Cambridge IGCSE0625

Simple phenomena of magnetism

Physics 0625 Chapter Notes

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Simple phenomena of magnetism
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In this chapter

  1. 1Properties of Magnets and Poles
  2. 2Understanding Magnetic Fields
  3. 3Induced Magnetism
  4. 4Magnetic Materials: Hard and Soft

1. Properties of Magnets and Poles

A magnet is an object that produces a magnetic field. Every magnet has two ends, called poles. These are the points where the magnetic force is strongest. The poles are named the north-seeking pole (N) and the south-seeking pole (S). This naming comes from how a freely suspended magnet aligns itself with the Earth's magnetic field, with its north-seeking pole pointing towards the Earth's geographic North Pole. The fundamental rule of interaction between poles is simple but crucial: like poles repel each other (N repels N, S repels S), and unlike poles attract each other (N attracts S).

Key term

Magnetic Pole: A region on a magnet, typically at its ends, where the magnetic force exerted by the magnet is strongest.

Examiner insight

Examiners look for the precise wording 'like poles repel, unlike poles attract'. Vague answers like 'opposites attract' may not receive full marks.

Common pitfall

Forgetting that repulsion is the only true test for a magnet. Attraction can occur between a magnet and an unmagnetised magnetic material, so it is not a conclusive test.

Fun fact

The Earth's geographic North Pole is actually a magnetic south pole. This is why the north-seeking pole of a compass needle is attracted towards it.

Worked example 13 marks

You have three identical-looking metal bars, X, Y, and Z. You test them by bringing their ends together. End X1 attracts end Y1. End Y1 repels end Z1. What can you deduce about the nature of bars X, Y, and Z?

  1. 1

    Step 1: Analyse the interaction between Y and Z. End Y1 repels end Z1. Repulsion is the only definitive test for a magnet.

  2. 2

    Step 2: Since repulsion occurs, both bar Y and bar Z must be permanent magnets.

  3. 3

    Step 3: Analyse the interaction between X and Y. End X1 attracts end Y1. Since we know Y is a magnet, X could be either another magnet (with an opposite pole at X1) or simply an unmagnetised magnetic material (attracted by induced magnetism).

  4. 4

    Step 4: Conclude the findings. Y and Z are definitely permanent magnets. X is either a permanent magnet or an unmagnetised piece of magnetic material (like iron or steel). We cannot be certain about X without more tests (e.g., testing if X repels another magnet).

Recap

  • A magnet has two poles: a North-seeking (N) pole and a South-seeking (S) pole.
  • The magnetic force is strongest at the poles.
  • Like poles repel each other.
  • Unlike poles attract each other.
  • Repulsion is the only sure test that an object is a magnet.

Quick check

  1. What force is experienced between the S pole of one magnet and the S pole of another?1 mark
  2. A bar magnet is broken in the middle. Will you have two pieces, one with a N pole and one with a S pole?1 mark

2. Understanding Magnetic Fields

A magnetic field is an invisible region around a magnet where a magnetic force can be exerted on other magnets or magnetic materials. We can visualise these fields using magnetic field lines. These are imaginary lines that show the direction and strength of the field. By convention, the direction of the field line at any point is the direction of the force that would be exerted on a tiny, isolated north pole placed at that point. Therefore, field lines always point away from a North pole and towards a South pole outside the magnet. The spacing of the lines indicates the field's strength: where the lines are close together, the field is strong; where they are far apart, the field is weak.

Key term

Magnetic Field: A region of space around a magnet or a current-carrying conductor in which a magnetic force can be detected.

Examiner insight

Marks are frequently awarded for drawing clear, non-crossing field lines with correctly indicated direction (N to S) and showing higher density at the poles.

Common pitfall

Drawing magnetic field lines that cross each other. Field lines represent the net force direction at a point, which can only be in one direction, so they can never intersect.

Worked example 13 marks

Draw the magnetic field pattern for a single bar magnet, labelling the poles and the direction of the field lines.

  1. 1

    Step 1: Draw a rectangle to represent the bar magnet. Label one end 'N' and the other end 'S'.

  2. 2

    Step 2: Draw at least three continuous loops of field lines starting from the N pole and ending on the S pole outside the magnet.

  3. 3

    Step 3: Ensure the lines are most concentrated (closest together) at the poles, indicating the strongest field strength.

  4. 4

    Step 4: Add arrows to each field line pointing away from the N pole and towards the S pole.

  5. 5

    Step 5: Make sure that none of the field lines cross each other.

Worked example 24 marks

Describe how you would use a small plotting compass to map the magnetic field around a bar magnet.

  1. 1

    Step 1: Place a bar magnet on a large sheet of paper and draw its outline.

  2. 2

    Step 2: Place the plotting compass near the N pole of the magnet.

  3. 3

    Step 3: Mark a dot on the paper at the position pointed to by the north end of the compass needle.

  4. 4

    Step 4: Move the compass so that its south end is now on the dot you just made.

  5. 5

    Step 5: Mark a new dot at the position of the north end of the needle. Repeat this process, creating a chain of dots.

  6. 6

    Step 6: Join the dots with a smooth line and add an arrow pointing from N to S. This represents one magnetic field line. Repeat from different starting points to map the entire field.

Recap

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

Quick check

  1. Where is the magnetic field around a bar magnet the strongest?1 mark

3. Induced Magnetism

When a magnetic material (like iron or steel) is brought into a magnetic field, it can become a magnet itself. This is called induced magnetism. The permanent magnet 'induces' (causes) magnetism in the nearby material. The end of the material closer to the magnet's pole becomes an opposite pole. For example, if you bring a N pole near an iron nail, the part of the nail closest to the magnet becomes a S pole, and the far end becomes a N pole. Because unlike poles attract, the nail is then attracted to the magnet. This is why a magnet can pick up a chain of paperclips; each paperclip becomes an induced magnet and, in turn, induces magnetism in the next.

