Download Ch 16 Magnetic Effect of a Current

06-07 CE Physics Ch 16 Magnetic Effect of a Current
Chapter 16 Magnetic Effect of a Current
16.1 Magnetic force and magnetic field
 Permanent magnet
 Magnetic force
 Magnetic field
16.2 Magnetic effect of electric current 電流的磁效應
 Current-carrying straight wire
 Current-carrying circular coil
 Current-carrying solenoid
16.3 Electromagnet 電磁鐵
 Factors affecting the strength of electromagnet
16.4 Force on current-carrying conductor in magnetic field
 Fleming’s left-hand rule
 Factors affecting the magnitude of force on current-carrying conductor
 Turning effect of coil
16.5 Simple d.c. motor
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
16.1 Magnetic force and magnetic field
(p. 105)
1. Permanent magnet (p. 106) Fig. 16.1 (p. 106)
(a) (i) Two thousand years age, the Chinese discovered a
“lodestone” which always points in the north-south
direction.
(ii) Anything which behaves as a lodestone is called a
magnet.
(b) Every magnet has two poles:
(i) the north pole (N-pole)
- which points to the north of the earth.
(ii) the south pole (S-pole)
- which points to the south of the earth.
2. Magnetic force (p. 106) Fig. 16.2 (p. 106)
(a) (i) Like poles repel.
(ii) Unlike poles attract.
(b) The earth is like a large magnet. Fig. 16.3 (p. 106)
If a bar magnet is hung by a thread,
(i) Its N-pole always points to the Arctic.
(ii) Its S-pole always points to the Antarctic.
(c) Conclusion:
(i) The Arctic is like a magnetic S-pole.
(ii) The Antarctic is like a magnetic N-pole.
Activity 1 Compass (p. 107)
(d) Compass: Fig. 16.4 (p. 107)
(i) It is used for finding direction.
(ii) It has a needle, which is a small magnet, pivoted
freely in a horizontal direction.
(iii)The N-pole of the compass needle points to the north
of the earth automatically.
(e) Magnets attract some objects: Fig. 16.5 (p. 107)
(i) Place an iron rod near a magnet. It is magnetized
and attracted by the magnet.
(ii) Magnets do not attract paper, silver, copper and
aluminium.
(iii)Not all the metals are attracted by magnets.
Class Practice 1 (p. 108)
3. Magnetic field (p. 108)
(a) The presence of a magnet produces a magnetic field in
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
the space around it.
(b) Magnets can exert magnetic forces on each other or on
other objects.
(c) The magnet that attracts more nails has a stronger
magnetic field.
4. Magnetic field lines (p. 108) Fig. 16.6 (p. 109)
(a) Magnetic field lines are used to show the strength and
direction of the magnetic field.
(b) Plotting magnetic field lines of a bar magnet:
(i) Place a bar magnet on a piece of paper and put a
compass near the N-pole of it.
(ii) Place another compass next to the first one that the
tip of the first compass needle points to the tail of the
second compass needle.
(iii)Repeat the above steps and mark the direction of the
compass needle in each case.
(iv)The pattern of magnetic field lines around a bar
magnet is drawn.
(c) Direction of magnetic field lines:
The magnetic field lines go out from the north pole,
loop round, and end at the south pole of a magnet.
5. Magnetic field patterns produced by different magnets (p.
109)
Experiment 16A Magnetic field of magnet (p. 109)
(a) Experimental procedures: Fig. 16.7 (p. 109)
(i) Place a bar magnet under a plastic board. Sprinkle
some fine iron filings on the board and tap the board
gently.
(ii) Observe the pattern formed by the iron filings.
(iii)Repeat the process by placing two bar magnets with
unlike poles and then like poles facing each other and
two slab-shaped magnets mounted on an iron yoke, with
opposite poles facing each other.
(b) Result and conclusion: Fig. 16.8 (p. 110)
(i) Like the electric field, the magnetic field cannot be
seen.
(ii) The iron filings can be magnetized by the magnets
and be aligned with the magnetic fields.
