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Cambridge IGCSE0625

Simple phenomena of magnetism

Physics 0625 Chapter Notes

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Simple phenomena of magnetism
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Simple phenomena of magnetism notes

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In this chapter

  1. 1Poles and magnetic materials
  2. 2Induced magnetism
  3. 3Temporary and permanent magnets
  4. 4Uses of magnets
  5. 5Magnetic field and its direction
  6. 6Field lines and plotting them
  7. 7Forces from interacting fields
  8. 8Field strength and spacing of lines

1. Poles and magnetic materials

A magnet has an N pole and an S pole. Unlike poles attract and like poles repel.

A magnetic material, such as iron, is attracted by a magnet. A non-magnetic material, such as copper or aluminium, is not.

Attraction alone does not show a magnet. An unmagnetised iron rod is attracted at both ends. A magnet is repelled at one end.

Examiner insight

“A surprisingly large number of candidates thought that tin would be attracted to the magnet, and that a magnet would have no effect on a bar of unmagnetised iron.” “Only the very best candidates gained full credit recognising that the test for a magnet was repulsion at one end.”

Worked example 11 mark

The north pole (N pole) of a magnet is moved close to each end of two different rods, X and Y. Both ends of rod X are attracted to the N pole. Only one end of rod Y is attracted to the N pole. Which statement is correct?

A X is a magnet and Y is an unmagnetised iron rod · B X is an unmagnetised iron rod and Y is a magnet · C X is a strong magnet and Y is a weak magnet · D X is a weak magnet and Y is a strong magnet

  1. 1

    B: X is an unmagnetised iron rod and Y is a magnet. [1]

2. Induced magnetism

A magnetic material placed in a magnetic field becomes a temporary magnet. This is induced magnetism.

Near the N pole of a magnet, the nearest end of an iron bar becomes an S pole. Near the S pole of the magnet, it becomes an N pole.

So an unmagnetised iron bar is attracted by both poles.

Examiner insight

“Many candidates struggled with the concept of induced magnetism.” “Very few candidates were able to explain that the magnet induced a N pole in the iron bar next to the S pole of the magnet.”

Worked example 11 mark

A soft-iron bar is not magnetised. It is held close to the N pole of a magnet and then to the S pole of the same magnet. What will be the result?

A N pole attracts, S pole attracts · B N pole attracts, S pole repels · C N pole repels, S pole attracts · D N pole repels, S pole repels

  1. 1

    A: the bar is attracted to both poles. [1]

3. Temporary and permanent magnets

A temporary magnet is made of soft iron. Soft iron is easily magnetised and demagnetised, so it does not keep its magnetism.

A permanent magnet is made of steel. Steel keeps its magnetism.

Examiner insight

“They should also be aware that iron does not retain its magnetism, whereas steel does.” “Many candidates suggested a magnet made of iron because it is easy to magnetise which suggested a misunderstanding of the application.”

Worked example 11 mark

Which row gives the metal used to make the core of an electromagnet and one property of the electromagnet?

A iron, permanent magnet · B iron, temporary magnet · C steel, permanent magnet · D steel, temporary magnet

  1. 1

    B: iron, temporary magnet. [1]

4. Uses of magnets

An electromagnet can be switched on and off. A crane uses one to pick up pieces of scrap iron and then drop them.

The core of an electromagnet is (soft) iron.

A permanent magnet is used where the magnet must keep its magnetism. Give a specific use, not a vague one.

Examiner insight

“Common answers that did not score included “that it could be magnetised”, “that it was strong(er)”.” “However, there were many answers that were too vague for credit, including “in a kitchen appliance” or “in a door” or “picking up metals in a scrapyard”.”

Worked example 11 mark

A crane uses an electromagnet to move heavy vehicles around in a scrap metal yard. Why does the crane use an electromagnet?

A Electromagnets are always stronger than all other magnets · B Electromagnets attract all metals · C Electromagnets attract all non-metals · D Electromagnets can be switched on and off

  1. 1

    D: electromagnets can be switched on and off. [1]

5. Magnetic field and its direction

A magnetic field is a region in which a magnetic pole experiences a force. The direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point.

Examiner insight

“Most stronger candidates answered this question correctly but weaker candidates incorrectly thought that the direction of the magnetic field is towards both the N pole and the S pole and therefore chose option D.”

Worked example 11 mark

State the effect that a magnetic field has on a magnetic pole.

  1. 1

    (exertsa) force [1]

6. Field lines and plotting them

Outside a magnet, field lines point away from N and towards S. Arrows show the direction. Field lines never cross.

Iron filings show the pattern of the field. A plotting compass shows its direction: the needle points in the direction of the field.

Examiner insight

“Many candidates correctly described the method to plot the shape of the magnetic field using iron filings but then did not describe how to find the direction using a plotting compass.” “Most candidates merely stated that the plotting compass would point to either the north or the south pole of the bar magnet.”

