Chapter 19 Magnetism - WordPress.comChapter 19 Magnetism Extension page 323 1. MRI is primarily used to detect pathological or other physiological alterations of living tissues and

Chapter 19 Magnetism

Learning Outcomes

After completing this chapter, students should be able to:

1. state the properties of magnets 2. describe induced magnetism 3. describe electrical methods of magnetisation and demagnetisation


4. draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets 5. describe the plotting of magnetic field lines with a compass 6. distinguish between the properties and uses of temporary magnets (e.g. iron) and permanent magnets

(e.g. steel)

19.1 Laws of Magnetism page 307 1. Students probably know that like poles repel and unlike poles attract. Emphasise 'north pole' is an

abbreviation for 'north-seeking pole'.

2. The Earth can be regarded as a large bar magnet with its south pole in the northern hemisphere.

Answers to Think Time question page 307

The two broken pieces still have two poles.

19.2 Magnetic Properties of Matter page 308 1. Magnetic materials are classified in terms of their magnetic properties and their uses. If a material is

easily magnetised and demagnetised then it is referred to as a soft magnetic material, whereas if it is difficult to demagnetise then it is referred to as a hard (or permanent) magnetic material. Materials in between hard and soft are almost exclusively used as recording media and have no other general term to describe them.

2. Emphasised to students that not all metals are magnetic.

3. Emphasised to students that only repulsion between a specimen and a magnet allows us to conclude that the specimen is a magnet.

4. The Theory of Magnetism (a) Iron bar unmagnetised

domains point all ways

(b) Iron bar partially magnetised

all domains point the same way

(cl Iron bar has reached magnetic saturation

The tiny arrows show the direction of magnetisation. The domains are actually much smaller than the ones shown here.

© Pearson Education South Asia Pte Ltd Science in Focus: Physics '0 ' Level


The mechanism of magnetism is still not fully understood. We know that substances are made up of a large number of atoms. There are electrons which move in orbits about the nucleus of the atom. It is believed that the movement of the electrons in the atoms of a magnetic material makes each atom a magnet. This atomic magnet is very small and weak. However, in a magnetic material, the millions of atoms arrange themselves in groups each with all its atomic magnets pointing in a certain direction. These groups are known as magnetic domains and each single domain is a strong magnet. In the urunagnetised state (Figure (a)) , the domains all point in different directions and their magnetic effects cancel out. When an iron bar is placed in a current-carrying solenoid (Figure (b)), the domains tend to align themselves until the magnet is at its greatest possible strength (Figure (c)). A magnet is thus produced.


This is because when the specimen experiences attraction, it may be a magnet of opposite pole or a magnetic substance.

Answers to Section Review questions page 312

1. Refer to page 310, Fig. 19.8. When the steel pin is brought near to the magnet, the steel pin is induced magnetically with the end nearer to the magnet having opposite polarity to that of the magnet. This results in attraction.

2. Suspend the three rods freely. The one, which always comes to rest in a north-south direction, is the steel magnet. Use the steel magnet to identify the other two rods. The rod it attracts will be the iron rod and the one it does not attract will be the aluminium rod.

3. The magnet will lose its magnetism.

19.3 Magnetic Fields page 312 1. Ask students to draw magnetic field pattern.

2. Emphasise the following rules: field lines do not cross; arrows point from N pole to S pole.

3. Emphasise that neutral point is a point where there is no magnetic effect. A compass needle placed at this position will not point to a specific direction.


Outside the magnet, the direction of the magnetic field is from N pole to S pole


1. Magnetic field is the region around a magnet in which magnetic effect can be detected. 2.

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3. The compass needle placed at neutral point will not point to a specific direction.

19.4 Uses of Temporary Magnets and Permanent Magnets page 315 1. Ask students to give examples where temporary magnets and pennanent magnets are used. Example:

motors, loudspeakers, microphones, generators, dynamos, electric meters, speedometers, medical scanners, maglev trains, audio and video tapes, credit card, A TM cards, television, electric bell, circuit breaker, floppy disk, hard disk, etc.


This is because soft magnetic materials can be magnetised and demagnetised easily. When there is a direct current flowing through a coil wounded on it, it becomes magnetised. When there is no current, it loses its magnetism.


1. Magnetic field is the region around a magnet in which magnetic effect can be detected. (a) Iron is more suitable for making an electromagnet. (b) Steel is more suitable for making a pennanent magnet.

2. Loudspeaker, electric meter, electric motor.

3. Sorting scrap metal, electric bell, recording on audio and video tape.

Physics in Societ)': Experimental and Theoretical Physicists page 319 Answers to Q

1. Ability to use sophisticated equipment to provide reliable experimental data to support theories or make new discoveries.

2. Ability to translate observations and physical insights into a formalized theoretical structure that could be verified by further experimentation.

