Table of Contents
IGCSE Physics Topic 4.1 – Understanding magnetic forces, fields, and materials
What is Magnetism? Magnetism is an invisible force that can attract or repel certain materials without touching them. You experience this force every time you stick a note to your fridge or use a compass to find direction. Magnetism is one of the fundamental forces in nature, and understanding it helps us create many useful devices from electric motors to computer hard drives.
Magnets have been fascinating humans for thousands of years. Ancient Greeks discovered natural magnetic rocks called lodestones that could attract iron. Today, we use magnets in countless ways – in speakers, motors, generators, credit cards, and even in hospital MRI scanners. This topic will help you understand how magnets work and why they behave the way they do.
Magnetic Poles and Forces #
Every magnet, no matter what shape or size, has two special regions where the magnetic force is strongest. These regions are called magnetic poles. Understanding how these poles interact with each other is the key to understanding all magnetic behavior.
North Poles and South Poles #
Every magnet has two poles:
- North pole (N pole): This is the end of a magnet that points toward Earth’s geographic North Pole when the magnet is free to rotate (like in a compass)
- South pole (S pole): This is the end of a magnet that points toward Earth’s geographic South Pole when the magnet is free to rotate
Important Rule: You can never have a magnet with only one pole. If you break a bar magnet in half, you don’t get a separate north pole and south pole – instead, you get two smaller magnets, each with its own north and south pole. This is because magnetism comes from the arrangement of tiny magnetic regions inside the material, not from having “north stuff” at one end and “south stuff” at the other.
Forces Between Magnetic Poles #
When you bring two magnets close together, they either pull toward each other or push away from each other. This behavior follows a simple rule based on which poles are facing each other.
There are two types of magnetic forces between poles:
The Rules of Magnetic Pole Interaction:
- Attraction (pulling together): Opposite poles attract each other
- North pole attracts south pole
- South pole attracts north pole
- Repulsion (pushing apart): Like poles repel each other
- North pole repels north pole
- South pole repels south pole
Remember this simple rule: Opposites attract, likes repel. This is easy to remember because it works the same way for electric charges. Unlike poles (N and S) attract each other. Like poles (N and N, or S and S) repel each other.
Try This: If you have two bar magnets, try bringing them close together with different poles facing each other. You’ll feel the force of attraction when opposite poles face each other, and you’ll feel the force of repulsion (it feels like they’re pushing against an invisible spring) when like poles face each other.
Forces Between Magnets and Magnetic Materials #
Magnets don’t just interact with other magnets – they also attract certain materials. These materials are called magnetic materials, and the most common example is iron. When you bring a magnet near a piece of iron (like a nail or paper clip), the magnet always attracts the iron, no matter which pole of the magnet you use.
This attraction works differently from the attraction between two magnets. A magnetic material like iron is attracted to both the north pole and the south pole of a magnet. This is because the iron becomes magnetized when the magnet comes near it.
Common Magnetic Materials:
- Iron: The most common magnetic material, used in nails, paper clips, and steel
- Nickel: A magnetic metal used in coins and some alloys
- Cobalt: A strong magnetic metal used in powerful magnets
- Steel: An alloy (mixture) of iron and carbon, which is also magnetic
Magnetised and Unmagnetised Materials #
Not all iron objects are magnets, even though iron is a magnetic material. A nail sitting on a table doesn’t attract other nails, but a magnetized nail would. Understanding the difference between magnetised and unmagnetised materials helps us understand how magnets can create other magnets.
Unmagnetised Materials #
An unmagnetised material is a magnetic material (like iron or steel) that is not currently acting as a magnet. Inside this material, there are tiny magnetic regions called domains. In an unmagnetised material, these domains point in random directions, so their magnetic effects cancel each other out. The material as a whole does not behave as a magnet.
Magnetised Materials #
A magnetised material is a magnetic material where the domains have been lined up to point in the same direction. When this happens, all the tiny magnetic effects add up, and the material becomes a magnet with its own north and south poles. This is what we mean when we say something has been “magnetized.”
