Making artificial magnets
A steel needle rubbed on a magnet starts picking up pins. Fridge magnets and cupboard-door magnets hold things in the same way.
Magnets are mostly made from alloys of iron, cobalt and nickel. Nipermag (iron, nickel, aluminium, titanium) and alnico (aluminium, nickel, cobalt) are alloys used for industrial magnets. Making a material like steel magnetic is magnetisation (चुंबकीकरण).
Rule 1: Single-touch method — Drag one pole of a magnet from one end of a steel bar to the other, in one direction only, 15-20 times, lifting the magnet each time. The magnetism so made is weak and lasts only a short time.
- Easy : The single-touch method uses one pole.
- Medium : Magnetism made by single touch is weak.
- Exam-type : A bar magnetised by single touch may stop picking up pins within a short time.
Rule 2: Double-touch method — Place opposite poles of two magnets at the middle of the bar and drag them apart towards the two ends, 15-20 times. The magnetism lasts longer.
- Easy : The double-touch method uses opposite poles of two magnets.
- Medium : Magnetism from double touch lasts longer than from single touch.
- Exam-type : If a longer-lasting magnet is needed, choose the double-touch method.
To test the new magnet, bring the bar near iron filings and hang it freely on a thread.
Thinking a magnet attracts every metal. It attracts iron, cobalt, nickel and similar materials, not copper or aluminium.
Memory clue : "One pole = single = weak; two poles = double = lasts longer" — count the poles used.
Spot the difference
| Point | Single touch | Double touch |
|---|---|---|
| Poles used | One | Two opposite poles |
| Strength | Low | Higher |
| Lasts | Short time | Longer |
Practice questions
- 1What are magnets made from?
- 2Which metals does alnico contain?
- 3What is magnetisation?
- 4How is the single-touch method done?
- 5Which poles are used in double touch?
- 6Which method gives a longer-lasting magnet?
- 7How will you check whether a new magnet has formed?
- 8Why is a copper strip not attracted by a magnet?
Show answers (try them yourself first)
- 1Alloys of iron, cobalt and nickel.
- 2Aluminium, nickel and cobalt.
- 3Making a material magnetic.
- 4By dragging one pole of a magnet along the bar in one direction 15-20 times.
- 5Opposite poles of two magnets.
- 6Double touch.
- 7See if it picks up iron filings and settles north-south when hung freely.
- 8Copper is not a magnetic material.
The Earth as a huge magnet
The needle of a compass shows roughly the north-south direction wherever you are.
A freely hanging magnet always settles in the north-south direction. William Gilbert showed through experiment that the Earth itself is a giant magnet.
Rule 1: Gilbert's experiment — Gilbert shaped a natural magnetic rock into a sphere and hung it so that it could turn freely. When the north pole of a bar magnet came near, the south pole of the sphere was attracted. From this he concluded that the Earth is a magnet.
- Easy : Opposite poles attract each other.
- Medium : The south pole of the sphere was drawn to the north pole of the bar magnet.
- Exam-type : A free magnet's north pole turns towards geographic north, so the Earth's magnet must have its south pole there.
Rule 2: The magnetic needle — A magnetised needle fixed on a card floating on water settles in the north-south direction, its north pole pointing to geographic north. That is why a compass uses a magnetic needle.
- Easy : The needle in a compass shows direction.
- Medium : A magnetic needle floating on water turns freely and settles north-south.
- Exam-type : If you lose direction, the side where the compass needle's north pole settles is north.
The south pole of the Earth's magnet is near geographic north and its north pole near geographic south.
Thinking there is a magnetic north pole at geographic north. There is a south pole of the Earth's magnet there.
Memory clue : "North turns north, so south sits there" — unlike poles attract.
Spot the difference
| Point | Geographic north | Geographic south |
|---|---|---|
| Earth magnet pole | South pole (S) | North pole (N) |
| Needle pole turning | North pole | South pole |
Practice questions
- 1In which direction does a freely hanging magnet settle?
- 2Who showed that the Earth is a magnet?
- 3What is there in a compass?
- 4How did Gilbert use a spherical magnet?
- 5Where is the south pole of the Earth's magnet?
- 6Towards which direction does the north pole of a magnetic needle turn?
- 7Why does the north pole of the needle turn to the north?
- 8How will a compass help if you lose direction in a forest?
Show answers (try them yourself first)
- 1North-south.
- 2William Gilbert.
- 3A magnetic needle.
- 4He hung it freely and brought the north pole of a bar magnet near it.
- 5Near geographic north.
- 6Geographic north.
