Simple Phenomena Of Magnetism

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Notes

Magnetic and Non-Magnetic Materials

Only iron, cobalt, nickel, and steel (an alloy containing iron) are **magnetic**.
Magnetic materials are **attracted** to a magnet; non-magnetic materials are not.
A magnetic material (not a magnet) is attracted to **both poles** of a permanent magnet.
To test a material: if repelled by a known magnet, it is a **magnet**; if attracted, it is a **magnetic material**; if neither, it is **non-magnetic**.

Forces Between Magnetic Poles

Every magnet has a **north pole** and a **south pole**; magnetic forces are strongest at the poles.
**Like poles repel**: north–north or south–south push apart.
**Opposite poles attract**: north–south pull together.
The force between a magnet and a magnetic material is always **attractive** due to induced magnetism.

Induced Magnetism

An **induced magnet** is a material (e.g., soft iron) that becomes a magnet temporarily in a magnetic field.
The end of the material nearest the magnet acquires the **opposite pole** to the magnet’s pole.
When removed from the field, the material **loses its magnetism** quickly.
Induced magnetism always causes **attraction** between the magnet and the material.

Permanent and Temporary Magnets

**Permanent magnets** (e.g., steel) produce their own magnetic field and do not lose magnetism easily.
**Temporary (induced) magnets** (e.g., soft iron) are magnetic only while in an external field.
An **electromagnet** consists of coils of wire around a soft iron core; it can be switched on/off.
Strength of an electromagnet can be increased by: increasing current, increasing number of turns, or adding a soft iron core.

Uses of Magnets

Permanent magnets are used in **compasses**, fridge magnets, toys, and school experiments.
Electromagnets are used in **MRI scanners**, speakers, recycling (lifting scrap metal), and Mag-Lev trains.

Magnetic Fields

A **magnetic field** is a region where a magnetic pole experiences a force.
Field lines go from **north to south**; arrows show the direction of force on a north pole.
Field is **strongest at the poles** (lines are closer together) and weakens with distance (lines spread out).
The direction of the field at a point is the direction of the force on a north pole placed there.

Particle arrangement in solids, liquids, and gases (for reference, though not magnetism-specific).

Practice questions

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1.Which of the following metals is magnetic?
Easy
- ACopper
- BAluminium
- CNickel
- DZinc
2.Two magnets are brought close together. The north pole of one magnet is placed near the north pole of the other magnet. What happens?
Easy
- AThey attract each other
- BThey repel each other
- CNo force is exerted
- DThey become demagnetised
3.A bar magnet is brought near a piece of metal. The metal is attracted to the magnet. Which of the following could be the metal?
Medium
- AAluminium
- BCopper
- CSteel
- DZinc
4.Magnetic materials are attracted to both poles of a permanent magnet.
Easy
True or false?
5.An induced magnet can be switched on and off.
Medium
True or false?
6.State the two magnetic metals (other than steel).
Easy
7.Explain why a magnetic material is always attracted to a magnet, regardless of which pole is held close to it.
Medium
8.Complete the sentence:
Easy
Like magnetic poles $____ and$ opposite magnetic poles ____.

Slides

Magnetism

Magnetism is a force that acts at a distance, caused by magnetic fields.
Only a few metals are magnetic: iron, cobalt, nickel, and steel (an alloy of iron).
Magnetic materials are attracted to magnets; non-magnetic materials are not.

Which of the following is a magnetic material?

Aluminium
Copper
Nickel
Zinc

Magnets and Magnetic Poles

Every magnet has two poles: a north pole and a south pole.
Magnetic forces are strongest at the poles.
Like poles repel each other; opposite poles attract.

Which of these are true about magnetic poles?

A north pole repels a north pole.
A north pole attracts a south pole.
A south pole repels a north pole.
Magnetic forces are weakest at the poles.

Induced Magnetism

When a magnetic material is placed near a magnet, it becomes temporarily magnetised – this is induced magnetism.
The end closest to the magnet becomes the opposite pole.
Induced magnetism always causes attraction.
When removed from the magnetic field, the material loses its magnetism quickly (especially soft iron).

A piece of iron is attracted to the north pole of a magnet. What pole is induced on the side of the iron closest to the magnet?

North
South
It depends on the iron
No pole is induced

Permanent and Temporary Magnets

Permanent magnets (e.g., steel) produce their own magnetic field and do not lose magnetism easily.
Temporary (induced) magnets (e.g., soft iron) become magnetic only in a magnetic field and lose it when removed.
Electromagnets are made by winding a coil around a soft iron core; they can be switched on/off.

An electromagnet can be made by winding coils of wire around $a ____$ core.

Comparing Permanent and Electromagnets

Permanent magnets are always magnetic; electromagnets can be turned on/off.
The strength of an electromagnet can be changed by altering the current, number of coil turns, or adding a core.
Uses of permanent magnets: compasses, fridge magnets, toys.
Uses of electromagnets: MRI scanners, speakers, recycling, Mag-Lev trains.

Which of these are true about electromagnets?

They can be switched on and off.
Their strength cannot be changed.
They usually have a soft iron core.
They are used in compasses.

Magnetic Fields

A magnetic field is a region where a magnetic pole experiences a force.
Field lines show the direction of the force on a north pole – from north to south.
The field is strongest where lines are closest together (at the poles).
The field weakens as distance from the magnet increases.

Where is the magnetic field of a bar magnet strongest?

At the centre
At the north pole only
At both poles
Far away from the magnet

The direction of a magnetic field line is from $the ____$ pole to $the ____$ pole.

Summary

Magnetic materials: iron, cobalt, nickel, steel.
Like poles repel, opposite poles attract.
Induced magnetism is temporary and always attractive.
Permanent magnets vs electromagnets: properties and uses.
Magnetic field lines go from north to south; strength indicated by line spacing.

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