Key term

Induced Magnetism: The process by which a material becomes a magnet when it is placed within a magnetic field.

Examiner insight

High-quality answers will clearly state that an opposite pole is induced, and it is this that causes the attraction. Simply saying 'it becomes a magnet and attracts' is not specific enough.

Common pitfall

Thinking that attraction is only possible between two permanent magnets. A key concept is that a magnet will always attract an unmagnetised magnetic material through induction.

Worked example 14 marks

A strong bar magnet is used to pick up a steel paperclip. The paperclip is then able to pick up a second paperclip. Explain these observations.

  1. 1

    Step 1: The strong bar magnet has a magnetic field around it.

  2. 2

    Step 2: When the steel paperclip is brought into this field, magnetism is induced in it. The end of the paperclip closer to the magnet becomes an opposite pole, resulting in a force of attraction.

  3. 3

    Step 3: Now magnetised, the first paperclip has its own magnetic field.

  4. 4

    Step 4: This field is strong enough to induce magnetism in the second paperclip, causing attraction and allowing the first paperclip to pick up the second.

Recap

  • A magnetic material placed in a magnetic field becomes a magnet itself.
  • This process is called induced magnetism.
  • The induced pole nearest the permanent magnet is always an opposite pole.
  • This is why magnets attract unmagnetised magnetic materials.
  • Induced magnetism explains how a magnet can pick up a chain of paperclips.

Quick check

  1. If the S pole of a magnet is brought near an unmagnetised iron bar, what pole is induced in the end of the bar closest to the magnet?1 mark

4. Magnetic Materials: Hard and Soft

Materials can be classified by how they respond to magnetic fields. 'Magnetically soft' materials, like soft iron, are easy to magnetise and also easy to demagnetise. They gain magnetism quickly when a field is applied but lose it almost immediately when the field is removed. This makes them ideal for temporary magnets, such as the cores of electromagnets and transformers. In contrast, 'magnetically hard' materials, like steel, are difficult to magnetise. However, once magnetised, they are also difficult to demagnetise and retain their magnetism for a long time. This property, called high retentivity, makes them perfect for making permanent magnets used in things like fridge magnets, compasses, and electric motors.

Key term

Retentivity: The ability of a magnetic material to retain its magnetism after the external magnetising field is removed.

Examiner insight

Examiners reward answers that explicitly link the property of the material (e.g., 'low retentivity' or 'easy to demagnetise') to the requirements of the application (e.g., 'so it can be switched off').

Common pitfall

Mixing up the properties of iron and steel. A good way to remember is: Steel is 'stubborn' – hard to change its magnetic state. Iron is 'fickle' – easy to change its magnetic state.

Fun fact

The data on a credit card's magnetic stripe is stored using tiny 'hard' magnetic particles, which are arranged to form a permanent magnetic code that the card reader can scan.

Worked example 14 marks

A school laboratory needs to build a powerful electromagnet that can be switched on and off. They also need to replace a weak bar magnet. They have a supply of soft iron bars and steel bars. Which material should be used for each purpose? Justify your choices.

  1. 1

    Step 1: For the electromagnet core, choose soft iron. The justification is that soft iron is a magnetically soft material. It is easily magnetised when current flows, creating a strong magnetic field, but also loses its magnetism almost instantly when the current is switched off.

  2. 2

    Step 2: For the permanent bar magnet, choose steel. The justification is that steel is a magnetically hard material. Although it is harder to magnetise initially, it retains its magnetism well (has high retentivity) and will therefore be a strong, long-lasting permanent magnet.

Recap

  • Magnetically soft materials (e.g., soft iron) are easy to magnetise and demagnetise.
  • Soft iron is used for temporary magnets like electromagnet cores.
  • Magnetically hard materials (e.g., steel) are hard to magnetise but retain magnetism well.
  • Steel is used for making permanent magnets.
  • Retentivity is the ability to stay magnetised.

Quick check

  1. Name one application of a magnetically soft material.1 mark
  2. Why is steel unsuitable for the core of a transformer?1 mark

End-of-chapter exercise

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

  1. Explain, using the concept of magnetic poles, why repulsion is considered the only definitive test for a magnet.3 marks
  2. Draw the magnetic field pattern produced when the North poles of two bar magnets are placed close to each other. Include a neutral point.4 marks
  3. Describe two different methods that can be used to demagnetise a steel bar magnet.2 marks
  4. A student claims that a bar magnet can be created with only a single North pole. Explain why this is incorrect.2 marks
  5. Explain in detail why a soft iron core is used in a device like a scrapyard crane electromagnet, whereas the tip of a screwdriver is often made of magnetised steel.4 marks
  6. A bar magnet is suspended by a thread so it can rotate freely. Describe and explain its final orientation with respect to the Earth.3 marks
  7. You are given an iron nail, a battery, and a long piece of insulated copper wire. Describe how you would turn the nail into a magnet.3 marks
  8. A magnet is brought near a pile of aluminium paperclips, but it does not pick them up. It is then brought near a pile of steel paperclips and picks them up. Explain this difference in behaviour.2 marks
  9. The diagram shows a bar magnet and four plotting compasses. On a copy of the diagram, draw an arrow inside each compass to show the direction the needle would point.4 marks
  10. Distinguish between a magnetic material and a permanent magnet. Give one example of each.3 marks

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