Magnetism
PhysicsΒ· 6(b) (6.2β6.7, 2017 spec)Β· 12 min read
1. Fundamental Magnetic Properties and Material Typesβ β ββββ± 3 min
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Magnets have two poles: north (N) and south (S). The basic rule of magnetic force is: like poles repel, unlike poles attract. Magnets also attract unmagnetised magnetic substances, including iron, steel, nickel, and cobalt.
Magnetic Hard and Soft Materials
Magnetically hard materials are difficult to magnetise and demagnetise, making them ideal for permanent magnets. Magnetically soft materials are easily magnetised and lose their magnetism quickly when removed from a field, ideal for temporary magnet applications.
Example:
Hard material = steel; Soft material = iron
A manufacturer wants to make a permanent fridge magnet and a core for an electromagnet. State which material (steel or iron) they should use for each, and justify your answer.
- 1
For the permanent fridge magnet, use steel (magnetically hard material).
- 2
Justification: Steel retains its magnetism long-term, so it will stay stuck to the fridge without an external magnetic field.
- 3
For the electromagnet core, use iron (magnetically soft material).
- 4
Justification: Iron loses its magnetism immediately when the electromagnet current is switched off, so the electromagnet can be turned on and off as required.
Exam tip:
Always link material choice directly to hard/soft magnetic properties, don't just list properties without applying to the given scenario to get full marks.
2. Magnetic Field Lines: Rules and Conventionsβ β ββββ± 3 min
A magnetic field is the region around a magnet where a magnetic force acts on magnetic materials or other magnets. We represent fields using magnetic field lines, which follow strict conventions for exams.
Magnetic Field Line
A line that shows the direction a free north pole would move if placed in the field, and the relative strength of the field at different points.
Field lines run from North to South outside the magnet, and South to North inside the magnet
Closer lines = stronger magnetic field; the strongest field is always at the poles of a magnet
Field lines never cross each other
Arrows on lines always point in the direction of the magnetic field
Sketch the field pattern around a single permanent bar magnet, and label the areas of strongest and weakest field.
- 1
Draw a rectangular bar magnet, label the left end N and right end S.
- 2
Draw curved lines from the N pole to the S pole outside the magnet, adding arrows pointing towards the S pole on every line.
- 3
Draw lines closest together at the N and S poles, label these regions strongest field.
- 4
Draw lines furthest apart far away from the magnet, label these regions weakest field.
Exam tip:
You will lose 1 mark per question if you omit arrows on field lines, or draw arrows pointing from S to N outside the magnet.
3. Induced Magnetismβ β β βββ± 2 min
When an unmagnetised magnetic material is placed in a magnetic field, temporary magnetism is induced in it, turning it into a temporary magnet. The end of the material nearest the permanent magnet always becomes the opposite pole, so attraction always occurs between the magnet and the material.
Induced Magnetism
Temporary magnetism created in an unmagnetised magnetic material when it is placed inside an external magnetic field. Induced magnetism always causes attraction, never repulsion.
Explain why a permanent bar magnet will attract an unmagnetised steel paper clip.
- 1
When the paper clip is brought near the permanent magnet, it enters the magnet's magnetic field.
- 2
Magnetism is induced in the steel paper clip, turning it into a temporary magnet.
- 3
The end of the paper clip nearest the permanent magnet's pole becomes the opposite pole (e.g. if the magnet's end is N, the clip's nearest end becomes S).
- 4
Unlike poles attract, so the paper clip is pulled towards the permanent magnet.
4. Practical Field Mapping and Uniform Magnetic Fieldsβ β β βββ± 4 min
You are expected to describe two methods to investigate magnetic field patterns for permanent bar magnets, and explain how to produce a uniform magnetic field using two permanent magnets.
Iron filings method: Place the magnet under a sheet of paper, sprinkle thin iron filings evenly over the paper, tap the paper gently to reduce friction. The filings will align along the magnetic field lines, showing the full pattern.
Plotting compass method: Draw around the magnet on a sheet of paper. Place the compass near one pole, mark the direction the needle points. Move the compass so the tail of the needle lines up with the previous mark, mark the new direction. Repeat until you reach the opposite pole, then draw a line through the marks. Repeat for multiple lines around the magnet.
Uniform Magnetic Field
A magnetic field where strength and direction are identical at all points, represented by straight, evenly spaced parallel field lines.
