1.0 Introduction to Magnetism
Magnetism is a fascinating force of nature. A substance that has the property of attracting iron or iron-like substances is called a Magnet. The phenomenon by which a magnet attracts magnetic substances is known as magnetism.
Magnetic vs. Non-Magnetic
- Magnetic Substances: Materials that are attracted by a magnet (e.g., Iron, Steel, Nickel, Cobalt).
- Non-Magnetic Substances: Materials that are not attracted by a magnet (e.g., Wood, Plastic, Copper, Aluminum, Gold).
1.1 Types of Magnets
Magnets are classified based on their origin into two main types:
- Natural Magnets: Found in nature in the form of ores. The most famous natural magnet is Lodestone (an ore of iron called Magnetite). They have low magnetic strength and irregular shapes.
- Artificial Magnets: Made by humans using magnetic materials. They are much stronger and come in various shapes like Bar magnets, Horseshoe magnets, and Magnetic needles.
1.2 Properties of a Magnet
Every magnet, regardless of its size or shape, exhibits these fundamental properties:
- Attractive Property: A magnet attracts magnetic substances. This attraction is maximum at the poles.
- Directive Property: A freely suspended magnet always points in the North-South direction.
- Poles always exist in pairs: You cannot separate the North pole from the South pole. If you break a magnet, each piece becomes a new magnet with its own N and S poles.
- Law of Magnetic Poles: Like poles (N-N or S-S) repel each other, while unlike poles (N-S) attract each other.
The Basic Law
Repulsion is the Sure Test of Magnetism
Attraction can happen between a magnet and an unmagnetized iron piece, but repulsion only happens between two magnets.
You are given two identical-looking metal bars. One is a magnet and the other is a simple iron bar. How can you identify the magnet using only a third known magnet?
Solution:
1. Bring the North pole of the known magnet near both ends of the first bar.
2. If repulsion occurs at any one end, the bar is a magnet.
3. If both ends are attracted, the bar is just a simple iron bar.
Ancient sailors used lodestones as the first Magnetic Compasses to find their way across the oceans because of the directive property of magnets!
2.0 Making Your Own Magnet
Magnetic properties can be induced in a piece of ordinary iron or steel. This process is called Magnetisation. While there are several ways to do this, the most common methods for students involve physical contact or electrical current.
2.1 Single Touch Method
In this method, an iron bar is rubbed with a permanent magnet. You must rub the magnet in one direction only, from one end of the bar to the other. When you reach the end, lift the magnet and return to the starting point. Repeating this 30-40 times turns the iron bar into a magnet.
2.2 Magnetic Field
A magnet does not need to touch an object to pull it. This is because every magnet is surrounded by an invisible region where its influence can be felt. This space is called the Magnetic Field.
Magnetic Field Lines
We represent the magnetic field using imaginary lines called Magnetic Field Lines. Their characteristics include:
- They start from the North Pole and end at the South Pole (outside the magnet).
- They never intersect (cross) each other.
- They are closer together where the magnetic field is strongest (at the poles).
A magnet can lose its properties (become demagnetised) if it is:
1. Heated to a very high temperature.
2. Hammered or dropped repeatedly.
3. Not stored properly with magnetic keepers.
While using the single touch method, if you start rubbing with the North pole of a magnet from end A to end B of an iron bar, what pole will be developed at end B?
Solution:
In the single touch method, the end where the rubbing process finishes (end B) always develops the opposite pole to the one used for rubbing.
Since we used the North pole to rub, end B will become a South pole.
3.0 Earth as a Giant Magnet
Have you ever wondered why a freely suspended magnet always points in the North-South direction? This happens because the Earth itself behaves like a giant bar magnet. Its magnetic field interacts with any magnet on its surface, forcing it to align with the Earth's magnetic lines of force.
Earth's Magnetic Poles
The Earth has two sets of poles:
- Geographic Poles: The actual North and South poles on the Earth's axis of rotation.
- Magnetic Poles: The points where the Earth's imaginary internal magnet is strongest.
Note: The Earth's Magnetic North Pole is actually near the Geographic South Pole, which is why the North pole of a compass is attracted toward the Geographic North!
3.1 The Magnetic Compass
A Magnetic Compass is a simple device used for finding directions. It consists of a tiny, light magnetic needle pivoted at its centre so that it can rotate freely in a horizontal plane. The needle is housed in a small brass or aluminium box with a glass top.
Features of a Compass
- The Needle: Usually painted red or marked with an arrow at the North-seeking end.
- The Dial: Marked with cardinal directions (North, South, East, West).
- The Casing: Made of non-magnetic material (like brass) so it doesn't interfere with the needle.
Why is the casing of a magnetic compass never made of iron or steel?
Solution:
Iron and steel are magnetic substances. If the casing were made of these materials, it would become magnetised by the Earth's field or attract the compass needle itself. This would prevent the needle from moving freely and pointing to the true magnetic North.
Do not use a magnetic compass near heavy machinery, high-voltage wires, or other strong magnets. These create local magnetic fields that "confuse" the needle, giving an incorrect reading.
4.0 Electromagnets and Magnet Care
Sometimes we need a magnet that we can turn on or off at will, or one that is much stronger than any permanent magnet. This is where an Electromagnet comes into play. It uses the magnetic effect of electric current to create temporary magnetism.
4.1 How an Electromagnet Works
When an electric current flows through a wire coiled around a soft iron core, the iron core behaves like a magnet. As soon as the current is switched off, the magnetic property disappears.
- Soft Iron Core: Used because it gains and loses magnetism quickly.
- Strength: Can be increased by increasing the number of turns in the coil or increasing the current flowing through it.
- Uses: Electric bells, cranes in scrap yards, and MRI machines.
4.2 Care of Magnets
Magnets can lose their power over time if not handled correctly. This loss of magnetic property is called self-demagnetisation. To prevent this, specific storage methods are used.
Magnetic Keepers
Magnets should be stored in pairs with their unlike poles on the same side. A soft iron piece, called a Keeper, is placed across the ends. This forms a closed loop for the magnetic field lines, preventing them from weakening.
List two differences between a permanent bar magnet and an electromagnet.
Solution:
1. Permanence: A bar magnet has permanent magnetism, while an electromagnet is temporary (works only with current).
2. Strength: The strength of a bar magnet is fixed, while the strength of an electromagnet can be changed by adjusting the current.
Never keep magnets near electronic devices like mobile phones, laptops, or credit cards. The strong magnetic field can disrupt electronic circuits or erase data stored on magnetic strips!
Maglev trains (Magnetic Levitation) use powerful electromagnets to float above the tracks. Because there is no friction with the rails, they can reach speeds of over 600 km/h!