Induced Magnetism #
Induced magnetism is a fascinating phenomenon where an unmagnetised magnetic material becomes a temporary magnet when a permanent magnet is brought near it. This explains why a magnet can attract an iron nail that wasn’t magnetic before.
Here’s what happens during induced magnetism:
How Induced Magnetism Works:
- Magnet approaches: A permanent magnet is brought close to an unmagnetised magnetic material (like an iron nail)
- Magnetic field affects material: The magnetic field from the permanent magnet reaches the iron nail
- Domains line up: The magnetic field causes the randomly arranged domains in the iron to line up and point in one direction
- Material becomes magnetised: The iron nail now has its own north and south poles – it has become a temporary magnet
- Attraction occurs: The induced poles are arranged so that the pole closest to the permanent magnet is always the opposite pole, causing attraction
- Magnet removed: When the permanent magnet is taken away, the domains in soft iron return to random arrangement, and the nail stops being magnetic
Why Magnets Always Attract Iron: When you bring either pole of a magnet near an iron nail, the nail becomes magnetized with the opposite pole facing the magnet. If you bring the north pole of a magnet near a nail, the nail develops a south pole at the end nearest the magnet. If you bring the south pole of a magnet near the nail, the nail develops a north pole at the end nearest the magnet. This is why magnets always attract iron, never repel it.
Everyday Example: When you use a magnet to pick up a chain of paper clips, each paper clip becomes temporarily magnetized through induced magnetism. The first paper clip is magnetized by the permanent magnet, the second paper clip is magnetized by the first one, and so on. When you remove the permanent magnet, the whole chain falls apart because the paper clips lose their magnetism.
Temporary and Permanent Magnets #
Not all magnets are the same. Some materials stay magnetized for a long time (years or even decades), while others lose their magnetism very quickly. Understanding this difference is important because we use different types of magnets for different purposes.
Temporary Magnets (Soft Iron) #
Soft iron is iron that has been made very pure. It is called “soft” not because it feels soft to touch, but because its magnetic properties are “soft” – meaning they can be changed easily.
Properties of Temporary Magnets (Soft Iron):
- Easy to magnetise: Soft iron becomes magnetised very quickly when placed in a magnetic field
- Easy to demagnetise: Soft iron loses its magnetism very quickly when the magnetic field is removed
- Weak magnetic strength: When magnetised, soft iron is a relatively weak magnet
- Does not retain magnetism: Soft iron does not stay magnetised for long after the magnetizing field is removed
Why Soft Iron Behaves This Way: In soft iron, the magnetic domains can move and rotate very easily. When a magnetic field is applied, the domains quickly line up. But when the field is removed, the domains just as quickly return to random positions. This is because pure iron has a very simple atomic structure with no obstacles to domain movement.
Permanent Magnets (Steel) #
Steel is an alloy – a mixture of iron with a small amount of carbon and sometimes other elements. This mixture changes the magnetic properties significantly.
Properties of Permanent Magnets (Steel):
- Difficult to magnetise: Steel requires a strong magnetic field and takes longer to become magnetised
- Difficult to demagnetise: Once magnetised, steel keeps its magnetism for a very long time (many years)
- Strong magnetic strength: Magnetised steel is a relatively strong magnet
- Retains magnetism: Steel stays magnetised even after the magnetizing field is removed
Why Steel Behaves This Way: In steel, the carbon atoms and other impurities act like obstacles that make it harder for magnetic domains to move and rotate. This means it takes more effort to line up the domains (harder to magnetize), but once they’re lined up, they tend to stay that way (harder to demagnetize). The domains get “locked” in position.
Comparison Table – Soft Iron vs Steel:
- Ease of magnetisation:
- Soft iron = very easy
- Steel = difficult
- Ease of demagnetisation:
- Soft iron = very easy
- Steel = difficult
- Magnetic strength when magnetised:
- Soft iron = weak
- Steel = strong
- How long magnetism lasts:
- Soft iron = very short time
- Steel = many years
- Type of magnet:
- Soft iron = temporary magnet
- Steel = permanent magnet
Practical Application: Fridge magnets are made from permanent magnetic materials (like magnetized steel or ceramic magnets) because they need to stay magnetic for years. The core of an electromagnet is made from soft iron because we want it to become magnetic when we switch the electricity on, and stop being magnetic when we switch the electricity off.
