Cambridge IGCSE0972

Simple phenomena of magnetism

Physics 0972 Chapter Notes

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Simple phenomena of magnetism
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In this chapter

  1. 1Magnetic Poles and Forces
  2. 2Magnetic vs Non-Magnetic Materials
  3. 3Induced Magnetism and Material Types
  4. 4Magnetic Fields

1. Magnetic Poles and Forces

Every magnet has two ends called poles. One is the North-seeking pole (N pole) and the other is the South-seeking pole (S pole). They are called this because if a magnet is freely suspended, its N pole will point towards the Earth's geographic North Pole. The fundamental rule of magnetism is that like poles repel each other (N repels N, S repels S), while unlike poles attract each other (N attracts S). The force of attraction or repulsion is strongest at the poles.

Key term

Magnetic Pole: A region at each end of a magnet where the external magnetic field is strongest.

Examiner insight

Examiners award marks for using the precise terms 'like poles repel' and 'unlike poles attract', rather than vague phrases like 'opposites attract'.

Common pitfall

Stating that a single N pole can exist by itself. Magnetic poles always come in N-S pairs; you can never isolate a single pole (a monopole).

Fun fact

The Earth itself acts like a giant bar magnet, which is why compasses work. However, the Earth's magnetic North Pole is actually a magnetic south pole, which is why the north-seeking pole of a compass needle is attracted to it!

Worked example 13 marks

The N pole of magnet X is brought near the end of a metal bar Y. They repel each other. What can you conclude about bar Y?

  1. 1

    Repulsion is the only sure test for magnetism. Attraction can happen between a magnet and a magnetic material, but repulsion only happens between two magnets.

  2. 2

    Since the N pole of magnet X repels bar Y, bar Y must also be a magnet.

  3. 3

    Furthermore, the end of bar Y near magnet X must be a N pole, because like poles repel.

Worked example 22 marks

A bar magnet is broken in half. Describe the magnetic properties of the two new pieces.

  1. 1

    When a bar magnet is broken in half, you do not get a separate N pole and a separate S pole.

  2. 2

    Instead, you get two new, smaller bar magnets.

  3. 3

    Each new magnet will have its own N pole and S pole. A new pole is created at the point of the break.

Recap

  • Magnets have a North-seeking (N) pole and a South-seeking (S) pole.
  • Like poles repel each other, and unlike poles attract each other.
  • Magnetic forces are strongest at the poles.
  • Repulsion is the only certain test to prove an object is a magnet.

Quick check

  1. What happens when a S pole is brought near another S pole?1 mark
  2. What is the only definitive test for magnetism?1 mark

2. Magnetic vs Non-Magnetic Materials

Materials can be grouped based on how they interact with magnets. Magnetic materials are strongly attracted to magnets. The most common examples are ferromagnetic metals like iron, steel, nickel, and cobalt. Non-magnetic materials, such as wood, plastic, glass, copper, and aluminium, are not attracted to magnets and show no magnetic effect.

Key term

Ferromagnetic: A material, such as iron or nickel, that is strongly attracted to a magnet and can be magnetised.

Examiner insight

Students should be able to name at least two examples for both magnetic and non-magnetic materials to secure full marks in classification questions.

Common pitfall

Mistakenly thinking all metals are magnetic. Copper, aluminium, and gold are common examples of non-magnetic metals.

Fun fact

The 'silver' part of modern UK pound coins is a nickel-plated steel alloy, making it magnetic, while the outer 'gold' part is a non-magnetic nickel-brass alloy.

Worked example 13 marks

A student has a pile of paper clips (made of steel), some copper coins, and a plastic ruler. She brings a bar magnet near each item. Describe what she will observe.

  1. 1

    The paper clips (steel) are made of a magnetic material. They will be strongly attracted to the magnet and will stick to it.

  2. 2

    The copper coins are made of a non-magnetic metal. The student will observe no force or attraction between the magnet and the coins.

  3. 3

    The plastic ruler is a non-magnetic insulator. The student will observe no force or attraction between the magnet and the ruler.

Recap

  • Magnetic materials are strongly attracted to magnets.
  • Examples of magnetic materials include iron, steel, nickel, and cobalt.
  • Non-magnetic materials are not attracted to magnets.
  • Examples of non-magnetic materials include plastic, wood, copper, and aluminium.
  • Not all metals are magnetic.

Quick check

  1. Is aluminium a magnetic or non-magnetic material?1 mark
  2. Name two ferromagnetic metals other than iron.2 marks

3. Induced Magnetism and Material Types

When a magnetic material (like an iron nail) is brought near a magnet, it becomes a magnet itself. This is called induced magnetism. The end of the nail closer to the magnet's pole develops the opposite polarity (e.g., a S pole is induced by the magnet's N pole), which is why there is always attraction. Magnetic materials are further classified as 'soft' or 'hard'. Magnetically soft materials, like pure iron, are easy to magnetise and demagnetise. They make strong temporary magnets. Magnetically hard materials, like steel, are harder to magnetise but retain their magnetism once magnetised. They are used to make permanent magnets.

Key term

Induced Magnetism: The process where an unmagnetised magnetic material becomes a magnet when it is placed in a magnetic field.

Examiner insight

Clearly linking 'soft' materials like iron to temporary magnets (e.g., electromagnets) and 'hard' materials like steel to permanent magnets demonstrates a strong understanding of their applications.

Common pitfall

Confusing the terms 'hard' and 'soft' with physical hardness. It refers to magnetic properties: 'soft' means easy to magnetise/demagnetise, 'hard' means difficult to magnetise/demagnetise.