(iii)From the special pattern of the iron filings formed
around different magnets, we note the presence of
magnetic fields.
(iv)The closer the magnetic field lines, the stronger the
magnetic field.
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(v) At the neutral point: Fig. 16.8(c) (p. 110)
- The magnetic fields due to the magnets cancel each
other and the resultant magnetic field is zero.
- When a compass is placed at the neutral point, the
compass needle shows no deflection.
Class Practice 2 (p. 111)
16.2 Magnetic effect of electric current
(p. 111)
6. Magnetic effect of electric current (p. 112)
(a) Electric current, which is the flow of electric charges,
can produce a magnetic field.
(b) It is called magnetic effect of an electric current.
7. Current-carrying straight wire (p. 112)
Experiment 16B Magnetic effect of current
A. Magnetic field due to current-carrying straight wire
(p. 112)
(a) Experimental procedures: Fig. 16.9 (p. 112)
(i) A straight vertical wire passing through a perspex
board is connected to a d.c. power supply.
(ii) Several plotting compasses are then placed around
the wire.
(iii)Observe the directions of the needles of the
compasses.
(iv) Remove all the compasses. Sprinkle some iron
filings evenly on the board. Tap the board gently.
(v) Observe the pattern formed by the iron filings.
(b) Result and conclusion:
(i) The results illustrate that the magnetic field consists
of concentric circles with the wire at the centre. Fig.
16.10 (p. 112)
(ii) When the direction of the current is reversed, the
magnetic field pattern is unchanged. However, the
direction of the magnetic field is reversed.
Fig. 16.11 (p. 113)
(iii)Symbol of the direction of current:
- A current coming out of the paper is as the pointed
head of an arrow.
- A current going into the paper is as the tail of an arrow.
(iv)The closer it is to the wire, the stronger the magnetic
field.
8. Right-hand grip rule for a current-carrying straight wire (p.
113) Fig. 16.12 (p. 113)
(a) If the direction of the current is known, the direction of
the magnetic field along the straight wire can be
predicted by the right-hand grip rule.
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
(b) The rule:
If the right hand thumb points in the direction of the
current, the direction of the curled fingers indicates the
direction of the magnetic field.
9. Current-carrying circular coil (p. 114)
Experiment 16B Magnetic effect of current
B. Magnetic field due to current-carrying circular coil
(p. 114)
(a) Experimental procedures: Fig. 16.13 (p. 114)
(i) A circular coil, with a few turns, passing through a
perspex board is connected to a d.c. power supply.
(ii) Several plotting compasses are then placed around
the wire.
(iii)Observe the directions of the needles of the
compasses.
(iv) Remove all the compasses. Sprinkle some iron
filings evenly on the board. Tap the board gently.
(v) Observe the pattern formed by the iron filings.
(b) Result and conclusion: Fig. 16.14 (p. 114)
(i) The magnetic field at the centre of the coil is
perpendicular to the plane of the coil.
(ii) When the direction of the current is reversed, that of
the magnetic field is also reversed.
(iii)The closer it is to the wire, the stronger the magnetic
field.
10. Right-hand grip rule for a current-carrying circular coil (p.
114) Fig. 16.15 (p. 115)
The rule:
If the right hand fingers curl and point in the same
direction as the current, the thumb points in the
direction of the magnetic field inside the coil.
11. Current-carrying solenoid (p. 115)
Experiment 16B Magnetic effect of current
C. Magnetic field due to current-carrying solenoid (p. 115)
A solenoid is a long closely wound coil.
(a) Experimental procedures: Fig. 16.16 (p. 115)
(i) A solenoid passing through a perspex board is
connected to a d.c. power supply.
(ii) Several plotting compasses are then placed around
the wire.
(iii)Observe the directions of the needles of the
compasses.
(iv) Remove all the compasses. Sprinkle some iron
filings evenly on the board. Tap the board gently.
(v) Observe the pattern formed by the iron filings.
(b) Result and conclusion: Fig. 16.17 (p. 116)
(i) The magnetic field pattern produced is similar to that
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
produced by a bar magnet.