Magnetic and Non-Magnetic Materials #
We often think of materials as either “magnetic” or “not magnetic,” but it’s important to understand exactly what this means. The term “magnetic material” has a specific scientific meaning that’s different from everyday language.
Magnetic Materials #
Magnetic materials are materials that are attracted to magnets and can be magnetized. There are only a few pure elements that are strongly magnetic at room temperature.
Magnetic Materials Include:
- Iron: The most common magnetic material, found in steel and many alloys
- Nickel: Used in coins and some electronic components
- Cobalt: Used in strong magnets and some alloys
- Steel and other iron alloys: Any mixture containing iron is usually magnetic
Non-Magnetic Materials #
Non-magnetic materials are materials that are not attracted to magnets and cannot be magnetized. This includes most materials around us.
Non-Magnetic Materials Include:
- Most metals: Aluminum, copper, gold, silver, brass, bronze
- Non-metals: Wood, plastic, glass, paper, rubber
- Other materials: Water, air, most gases and liquids
Common Misconception: Many students think that all metals are magnetic because iron and steel are metals. This is not true! Most metals are non-magnetic. Aluminum is a metal, but it is not attracted to magnets at all. Copper wire is not attracted to magnets. Gold and silver jewelry is not magnetic. Only a few specific metals (iron, nickel, cobalt) are magnetic.
Testing Materials: You can test whether a material is magnetic by bringing a magnet close to it. If the material is pulled toward the magnet, it’s magnetic. If nothing happens, it’s non-magnetic. This is a simple test you can do at home – try testing different objects like aluminum foil, copper coins, steel nails, and plastic rulers.
Magnetic Fields #
The force from a magnet doesn’t require touching – it works through empty space. To understand this, we use the concept of a magnetic field. A magnetic field is like an invisible area of influence around a magnet where magnetic forces can be felt.
What is a Magnetic Field? #
Definition: A magnetic field is a region in which a magnetic pole experiences a force. In simpler terms, if you place a magnetic pole (like the north pole of a small magnet) somewhere and it feels a push or pull, then that location is inside a magnetic field.
Magnetic fields exist around all magnets, and they extend out into the space surrounding the magnet. The field is strongest near the poles of the magnet and gets weaker as you move further away. Even though we can’t see magnetic fields, we can detect them and show their effects.
Think of a magnetic field like the area around a fire where you can feel heat. You don’t have to touch the fire to feel warm – the heat spreads out through the surrounding space. Similarly, a magnet doesn’t have to touch an iron nail to attract it – the magnetic force spreads out through the surrounding space via the magnetic field.
Magnetic Field Lines #
To help us visualize and understand magnetic fields, we draw magnetic field lines. These lines are imaginary lines that show us the shape and direction of the magnetic field around a magnet.
Rules for Drawing Magnetic Field Lines:
- Direction: Field lines always go from the north pole to the south pole outside the magnet
- Never cross: Magnetic field lines never cross each other
- Closer together = stronger field: Where lines are close together, the magnetic field is strong. Where lines are far apart, the field is weak
- Always form closed loops: Field lines continue through the magnet from south pole to north pole, forming complete loops
- Come out perpendicular from poles: Field lines emerge from the north pole and enter the south pole at right angles to the magnet’s surface
IMAGE NEEDED: Clear diagram of magnetic field lines around a bar magnet, showing lines emerging from N pole, curving around, and entering S pole, with arrows showing direction
Google Images Search: “bar magnet magnetic field lines diagram arrows labeled IGCSE physics”
Direction of a Magnetic Field #
Important Definition: The direction of a magnetic field at any point is the direction of the force that would act on a north pole placed at that point. This means if you placed a tiny north pole at any location in a magnetic field, the direction it would be pushed is the direction of the field at that location.
This is why magnetic field lines have arrows on them – the arrows point in the direction that a north pole would move. Since opposite poles attract, the arrows point from north poles toward south poles. A north pole would be pushed away from other north poles and pulled toward south poles.