Worked example 12 marks

Three plotting compasses are placed in the magnetic field of a bar magnet. The circles in Figure 4.1 show the positions of the plotting compasses. The needle points in the direction of the magnetic field. On Figure 4.1, draw an arrow in each numbered circle to show the direction in which the compass needles point.

  1. 1

    arrow 1 horizontal, to the left (away from N) [1]

  2. 2

    arrows 2 and 3 horizontal, to the right [1]

7. Forces from interacting fields

Extended: magnetic forces are due to interactions between magnetic fields.

The field of a magnet makes a soft-iron bar an induced magnet. The two fields interact. The end of the bar nearest the N pole is an opposite pole, so the bar and the magnet attract.

Examiner insight

“Most candidates determined the correct polarity of end X, but others believed that the force on the soft iron was to the right rather than to the left, possibly because they confused it with the direction of the magnetic field at the N pole.”

Worked example 12 marks

The student places a bar magnet near to a soft-iron bar PQ, as shown in Figure 9.2. Explain why the bar magnet and the soft-iron bar move towards each other.

  1. 1

    the (soft-iron) bar becomes an induced magnet [1]

  2. 2

    the side nearest the N pole is an S pole (opposite pole), and unlike poles attract [1]

8. Field strength and spacing of lines

Extended: the relative strength of a magnetic field is shown by the spacing of the field lines.

Where the lines are closer together, the field is stronger. Where they are further apart, it is weaker.

Compare how close the lines are to each other, not how close they are to a pole.

Examiner insight

“The strongest answers clearly explained that the field is stronger when the lines are closer together (or vice versa).” “A few candidates tried to answer this question in terms of the proximity of field lines to the poles of the magnet rather than proximity to other field lines.”

Worked example 11 mark

State how the magnetic field at circle 3 is different from the magnetic field at circle 2.

  1. 1

    weaker (at 3) [1]

Recap

  • Unlike poles attract. Like poles repel. A magnet is repelled at one end.
  • Magnetic materials are attracted by a magnet. Non-magnetic materials are not.
  • Induced magnetism: a magnetic material in a field becomes a temporary magnet.
  • Soft iron: temporary magnet. Steel: permanent magnet.
  • An electromagnet can be switched on and off.
  • Magnetic field: a region where a magnetic pole experiences a force. Direction: the force on an N pole.
  • Field lines run from N to S and never cross. A plotting compass shows direction.
  • Extended: forces between magnets and iron come from interacting fields.
  • Extended: lines closer together mean a stronger field.

End-of-chapter exercise

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

  1. The N pole of one bar magnet is brought close to the N pole of a second bar magnet, and then to its S pole. State what happens each time.2 marks
  2. An iron nail is attracted to the N pole of a magnet. A student says the nail must be a magnet. Explain why he may be wrong and state a result that would show the nail is a magnet.2 marks
  3. A magnet is held close to an iron nail, a copper wire and a piece of tin. State which are attracted to the magnet and which are not.2 marks
  4. A soft-iron nail is not magnetised. The S pole of a bar magnet is held against one end of the nail. State the pole at that end of the nail and explain why the nail is attracted.3 marks
  5. (a) Name the metal used for the core of an electromagnet and give one reason. (b) State why iron is not used to make a permanent magnet.3 marks
  6. State what is meant by a magnetic field. State what the direction of the field at a point is.2 marks
  7. State the direction of the magnetic field lines outside a bar magnet and whether two field lines can cross.2 marks
  8. A student has a bar magnet, iron filings and a plotting compass. Describe how to find the pattern and the direction of the magnetic field.3 marks
  9. A student plots the field lines around a bar magnet. The N pole is at the left end of the magnet. He measures the gap between neighbouring field lines at three points. (a) Y is close to both poles. State whether the field is strongest at Y and give a reason. (b) Compare the strength of the field at X with the strength at Z. (c) A plotting compass is placed at each point in turn. State the direction its needle points at X, at Y and at Z. | point | position | gap between lines / mm | | --- | --- | --- | | X | beyond the left end, in line with the magnet | 3 | | Y | above the middle of the magnet | 9 | | Z | beyond the right end, in line with the magnet | 3 |6 marks
  10. A bar magnet hangs with its N pole at the bottom. An unmagnetised soft-iron rod and an unmagnetised steel rod stand upright on a bench. The magnet is lowered until it is just above the top of the soft-iron rod. (a) State the pole induced at the top of the soft-iron rod. (b) Just below the N pole, the magnetic field of the magnet points downwards. State the direction of the force on the rod and explain why there is a force. (c) The magnet is lowered over each rod in turn. Each rod is then taken far away and dipped into unmagnetised iron pins. State what happens for each rod and give the reasons. (d) The steel rod is turned upside down. Its new top end is brought close to the N pole of the magnet. State what the steel rod now experiences and explain why.8 marks

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