CIE IGCSE Physics (0625) Simple phenomena of magnetism Study Notes - New Syllabus
CIE IGCSE Physics (0625) Simple phenomena of magnetism Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Simple phenomena of magnetism
Key Concepts:
- Magnetic Poles and Magnetic Forces
- Temporary and Permanent Magnets
- Magnetic Field & Magnetic Field Lines
- Uses of Permanent & Electromagnets
Magnetic Poles and Magnetic Forces
Magnetic Poles and Magnetic Forces
Key Concepts:
- Every magnet has two poles: a north pole (N pole) and a south pole (S pole).
- Like poles repel: N–N or S–S → repulsion.
- Unlike poles attract: N–S or S–N → attraction.
Magnetic forces are non-contact forces that act at a distance due to interactions between magnetic fields.
Interaction with Magnetic Materials
- Magnetic materials include: iron, nickel, cobalt (and some of their alloys).
- Magnets attract magnetic materials – regardless of pole.
- Magnets do not repel magnetic materials – attraction only.
- This attraction is due to induced magnetism (explained below).
Magnetised vs Unmagnetised Materials
- Magnetised: the material behaves like a magnet and has a north and south pole.
- Unmagnetised: the material has magnetic domains pointing randomly, so it shows no overall magnetic effect.
Induced Magnetism
Induced magnetism occurs when a magnetic material (like iron) becomes temporarily magnetised due to the influence of a nearby magnet.
- When a magnet is brought near a magnetic material, the material becomes an induced magnet.
- Its atoms align temporarily, creating poles that face opposite the nearby pole.
- This causes attraction between the magnet and the material.
- Once the magnet is removed, the material usually loses its magnetism (unless it is a hard magnetic material).
Magnetic Forces and Fields
Magnetic forces arise from interactions between magnetic fields:
- Each magnet has an invisible field around it: the magnetic field.
- When two magnets are near each other, their fields interact → forces arise.
- Where fields are in the same direction → attraction; where they oppose → repulsion.
- Magnetic materials become magnetised by the external magnetic field inducing alignment of their domains.
Example:
A bar magnet is held near three different objects: another bar magnet, a piece of iron, and a wooden block. Predict what kind of force (if any) will act between the magnet and each object, and explain why.
▶️ Answer/Explanation
1. Magnet near another bar magnet:
If the magnets are aligned with opposite poles facing each other (e.g. North facing South), they will attract due to opposite poles. If like poles face (e.g. North facing North), they will repel. This interaction occurs because both objects have magnetic fields that interact directly.
2. Magnet near a piece of iron:
The iron is a magnetic material. It will be attracted to the magnet regardless of which pole is near it. This is because the magnetic field of the magnet induces magnetism in the iron (aligning its domains), leading to an attractive force.
3. Magnet near a wooden block:
Wood is a non-magnetic material. It does not respond to magnetic fields, so no force will act between the magnet and the wood.
Temporary and Permanent Magnets
Differences Between Temporary and Permanent Magnets
| Property | Temporary Magnet (e.g. Soft Iron) | Permanent Magnet (e.g. Steel) |
|---|---|---|
| Ease of Magnetisation | Easily magnetised | Takes longer to magnetise |
| Retention of Magnetism | Loses magnetism quickly | Retains magnetism permanently |
| Material Example | Soft Iron | Steel |
| Common Uses | Electromagnets (relays, cranes) | Fridge magnets, compasses |
Magnetic vs Non-Magnetic Materials
| Property | Magnetic Material | Non-Magnetic Material |
|---|---|---|
| Response to Magnetic Field | Attracted by a magnet | Not affected by a magnet |
| Examples | Iron, Nickel, Cobalt | Copper, Wood, Plastic |
| Can Be Magnetised? | Yes | No |
Example :
A crane uses an electromagnet to lift scrap iron. The electromagnet loses its magnetism immediately when the current is switched off. Explain why soft iron is used in the core of the electromagnet instead of steel.
▶️ Answer/Explanation
Soft iron is a temporary magnet – it is easily magnetised and loses its magnetism quickly when the current is turned off.
This property is useful in cranes so that the iron can be picked up and released quickly and efficiently.
