When you bring a magnet close to different objects, some are attracted while others are not.

Magnetic materials are those that can be attracted by a magnet. The most common magnetic materials are iron, nickel, and cobalt — these are called ferromagnetic materials.

Non-magnetic materials are not attracted by a magnet. Examples include copper, brass, aluminum, plastic, wood, glass, and water.

Why Some Materials Are Magnetic and Others Are Not

In most atoms, the magnetic effects of electron motion cancel each other out. However, in ferromagnetic materials like iron, these effects do not cancel completely, so the atoms behave like tiny magnets called atomic dipoles.

Temporary Magnets

These magnets lose their magnetism when the external magnetic field is removed. Iron nails and paper clips are common examples. Electromagnets are also temporary magnets — they only work when electric current flows through them.

Permanent Magnets

These retain their magnetism even after the external magnetic field is removed. Steel and special alloys like Alnico are used to make permanent magnets.

1. Every magnet has two poles: a North (N) pole and a South (S) pole.
2. Like poles repel each other; unlike poles attract each other.
3. The magnetic force is strongest at the poles.
4. A freely suspended bar magnet always points in a north-south direction.
5. Magnetic force can act at a distance (action-at-a-distance force).

Magnetisation is the process of aligning atomic dipoles in a material in one direction to make it magnetic.

Methods of Magnetisation

Method 1: Stroking

1. Hold a bar magnet with its north pole.
2. Stroke the magnet along the steel rod from one end to the other, always in the same direction.
3. Repeat about 10 times.
4. The end where you finish stroking becomes the south pole, and the starting end becomes the north pole.

Method 2: Using a Solenoid (Electric Method)

1. Wrap copper wire around the material to form a coil (solenoid).
2. Connect the wire to a battery.
3. Pass direct current through the wire for a few seconds.
4. The material becomes magnetised. The polarity follows the right-hand grip rule: if you grip the coil in the direction of current flow, your thumb points to the north pole.

Method 3: Heating and Vibration

Place the material in an external magnetic field while heating or vibrating it. The thermal energy and vibration help the atomic dipoles align with the field.

Demagnetisation is the process of destroying the magnetic property of a material. This can be done by:

1. Heating — heating a magnet makes the dipoles vibrate randomly and lose alignment
2. Hammering or dropping — physical impact disturbs the aligned dipoles
3. Using alternating current — passing AC through a coil wrapped around the magnet repeatedly changes the direction of alignment until they become random
4. Random stroking — stroking with magnets of different polarities in no particular order

Worked Example

A student has an iron nail and wants to make it temporarily magnetic to pick up small paper clips. Describe how the student can magnetise the nail and explain why it will only be temporary.

Solution:

The student should place the nail inside a solenoid (a coil of wire) and pass a direct current through the wire for a few seconds. This will align the atomic dipoles in the nail, making it magnetic.

The nail will be temporary because iron is a soft magnetic material — it loses its magnetism easily once the current is switched off. If the student had used steel instead, the magnetisation would be more permanent.

A magnetic field is the region around a magnet where magnetic force can be felt by magnetic materials. The field has both strength and direction.

Magnetic Lines of Force

Iron filings sprinkled on paper placed over a magnet arrange themselves in patterns called magnetic lines of force. These lines show the direction and strength of the magnetic field.

Properties of magnetic lines of force:

1. They emerge from the north pole and enter the south pole
2. They are continuous and form closed loops
3. They are crowded together at the poles (where the field is strong)
4. They are far apart away from the poles (where the field is weak)
5. They never cross each other
6. They pass through both magnetic and non-magnetic materials

Neutral Points

A neutral point is a location where two magnetic fields cancel each other out, resulting in zero magnetic field. For example, when two bar magnets are placed with their like poles facing each other, there is a point between them where no magnetic force is felt.

The Earth behaves as if it has a bar magnet inside it. The Earth has:

• Geographic poles (North and South) — based on Earth's axis of rotation
• Magnetic poles — based on Earth's magnetic field

The Earth's magnetic field protects us from harmful cosmic radiation from the sun.

Important Terms

• Angle of declination — the angle between the geographic north pole and the magnetic north pole
• Angle of inclination (dip) — the angle between the Earth's magnetic field and the horizontal plane. This angle is 0° at the magnetic equator and 90° at the magnetic poles.

In Tanzania, magnets are commonly used in jiko cha umeme (electric cookers) and vip灌灌 (induction cookers) found in many homes and restaurants. The electromagnets in these devices generate heat for cooking. Additionally, magnets are used in baiskeli za umeme (electric bikes) and in vifaa vya umeme like speakers and generators. A student might also encounter magnets in kadi za benki (bank ATM cards), which have a magnetic strip that stores account information.