(ii) The magnetic field inside the solenoid is uniform
(straight and evenly spaced between lines).
12. Right-hand grip rule for a current-carrying solenoid
(p. 116) Fig. 16.18 (p. 116)
(a) The rule:
If the right hand fingers curl and point in the same
direction as the current, the thumb points in the direction
of the magnetic field inside the solenoid.
(b) Consider the solenoid as a bar magnet:
The end at which the thumb points is the north pole
whereas the opposite end is the south pole.
(c) The magnetic field lines outside the solenoid are
directed from the north pole to the south pole.
Example 1 (p. 117)
16.3 Electromagnet (p. 117)
13. Electromagnet (p. 118) Fig. 16.19 (p. 118)
(a) Disadvantage of permanent magnet:
Its magnetic field strength cannot be changed easily.
(b) (i) When a solenoid is wound on a soft-iron core, this
is called an electromagnet.
(ii) It works as a magnet when there is a flow of current.
(iii)Its magnetic field strength can be changed easily.
(c) Soft-iron core is used:
Reason:
(i) It can be strongly magnetized and immediately
demagnetized when the current in the solenoid is
switched on and off respectively.
(ii) It can increase the magnetic field strength of an
electromagnet significantly.
14. Magnetism of electromagnet (p. 118)
Experiment 16C Electromagnet (p. 118)
(a) Experimental procedures: Fig. 16.20 (p. 118)
(i) Wind several turns of wire (solenoid) round on a
soft-iron rod that acts as an electromagnet.
(ii) Connect the wire to a d.c. power supply. Place some
paper clips under the electromagnet. Observe the motion
of paper clips.
(iii)Repeat the above process by increasing the number
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
of turns of wire and the current.
(b) Result and conclusion: Fig. 16.21(a) (p. 119)
(i) The electromagnet attracts paper clips only when the
power supply is turned on.
(ii)When the power supply is turned off, the attractive
force disappears.
(c) Direction of magnetic field: Fig. 16.21(b) (p. 119)
(i) Place a compass between the poles of an
electromagnet, the compass needle points in one
direction.
(ii) When the direction of the current is reversed, the
needle points in the opposite direction.
(iii)The direction of the magnetic field of the
electromagnet can be changed easily.
(d) Factors affecting the strength of electromagnet:
Fig. 16.22 (p. 119)
The strength can be increased by:
(i) Increasing the number of turns of wire of the
solenoid.
(ii)Increasing the current.
16.4 Force on current-carrying conductor in
magnetic field (p. 120)
15. Force on current-carrying conductor (p. 120)
A current-carrying conductor experiences a force in a
magnetic field.
Experiment 16D Magnetic force on a current-carrying conductor (p.
121)
To study the relation between current, magnetic field
and magnetic force of a current-carrying conductor with
the Fleming’s apparatus.
(a) Experimental procedures: Fig. 16.23 (p. 121)
(i) Connect the apparatus to a d.c. power supply.
(ii) Observe the motion of the rider.
(iii)Repeat the above process by reversing the direction
of the current at first, then the poles of the magnet and
both of them at the same time.
(b) Result and conclusion: Fig. 16.24 (p. 121)
(i) When a current flows through the rider, it moves.
(ii) When either the direction of the current or the poles
of the magnets is reversed, the rider moves in the
opposite direction.
(iii)The rider does not move when the magnetic field is
parallel to the current flowing in it.
(iv)The rider experiences a force in a magnetic field.
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16. Fleming’s left-hand rule (p. 121) Fig. 16.25 (p. 122)
(a) (i) If a current-carrying conductor is placed in a
magnetic field, the conductor experiences a force.
(ii) The directions of the field, current and force are
mutually perpendicular.
(iii)The wire will move in a direction according to the
Fleming’s left hand rule.
(b) The rule:
(i) Hold the thumb and the first two fingers of the left
hand at right angles to each other.
(ii) Point the first finger in the direction of the magnetic
field and the second finger in the direction of the electric
current.