Steel, on the other hand, is a permanent magnet — once magnetised, it would hold onto the scrap metal even after the power is turned off, which is not desirable in this application.
Example :
A student brings a magnet close to four different materials: copper, aluminium, iron, and plastic. Only one of them is attracted to the magnet. Identify the magnetic material and explain your answer.
▶️ Answer/Explanation
Iron is the magnetic material.
Magnetic materials are those that can be attracted by a magnet. These include iron, cobalt, and nickel.
Copper, aluminium, and plastic are non-magnetic materials and will not be attracted to the magnet.
Magnetic Field & Magnetic Field Lines
Magnetic Field
A magnetic field is the region around a magnet where a magnetic material or a magnetic pole experiences a force.
In other words, a magnetic field is a force field created by moving charges or magnetic materials that acts on other magnets or magnetic materials within that region.
Magnetic Field Lines Around a Bar Magnet
- Magnetic field lines show the shape and strength of a magnetic field.
- Field lines are drawn from the north pole to the south pole outside the magnet.
- The lines are:
- Closer together where the field is stronger (near poles)
- Never crossing one another
Direction of the Magnetic Field
- The direction of a magnetic field at any point is the direction in which the north pole of a magnet would move if placed at that point.
- This is also the same direction in which a compass needle points.
Plotting Magnetic Field Lines
Using Iron Filings:
- Place a sheet of paper over a bar magnet.
- Sprinkle iron filings lightly over the paper.
- Tap the paper gently → filings align along the magnetic field lines.
Using a Compass:
- Place a compass at various points around a magnet.
- The compass needle aligns with the magnetic field at each point.
- Mark the direction, then move the compass along the path to plot continuous lines.
- This method shows both the shape and direction of the magnetic field.
Example:
A student places a small plotting compass near the north pole of a bar magnet and observes that the compass needle points away from the magnet. Explain what this tells us about the direction of the magnetic field and why this happens.
▶️ Answer/Explanation
The direction of the magnetic field is defined as the direction a north pole would move if placed in the field.
The compass needle itself has a small magnet, and the north-seeking end of the needle points in the direction of the magnetic field.
Since the compass points away from the north pole of the bar magnet, the field is directed from the north pole outward into space, following field lines from N to S outside the magnet.
Thus, the observation confirms that magnetic field lines go from the north to the south pole.
Example :
A bar magnet is placed under a sheet of paper and iron filings are sprinkled on top. The filings arrange themselves in curved patterns that are dense near the poles and spread out further away. What does this reveal about the nature of the magnetic field?
▶️ Answer/Explanation
Iron filings become temporarily magnetised and align themselves with the magnetic field.
The curved patterns formed by the filings represent the shape of the magnetic field lines.
The fact that the filings are dense near the poles shows that the field is strongest there. Where the filings are more spread out, the field is weaker.
This experiment visually maps the magnetic field, showing both the shape and relative strength of the field.
Uses of Permanent & Electromagnets
Uses of Permanent Magnets:
Permanent magnets are objects that produce their own magnetic field. They are made of materials like steel that retain magnetism for a long time.
- Used in fridge doors to keep them closed using magnetic strips.
- Used in loudspeakers and headphones to interact with coils and produce sound.
- Used in electric motors (small ones like in toys or fans) to provide a constant magnetic field.
- Used in compasses – the needle is a small permanent magnet that aligns with Earth’s magnetic field.
Uses of Electromagnets:
Electromagnets are made by passing current through a coil wrapped around a soft iron core. The magnetism can be turned on and off.
- Used in scrapyard cranes to lift heavy iron and steel objects – can be switched on/off as needed.
- Used in electric bells and relays – the magnetic field pulls armatures to complete or break a circuit.
- Used in magnetic locks in security systems where the current controls locking mechanisms.
- Used in MRI machines in hospitals – strong controllable magnetic fields are needed for scanning.
Example:
Why is a crane in a scrapyard more likely to use an electromagnet instead of a permanent magnet?
▶️ Answer/Explanation
A crane needs to be able to pick up and release scrap metal on demand.
An electromagnet is ideal because it becomes magnetic only when current flows through the coil.
When the current is switched off, the magnetic field disappears, and the crane can release the metal easily.
Permanent magnets cannot be turned off, so they are not suitable for this application.