a table in one straight line with a gap between them, their north poles facing each other. Draw the arrangement, mark the likely position of the neutral point with a cross, and give a reason for the position you have chosen. (4 marks)
Fig. 10.4-2 shows two field lines drawn crossing each other at a point \(P\). Copy the figure, mark the point \(P\), and state with a reason why no field diagram may ever be drawn in this way. (3 marks)
Draw a labelled diagram of the earth showing it as a huge bar magnet placed at its centre. Mark the geographic poles, the magnetic poles, the magnetic axis and the axis of rotation, and show the angle between the two axes. (5 marks)