2014 Sec 4 Physics Chapter 16
Magnetism
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(
) Class: 4 / ___
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16
Magnetism
16.1
16.2
16.3
Laws of magnetism
Magnetic properties of matter
Magnetic field
Specific Instructional Objectives
(a)
state the properties of magnets
(b)
describe induced magnetism
(c)
describe electrical methods of magnetisation and demagnetisation
(d)
draw the magnetic field pattern around a bar magnet and between the poles of
two bar magnets
(e)
describe the plotting of magnetic field lines with a compass
(f)
distinguish between the properties of magnetic and non-magnetic materials
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(g)
distinguish between the properties and uses of temporary magnets (e.g. iron) and
permanent magnets (e.g. steel)
Read up:
• Textbook: “All About Physics ‘O’ Level” Chapter 20, page 396-413.
• http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html
16.1
Laws of Magnetism
Magnets attract objects made from iron, nickel, cobalt, gadolinium or certain other
ferromagnetic alloys and compounds.
A magnet has two poles, at which the attractive force seems greatest. The poles are
called north-seeking, or north (N), and south-seeking, or south (S). Like poles repel each
other and unlike poles attract each other.
Only repulsion between a specimen and a magnet allows
specimen is a magnet.
us to conclude that the
Example 16.1
Fig. 16.1a shows a compass pointing to the Earth’s magnetic north pole.
N
N
W
E
S
compass
Fig. 16.1a
Fig. 16.1b below shows what happens when a magnet is placed near the compass
N
W
E
S
Fig. 16.1b
Expain why the compass needle changes its direction in the presence of the magnet. [2]
The end of the magnet near to the compass is a south pole 
since unlike poles attact. 
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16.2
Magnetic properties of matter
When the magnetic domains in a ferromagnetic material are aligned in the same
direction, it becomes magnetised.
Ferromagnetic materials are classified in terms of their magnetic properties:
i.
a material that is easily magnetised and demagnetised is referred to as a soft
magnetic material, whereas
ii.
a material that is difficult to demagnetise is referred to as a hard (or permanent)
magnetic material.
In the presence of a magnet, an object made from ferromagnetic materials will itself
become a magnet. (This process is called magnetic induction.) The magnet attracts
the object because the pole of the magnet closest to the object produces an unlike pole
in the nearest part of the object. For example, the N pole of the magnet will produce an
S pole in the part of the object closest to it. (In this example, the most distant part of the
object would become an N pole.)
When a magnet is cut into two or more pieces, each piece becomes a new magnet.
Example 16.2
Explain why two steel needles placed at the ends of a bar magnet tend to incline towards
one another as shown in Fig. 16.2.
N
S
Fig. 16.2
Steel is a magnetic material and the needles will be induced with
unlike poles at its ends. 
Since unlike poles attract, the ends will be attracted to each other.
The best method of making powerful magnet is by the electrical method. You should be
be able to describe how it is done with the help of a diagram.
To demagnetise a magnet is to disturb the alignment of the magnetic domains so that
they all point in different directions. You should be able to describe how it is done with
the help of a diagram.
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16.3
Magnetic field
Magnetic fields, regions where a magnetic material will experience a force, exist around
magnets.
The field pattern can be observed as a series of magnetic lines of force with the
following properties:
• They attempt to form closed loops from pole to pole.
• They never cross one another as they are mutually repulsive and have lateral
tension.
• Their density (field strength) decreases with increasing distance from the poles.
• They flow from the south pole to the north pole within a material and north pole to
south pole in air.
Example 16.3
For each of the following set-ups, draw the magnetic field set up by the magnet.
(a)
X is the neutral point
(b)
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16.4
Magnetic screening
Mgnetic field lines cannot be destroyed or absorbed but they may be channelled away to protect specific
region; materials which do this are themselves magnetic.
Example 16.4
Fig.16.4 below shows a magnet placed on one side of an iron sheet. Draw the resultant
magnetic field pattern around the magnet.
Fig. 16.4
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Exercises
1
2
3
A metal bar X-Y hung by a thread always settles in no definite direction. Another
bar P-Q of the same metal always comes to rest with end P pointing North. What is
the result if the two bars are brought near one another?
A.
B.
P attracts X but repels Y.
P neither attracts nor repels X.
C.
D.
P attracts X but Q repels Y.
P and Q both attract X.
Which of the following statements describes an example of induced magnetism?
A. A bar magnet, swinging freely, comes to rest pointing North-South.
B. A bar magnet loses its magnetism if it is repeatedly dropped.
C. A bar magnet attracts a piece of soft iron.
D. It is hard to magnetise steel, but easy to magnetise soft iron.
As shown in the diagram, there is a magnetic field within a region of space.
The magnetic field strength is the greatest at
A. P.
B. Q.
C. R.
D. S.
4
Amy attaches one end of a string to a steel paper clip and the other end to a table.
The string is very light. She then uses a magnet to attract it so the clip seems to
float in air. However, when she lifts the magnet, the paper clip falls. What is the
reason for this?
A.
B.
C.
D.
The potential energy of the clip decreases.
The gravitational force near the magnet increases.
The magnetic properties of the clip decreases.
The magnetic field strength near the clip decreases.
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5
As shown in the diagram, three compasses are placed near a bar magnet.
Which of the compasses has / have the needle pointing in the correct direction?
(1) Compass P
(2) Compass Q
(3) Compass R
A.
B.
C.
D.
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(1) only
(2) only
(1) and (2) only
(2) and (3) only
Amy has a compass and wants to check whether it works or not. She puts the
compass next to a bar magnet as shown.
If the compass works well, which of the following shows the correct position of
the needle?
A.
B.
C.
D.
7
As shown in the diagram, two bar magnets face each other with their S-poles.
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Which of the following about the point P is / are correct?
(1) The strength of the magnetic field at point P is double that of only one bar
magnet.
(2) The resultant magnetic field at point P is zero.
(3) At point P, the magnetic field due to the left magnet alone is zero.
A.
B.
C.
D.
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(1) only
(2) only
(1) and (2) only
(2) and (3) only
The diagram shows three sheets of material placed between magnets and iron
nails.
Which of the following is a correct description of the force on the nail for each
material?
Lead
Iron
Aluminum
A.
Attraction
Attraction
Repulsion
B.
Attraction
No force
Attraction
C.
No force
No force
No force
D.
Repulsion
Attraction
Repulsion
Discussion
What is the nature of magnetism? Discuss.
Source: http://www.youtube.com/watch?feature=player_detailpage&v=hFAOXdXZ5TM
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