Plotting Magnetic Field Lines #
We can make magnetic fields visible using two main methods: plotting with a compass or using iron filings. Both methods help us see the pattern of the invisible magnetic field.
Method 1: Plotting with a Compass #
A compass contains a small magnetized needle that can rotate freely. The north pole of the compass needle always points in the direction of the magnetic field at its location.
Steps to Plot Field Lines with a Compass:
- Place magnet on paper: Put a bar magnet on a large sheet of paper and draw around it to mark its position
- Start at north pole: Place a compass near the north pole of the magnet
- Mark compass direction: Mark two dots on the paper – one at the north end of the compass needle and one at the south end
- Move compass forward: Move the compass so that its south end is where the north end was before
- Mark again: Mark the new position of the north end of the compass needle
- Repeat the process: Keep moving the compass and marking positions until you reach the south pole of the magnet
- Join the dots: Draw a smooth curve through all the dots with an arrow showing the direction from north to south
- Repeat for more lines: Start at different positions around the north pole to plot several field lines
Method 2: Using Iron Filings #
Iron filings are tiny pieces of iron that act like miniature compass needles. When sprinkled around a magnet, each filing becomes magnetized and lines up with the magnetic field.
Steps to Show Field Pattern with Iron Filings:
- Prepare setup: Place a bar magnet on a table and cover it with a sheet of stiff paper or thin plastic
- Sprinkle iron filings: Gently sprinkle iron filings evenly over the paper
- Tap the paper: Gently tap the paper to help the filings move and line up with the field
- Observe pattern: The iron filings will arrange themselves along the magnetic field lines, creating a visible pattern
- Photograph or sketch: Record the pattern by taking a photo or carefully drawing what you see
Why Iron Filings Work: Each tiny piece of iron becomes a temporary magnet through induced magnetism. The north pole of one filing attracts the south pole of the next filing, causing them to line up in chains along the magnetic field lines. This creates a beautiful pattern that shows the shape of the invisible magnetic field.
Comparing the Methods: Iron filings show the overall pattern quickly and clearly, which is useful for seeing the complete field shape at once. However, they don’t show the direction of the field. A compass shows the direction very clearly and is more precise for plotting individual field lines, but it takes longer to map out the whole field pattern.
Field Strength and Line Spacing (Supplement) #
The spacing of magnetic field lines tells us important information about the strength of the magnetic field. Understanding this relationship helps us identify where magnetic forces will be strongest.
Important Rule: The relative strength of a magnetic field is represented by the spacing of the magnetic field lines. Where field lines are close together (closely spaced), the magnetic field is strong. Where field lines are far apart (widely spaced), the magnetic field is weak.
This is why magnetic field lines are always drawn closest together near the poles of a magnet – that’s where the magnetic field is strongest. As you move away from the magnet, the field lines spread out, showing that the field gets weaker with distance.
Why Magnetic Forces Occur (Supplement) #
At a deeper level, we can understand that magnetic forces arise from the interaction between magnetic fields. This explanation helps us understand why magnets attract and repel each other.
Fundamental Principle: Magnetic forces are due to interactions between magnetic fields. When two magnets are brought close together, their magnetic fields overlap and interact. This interaction between the fields creates the forces we observe.
When two north poles or two south poles are brought together (like poles), their magnetic fields interact in a way that creates repulsion – the magnets push apart. When a north pole and south pole are brought together (opposite poles), their magnetic fields interact in a way that creates attraction – the magnets pull together.
Understanding Field Interactions: Think of magnetic fields as having a direction and strength at every point in space. When two fields overlap, they combine together. If they’re pointing in similar directions (like poles facing each other), they try to push apart. If they’re pointing in opposite directions (opposite poles facing each other), they pull together. The force you feel between magnets is really the interaction between their overlapping magnetic fields.
Uses of Magnets #
Understanding how magnets work helps us appreciate the many ways they’re used in everyday life and in technology. Different types of magnets are used for different purposes based on their properties.
Uses of Permanent Magnets #
Permanent magnets are used in applications where we need a constant magnetic field that lasts for many years without needing any power supply.