- 7It is attracted by the south pole of the Earth's magnet located there.
- 8The direction where the needle's north pole settles is north.
Magnetic field and lines of force
A magnet pulls pins towards itself from a distance without touching them.
The region around a magnet where its force acts is the magnetic field (चुंबकीय क्षेत्र). Michael Faraday called the imaginary lines that show this field magnetic lines of force (चुंबकीय बलरेषा).
Rule 1: Magnetic force acts without touching — Sprinkle iron filings around a bar magnet and tap the card gently; the filings arrange themselves along curved lines showing the shape of the field.
- Easy : A magnet attracts pins from a distance.
- Medium : The pattern of iron filings shows the shape of the field.
- Exam-type : As the magnet is moved away from the pins the pull weakens because the field is weaker farther away.
Magnetic field and lines of force
In the app you can move this figure with your finger.
Open in the appRule 2: Closer lines, stronger field — The number of lines passing perpendicularly through a unit area shows the strength of the field. Where lines are crowded the field is strong; where they are sparse it is weak. Lines are crowded near the poles.
- Easy : The grip of a magnet is strongest near its poles.
- Medium : Where lines are far apart the field is weak.
- Exam-type : Fewer pins stick to the middle of a bar magnet because the field is weaker there.
Lines of force are imaginary and cannot be seen; iron filings only show their shape.
Thinking lines of force are real threads. They are an idea used to describe the field.
Memory clue : "Crowded lines = strong field" — a crowd of lines means strength.
Spot the difference
| Point | Crowded lines | Sparse lines |
|---|---|---|
| Field | Strong | Weak |
| Where | Near poles | Far from poles |
Practice questions
- 1What is a magnetic field?
- 2Who introduced the idea of lines of force?
- 3What do iron filings show?
- 4How can we tell the strength of a field?
- 5Why is the field strong near the poles?
- 6How does a magnet pull pins from a distance?
- 7Why are lines of force called imaginary?
- 8Why does the pull weaken as we move away from the magnet?
Show answers (try them yourself first)
- 1The region around a magnet where its force acts.
- 2Michael Faraday.
- 3The shape of the magnetic field.
- 4By how crowded the lines of force are.
- 5The lines of force are crowded there.
- 6Because of its magnetic field.
- 7They are not seen; they are an idea to explain the field.
- 8The field gets weaker with distance.
Properties of lines of force and uses of magnets
A cupboard door has a small magnet; at an airport a metal detector finds metal.
Faraday argued that lines of force have certain properties which explain attraction, repulsion and how the field behaves.
Rule 1: Six properties of lines of force — (1) They are imaginary. (2) They run from the north pole to the south pole. (3) They are in tension like a stretched spring. (4) They repel each other. (5) They never cross. (6) The number at a place decides the strength of the field there.
- Easy : Lines of force never cross each other.
- Medium : Stretched lines pull unlike poles towards each other.
- Exam-type : Lines repel each other, so like poles push each other away.
Rule 2: The field passes through; repulsion is the real test — A magnetic field passes through cardboard, plastic and water, but each layer weakens it. The real test of a magnet is repulsion; attraction can also happen with a plain piece of iron.
- Easy : A magnet lifts pins from below a cardboard.
- Medium : More layers of cardboard weaken the pull of the magnet.
- Exam-type : Which of two iron pieces is the magnet? The one that is pushed away by another magnet's pole.
Metal detectors are used at airports, bus stations and temples, and in food industries to find stray iron pieces.
Thinking attraction proves a thing is a magnet. Only repulsion proves it.
Memory clue : "Attraction = maybe, repulsion = proof" — the test of a true magnet.
Spot the difference
| Point | Attraction | Repulsion |
|---|---|---|
| Between | Magnet-magnet or magnet-iron | Magnet-magnet only |
| As a test of a magnet | Not enough | The real test |
Practice questions
- 1From which pole to which pole do the lines of force run?
- 2Do lines of force cross each other?
- 3What is the real test of a magnet?
- 4Through what does a magnetic field pass?
- 5Why do like poles push each other away?
- 6State two uses of a metal detector.
- 7What happens as layers of cardboard are increased?
- 8How will you find the magnet among two similar-looking bars?
Show answers (try them yourself first)
- 1From the north pole to the south pole.
- 2No.
- 3Repulsion.
- 4Cardboard, plastic and water.
- 5Lines of force repel each other.
- 6Checking at airports; finding iron bits in food.
- 7The field weakens and the pull decreases.
- 8See if one bar's pole repels the other; if it does, they are both magnets.