Describe how you would produce a uniform magnetic field using two permanent bar magnets, and state the appearance of its field lines.
- 1
Take two identical permanent bar magnets.
- 2
Place them parallel to each other, 2-3 cm apart, with the North pole of one magnet facing the South pole of the other.
- 3
The region between the two magnets will have a uniform magnetic field.
- 4
The field lines in this region are straight, parallel, evenly spaced, and point from the North pole to the South pole.
Exam tip:
When describing the iron filings practical, always mention tapping the paper to reduce friction between the filings and the paper, as this is a common mark point.
5. Common Pitfalls
Wrong move:
Drawing magnetic field lines pointing from South to North outside the magnet
Why:
The standard convention is lines run North to South outside the magnet, only running South to North inside the magnet
Correct move:
Add arrows pointing from North to South on all field lines drawn outside of a permanent magnet
Wrong move:
Stating repulsion can occur between a permanent magnet and an unmagnetised magnetic material
Why:
Induced magnetism always creates an opposite pole nearest the permanent magnet, so only attraction occurs in this case
Correct move:
Remember repulsion is the only test for a permanent magnet; attraction can happen between a magnet and any unmagnetised magnetic material
Wrong move:
Using iron (soft magnetic material) to make a permanent magnet
Why:
Iron loses its magnetism quickly when removed from an external field, so it cannot retain magnetism long-term for permanent applications
Correct move:
Use steel (hard magnetic material) for permanent magnets, as it retains its magnetism for long periods
Wrong move:
Drawing curved, unevenly spaced lines for a uniform magnetic field
Why:
Uniform fields have identical strength and direction at all points, so lines must be straight, parallel and equally spaced
Correct move:
When drawing a uniform field between two opposite poles, draw straight, evenly spaced parallel lines with arrows pointing from N to S
Wrong move:
Forgetting to mention tapping the paper when describing the iron filings field mapping practical
Why:
Iron filings have friction with the paper, so they will not align with field lines unless the paper is tapped to reduce friction
Correct move:
Always state that you tap the paper gently after sprinkling iron filings to allow them to align with the magnetic field lines
6. Quick Reference Cheatsheet
Term/Rule | Key Property | Exam Application |
|---|---|---|
Magnetic force | Like poles repel, unlike attract | Identify permanent magnets if repulsion is observed |
Hard magnetic material (steel) | Hard to magnetise/demagnetise | Used for permanent fridge magnets, compass needles |
Soft magnetic material (iron) | Easily magnetised/demagnetised | Used for electromagnet cores, temporary magnets |
Magnetic field lines | NβS outside, closer = stronger, no crossing | Draw correct field patterns for bar magnets |
Induced magnetism | Temporary, always causes attraction | Explain why magnets attract unmagnetised metal objects |
Uniform field | Straight, parallel, evenly spaced lines | Produce using two opposite parallel poles facing each other |
7. Frequently Asked
What is the only reliable test for a permanent magnet?
Repulsion is the only test for a permanent magnet. A permanent magnet will attract both other permanent magnets (opposite poles) and unmagnetised magnetic materials via induced magnetism, so attraction alone does not confirm an object is a permanent magnet. Only repulsion between two objects confirms both are permanent magnets.
How do I draw a uniform magnetic field correctly?
To draw a uniform field:
- Draw two parallel straight lines to represent the gap between two opposite facing poles (N on left, S on right)
- Add straight, evenly spaced parallel lines between the poles
- Add arrows pointing from the N pole to the S pole on every line Do not draw curved lines in the uniform region, as this would indicate a non-uniform field.
Going deeper
What's Next
Now that you have mastered permanent magnetism fundamentals, you are ready to move on to electromagnetism, the next subtopic in the Edexcel IGCSE Physics Magnetism and Electromagnetism unit. You will learn how electric currents produce magnetic fields, how electromagnets work, and their real-world applications. This foundation in magnetic fields and material properties will be critical to understanding how electromagnet strength can be controlled, and how they are used in devices like electric bells and circuit breakers. After electromagnetism, you will cover the motor effect and electromagnetic induction, including generators and transformers, which are high-weightage topics in both Paper 1 and Paper 2 exams. Make sure you can correctly draw and label magnetic field patterns before moving on, as these are frequently assessed alongside electromagnet content.