Common Uses of Permanent Magnets:
- Compasses: The magnetized needle in a compass is a permanent magnet that aligns with Earth’s magnetic field to show direction
- Fridge magnets: Used to stick notes and photos to metal fridge doors
- Magnetic door catches: Keep cupboard doors closed without the need for mechanical latches
- Loudspeakers and headphones: Permanent magnets interact with electromagnets to produce sound
- Electric motors (small): Some small motors use permanent magnets to create the magnetic field
- Magnetic toys: Many toys use permanent magnets for construction or play
- Magnetic strips on cards: Credit cards and ID cards have magnetic strips that store information
- Computer hard drives: Use tiny permanent magnets to store data
Uses of Electromagnets #
Electromagnets are magnets that use electricity to create a magnetic field. They have a huge advantage over permanent magnets: they can be switched on and off, and their strength can be controlled. Electromagnets are made by wrapping wire around a soft iron core and passing an electric current through the wire.
Common Uses of Electromagnets:
- Electric bells and buzzers: An electromagnet rapidly attracts and releases a metal hammer to create sound
- Scrapyard cranes: Large electromagnets lift heavy pieces of scrap metal; when the current is switched off, the metal drops
- Circuit breakers: Electromagnets detect dangerous currents and automatically switch off the circuit
- Electric motors (large): Electromagnets in motors can be controlled to create rotation
- Relays: An electromagnet switches a separate circuit on or off, allowing a small current to control a large current
- Magnetic resonance imaging (MRI) scanners: Very powerful electromagnets create detailed images of the inside of the body
- Maglev trains: Electromagnets levitate and propel trains at very high speeds
- Sorting machines: Separate magnetic materials from non-magnetic materials in recycling
Why Use Soft Iron in Electromagnets? Remember that soft iron is easy to magnetize and easy to demagnetize. This makes it perfect for electromagnets because we want the magnetism to appear when we turn the electricity on and disappear when we turn it off. If we used steel (a permanent magnet material), the magnetism would remain even after we switched off the current, which would be a problem for most applications.
Comparing Uses: Use a permanent magnet when you need constant magnetism without electricity (like a compass or fridge magnet). Use an electromagnet when you need to control the magnetism by switching it on and off or adjusting its strength (like a scrapyard crane or electric bell).
Summary: Key Concepts in Magnetism #
Understanding magnetism requires knowing about magnetic poles, how materials respond to magnetic fields, and how we represent magnetic fields. Let’s review the most important concepts you need to remember.
Essential Magnetic Principles:
- Magnetic poles: Every magnet has a north pole and south pole. Like poles repel, opposite poles attract
- Magnetic attraction: Magnets attract magnetic materials (iron, nickel, cobalt, steel) regardless of which pole is used
- Induced magnetism: Unmagnetised magnetic materials become temporary magnets when near a permanent magnet
- Soft iron vs steel: Soft iron is easy to magnetize and demagnetize (temporary magnet). Steel is hard to magnetize but stays magnetic (permanent magnet)
- Magnetic field: A region where a magnetic pole experiences a force
- Field direction: The direction a north pole would be pushed at that point
- Field lines: Go from north to south outside the magnet. Closer spacing means stronger field
- Plotting fields: Use a compass to find direction, or iron filings to show the pattern
Exam Success Tips: When answering questions about magnetism, always think about whether you’re dealing with magnetic or non-magnetic materials, temporary or permanent magnets, and whether magnetism is being induced. Draw clear diagrams showing poles and field lines. Remember that magnetic forces work through fields, not by direct contact.
Common Exam Questions: You should be able to: (1) Explain why magnets attract all magnetic materials but repel or attract other magnets, (2) Describe how to plot magnetic field lines, (3) Explain the difference between soft iron and steel, (4) Describe induced magnetism, (5) Give examples of uses for permanent magnets and electromagnets.
Connecting to Real Life: Magnetism is all around us in technology we use every day. From the speakers in your headphones to the motor in electric cars, from the magnetic strip on your ID card to the MRI scanners in hospitals, understanding magnetism helps you understand how much of modern technology works. The principles you’ve learned here are fundamental to many devices that make modern life possible.