Worked example 14 marks

An unmagnetised steel paperclip is hung from the N pole of a strong bar magnet.(a) What pole is induced at the top of the paperclip (closest to the magnet)?(b) Another identical paperclip is brought near the bottom of the first one. Will it be attracted or repelled? Explain your answer.(c) What happens to the paperclips if the bar magnet is removed?

  1. 1

    (a) A South (S) pole is induced at the top of the paperclip. This is because unlike poles attract.

  2. 2

    (b) The first paperclip is now a magnet, so a North (N) pole is induced at its bottom end. This N pole will then induce a S pole at the top of the second paperclip, causing attraction.

  3. 3

    (c) Steel is a magnetically hard material. When the bar magnet is removed, the paperclips will retain some of their induced magnetism and may remain stuck together as weak permanent magnets.

Worked example 23 marks

Explain why iron is used for the core of an electromagnet, but steel is used for a permanent fridge magnet.

  1. 1

    Iron is a magnetically soft material. This means it can be magnetised strongly but loses its magnetism almost instantly when the current is switched off. This switchable property is essential for an electromagnet.

  2. 2

    Steel is a magnetically hard material. This means it is difficult to magnetise, but once it is magnetised, it retains its magnetism for a long time. This makes it ideal for making permanent magnets.

Recap

  • Induced magnetism causes a magnetic material to become a magnet in a magnetic field.
  • The induced pole closest to the magnet is always the opposite pole, resulting in attraction.
  • Magnetically 'soft' materials (like iron) are easy to magnetise and demagnetise, making temporary magnets.
  • Magnetically 'hard' materials (like steel) are hard to magnetise but retain magnetism, making permanent magnets.

Quick check

  1. What is the difference between a magnetically 'hard' and 'soft' material?2 marks
  2. A paperclip is attracted to a magnet. Is this an example of induced magnetism?1 mark

4. Magnetic Fields

A magnetic field is a region around a magnet where a magnetic force can be detected. We cannot see magnetic fields, but we can visualise them using magnetic field lines. These are imaginary lines that show the direction and strength of the field. The direction of a field line at any point is the direction a North pole of a compass would point if placed there. By convention, field lines always travel from the North pole to the South pole outside the magnet. The strength of the field is shown by how close the lines are to each other; where the lines are closest together (at the poles), the field is strongest.

Key term

Magnetic Field Line: An imaginary line used to represent a magnetic field, where its direction shows the force on a north pole and its density indicates the field's strength.

Examiner insight

Marks are consistently awarded for drawing field lines that do not cross, have clear arrowheads pointing from North to South, and are more concentrated at the poles.

Common pitfall

Drawing magnetic field lines that cross each other. Field lines can never cross, as this would imply the field points in two directions at once.

Fun fact

Some animals, like pigeons and sea turtles, have a built-in magnetic sense (magnetoreception) that allows them to use the Earth's magnetic field for navigation during long migrations.

Worked example 13 marks

Draw the magnetic field pattern around a single bar magnet. Label the poles and show the direction of the field lines with arrows.

  1. 1

    Draw a rectangle to represent the bar magnet. Label one end 'N' and the other 'S'.

  2. 2

    Draw at least three continuous loops of field lines emerging from the N pole and entering the S pole.

  3. 3

    The lines should be drawn closer together near the poles to show the field is stronger there.

  4. 4

    Draw arrows on each line pointing away from the N pole and towards the S pole.

  5. 5

    Ensure no lines cross each other or touch except at the poles.

Worked example 23 marks

The diagram shows two bar magnets placed with their N poles facing each other. Copy the diagram and sketch the magnetic field pattern between the poles. Mark any neutral points.

  1. 1

    Draw the field lines emerging from the N pole of the left magnet and curving away from the N pole of the right magnet.

  2. 2

    Similarly, draw lines from the right magnet's N pole curving away from the left one. The lines from the two like poles will appear to 'push' against each other.

  3. 3

    Draw arrows on all lines pointing away from both N poles.

  4. 4

    In the centre, exactly between the two poles, there is a point where the fields cancel out. Mark this with an 'X' and label it as a 'Neutral Point'.

Recap

  • A magnetic field is a region where magnetic forces are felt.
  • Field lines show the direction and strength of a magnetic field.
  • Field lines point from North to South outside the magnet.
  • Closer field lines indicate a stronger magnetic field.
  • Field lines never cross.

Quick check

  1. In which direction do magnetic field lines point by convention?1 mark
  2. Where on a bar magnet is the magnetic field strongest? How do field lines show this?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define a magnetic pole.1 mark
  2. You have a bar of steel and a bar of soft iron. Which would be better for making a compass needle, and why?2 marks
  3. Describe how you could use a plotting compass to map the magnetic field around a bar magnet.3 marks
  4. The diagram shows three identical iron nails hanging in a chain from the S pole of a strong magnet. Explain why the nails hang from each other and why their tips spread apart.4 marks
  5. A student claims that all metals are magnetic. Explain, with examples, why this statement is incorrect.3 marks
  6. Draw a diagram to show the magnetic field pattern produced when the South pole of one bar magnet is placed near the North pole of another. Include direction arrows on the field lines.3 marks
  7. You are given three identical-looking metal bars, labelled A, B, and C. You test them by bringing their ends together and find that: End A1 attracts end B1. End A1 repels end C1. What can you deduce about the magnetic properties of bars A, B, and C? Explain your reasoning fully.5 marks
  8. State the law of magnetism that describes the forces between poles.2 marks
  9. Explain the difference between a magnetically 'hard' and a magnetically 'soft' material, giving an example of a use for each.4 marks
  10. A strong permanent magnet is dropped repeatedly on a hard floor. It is noticed that its ability to pick up paperclips becomes weaker. Using the idea of atomic magnets (domains), explain this observation.3 marks

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