3. Even during his last year of life (he died of cancer), he was still actively involved in the study of cosmic rays.

Answers to Misconception Analysis - page 321

1. True 2. True 3. True 4. False. It may also be a magnet

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5. False. Magnetism cannot be induced on non magnetic material 6. True 7. False. It is a point where there is no magnetic effect 8. True 9. False. A hard magnetic material is difficult to demagnetise 10. True

Answers to Multi Ie Choice Questions · pa~e 321, 322

1.D 2.C 3.A 4. A 5. D 6. C 7. B 8. C 9. C 10. C

Answers to Structured Questions pa~c 322, 323

1. Refer page 307 Figure 19.2

2. If the metal rod is a magnetic material but not magnetised, both ends of the rod will attract the north pole (or south pole) of a magnet. If the metal rod is magnetised, one end of the rod will attract the north pole (or south pole) of a magnet whereas the other end of the rod will repel the north pole (or south pole) of a magnet. (Refer page 309 Table 19.1)

3. (a) Magnetic lines of force are paths along which a free north pole would travel. Refer to page 312, Figure 19.12.

(b) For the sketches, refer to page 313, Figure 19.15(b) and Figure 19.15(a) respectively. The neutral point is the point in a combined magnetic field where there is no magnetic effect. Refer to page 313, Figure 19.15(b) for neutral point.

4. Refer to page 315, Section 19.4.

5. Refer to page 310-311 - Methods of magnetisation and demagnetisation. Refer to page 308 -Checking of poles

6. (a) The soft iron nails are magnetized by induction. (b) It is because the soft iron has been magnetized nearly as strongly as the bar magnet itself. (c) The nails will fall off because the soft iron would have lost its magnetism. (d) The two nails are magnetized weakly by the bar magnet at a distance through induction.

7. (a) Refer to page 310, Figure 19.8. (b) The N-pole is attracted to the north but the S-pole is repelled strongly away from the north with an

equal force . (c) Both the two lower ends of the needles are magnetized as N-poles, and like poles repel.

Answers to Critical Thinkin~ Questions pa~e 323

1. If the magnetic field lines cross, it means that at the same point the magnetic field will be pointing to two directions. If the magnetic field lines cross, the compass needle placed at that point will point to two possible directions.

2. If it is attraction, it may be due to attraction of unlike poles of magnets, it may also be due to attraction of a magnet and a magnetic material.

3. With the 'keepers' across the ends, the magnetic field will be continuous throughout. This will help the magnet to keep its magnetism. A magnet arranged like this is not good for pick up iron filings because there is no magnetic field outside the loop.

4. The ends of needles are induced with unlike poles and attract each other.

5. The forces are the same.

6. The pigeons will be able to identify the north and south direction like the compass .

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Extension page 323

1. MRI is primarily used to detect pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging. MRI uses radio frequency signals and a magnet to acquire its images. The use of MRI enables the doctor to obtain detailed information concerning internal structures of the body. This is achieved without the need of the surgeon's knife or an endoscope and is described as a non-invasive technique. Non-invasive techniques are much safer and far less traumatic than surgery.

MRI is harmless according to all present knowledge. The method does not use ionising radiation, in contrast to X-ray examinations. However, patients with magnetic metal in the body or a pacemaker cannot be examined with MRl due to the strong magnetic field , and patients with claustrophobia may have difficulties undergoing MRI.

Today, MRl is used to examine almost all organs of the body. The technique is especially valuable for detailed imaging of the brain and the spinal cord. Nearly all brain disorders lead to alterations in water content, which is reflected in the MRI picture.

Another example is prolonged lower back pain, leading to great suffering for the patient and to high costs for the society. It is important to be able to differentiate between muscle pain and pain caused by pressure on a nerve or the spinal cord. MRI examinations have been able to replace previous methods which were unpleasant for the patient. With MRI, it is possible to see if a disc herniation is pressing on a nerve and to determine if an operation is necessary.

Refer to website: http://en. wikipedia.orglwikilMRI; http://www.cis.rit.edulhtbooks/mrilinside.htm http://nobe1prize.orginobeLprizes/medicine/laureates/2003/press.htrnl

2. Magnetotactic bacteria are motile, mostly aquatic prokaryotes that swim along geomagnetic field line. Magnetotactic bacteria usually mineralize either iron oxide magnetosomes, which contain crystals of magnetite, or iron sulphide magneto somes, which contain crystals of greigite. Magnetotactic bacteria use chains of magnetosome crystals as a kind of compass to orient themselves in their environment. They use these compasses to detect the dip of the Earth's magnetic field. Equipped with a sense of direction underground, the bacteria are able to swim away from areas with greater than optimal oxygen content and toward areas with less than optimal oxygen content. These bacteria exhibit a negative growth response to atmospheric concentrations of oxygen.

Refer to website: http://www.calpoly.edu/-rfrankellNatRevMicro.pdf.

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Chapter 19 Magnetism - WordPress.comChapter 19 Magnetism Extension page 323 1. MRI is primarily used to detect pathological or other physiological alterations of living tissues and