(iii)Then the thumb shows the direction of the force.
(c) The direction of the magnetic field is from the N-pole to
the S-pole.
17. Factors affecting the magnitude of force on current-carrying
conductor (p. 122)
The magnetic force can be increased by:
(i) Using a stronger magnet.
(ii) Increasing the number of turns of wire in the
magnetic field.
(iii)Increasing the current.
18. Turning effect of coil (p. 123) Fig. 16.26 (p. 123)
Turning effect of a coil:
(a) Consider a rectangular current-carrying coil ABCD
placed between the poles of a magnet:
As a current flows through the coil from A to D,
according to Fleming’s left hand rule:
(i) An upward force acts on the arm CD.
(ii) A downward force acts on the arm AB.
(iii)As a result, the coil rotates.
(b) Application:
(i) Electric motors
(ii) Meters
Example 2 (p. 123) , Class Practice 3 (p. 124)
16.5 Simple d.c. motor (p. 124)
19. Model d.c. electric motor (p. 125)
Experiment 16E Model electric motor (p. 125)
(a) Experimental procedures: Fig. 16.28 (p. 125)
(i) Connect the motor to a d.c. power supply and give a
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06-07 CE Physics Ch 16 Magnetic Effect of a Current
slight push to the armature.
(ii) Observe the motion of the armature.
(iii)Repeat by increasing the current flowing through the
armature, the number of turns of the coil in the armature
and using stronger magnets.
(b) Result and conclusion:
The armature rotates in the same direction.
20. Motors (p. 125) Fig. 16.29 (p. 126)
(a) Motors are found in many electrical appliances.
Example:
Small electric fans and electric drills
(b) A typical d.c. motor consists of:
(i) A permanent magnet or an electromagnet.
(ii) An armature, which is a coil wound on a soft-iron
core and placed between the poles of the magnet. The
framework of it is mounted on an axle.
(iii)The ends of the coil are connected to two copper
half-rings called a commutator 換向器.
(iv)A pair of carbon brushes 碳電刷 is fixed to push
against the commutator. They are connected to a d.c.
power supply.
(c) When a direct current flows to the coil through the
carbon brushes and the commutator, the coil together
with the commutator rotate in the magnetic field.
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21. Motion of the coil in a motor (p. 127)
Fig. 16.30 (p. 127)
Four stages of motion of the coil:
(Stage 1) The plane of the coil is parallel to the
magnetic field lines:
According to Fleming’s left hand rule:
(i) Arms PQ and RT of the coil are turned upwards and
downwards respectively.
(ii) The coil rotates in a clockwise direction.
(Stage 2) The plane of the coil is perpendicular to the
magnetic field lines:
(i) The circuit is broken at the junctions between the
commutator and the carbon brushes. No current flows
through the coil.
(ii) The coil continues to rotate due to its inertia.
(iii)After passing the vertical position, the contacts
between the commutator and the brushes have been
changed.
(Stage 3) The plane of the coil is parallel to the
magnetic field lines again (as stage 1):
According to Fleming’s left hand rule:
(i) Arms PQ and RT are turned downwards and
upwards respectively.
(ii) The coil continues to rotate in a clockwise direction.
(Stage 4) The plane of the coil is perpendicular to the
magnetic field lines again (as stage 2):
(i) No current flows through the coil.
(ii) The coil continues to rotate due to its inertia.
(iii)This goes on until the plane of the coil is parallel to
the magnetic field lines again.
22. Factors affecting the motion of the coil in motor
(p. 128)
(a) Arrangement of the commutator and carbon brushes:
Fig. 16.31 (p. 128)
Ensure the coil continues to rotate in the same direction.
Reason:
The direction of the current is reversed for each half
revolution of the armature.
(b) The magnitude of turning effect of a motor depends on:
(i) the magnitude of the current.
(ii) the number of turns of the coil.
(iii)the strength of the magnet.
(iv)the area of the coil.
STS Corner 1 Applications of electromagnet (p. 129)
STS Corner 2 Magnetic field of power line (p. 131)
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