Transcript F5 e Magnetism_2009_11_11

Magnetism
1
S
Magnetic south
pole
磁南極
N
Magnetic north pole
磁北極
2
Law of magnetism:
Like poles repel.
同性磁極相斥。
Unlike poles attract
異性磁極相吸。
3
Magnetic field (磁場的概念)
Magnetic field :N- pole to S- pole.
磁場線是從磁北極繞向磁南極的。
field lines closer
磁場線愈密
field lines further apart
磁場線愈疏
stronger field
磁場愈強
weaker field
磁場愈弱
4
Plotting compasses.
用多個指南針把磁場顯示出來。
line up along the direction of magnetic field lines
指南針的指針自動排列並顯示出磁場。
5
Magnetic field pattern of 2
opposite poles
兩根異極相對的磁棒所產生的
磁場圖形：
Magnetic field pattern of 2 like
poles
X
兩根同極相對的磁棒所產生的
磁場圖形
neutral point X : magnetic field = 0
中和點 :磁場的合量為零
6
right-hand grip rule.
右手握拳定則
direction
of current 電流方向
direction of
magnetic
磁場方向
field
7
B-field due to a straight current wire--1
載電流直導線周圍的磁場圖形--1
8
B-field due to a straight current wire--2
載電流直導線周圍的磁場圖形--2
9
B-field inside a solenoid
uniform magnetic field
螺線管中央
磁場線分佈均勻
CW:
current
flow
P.101
電流 方向
Outside the solenoid螺線管外
1. B ~ bar magnet (與磁棒周圍的磁場線相類似)
2. Solenoid ~ bar magnet
10
Electromagnet
電磁鐵
can be ‘switched’ on and off.
能夠隨時 啟動 和 關閉
magnetic field direction can be reversed
磁場方向 隨電流方向改變。
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Strength of an electromagnet can be increased by:
以下方法加強電磁鐵的磁場:
1. a soft iron core (插入軟鐵心)
)
2. Increasing current (電流
3. Increasing the number of turns per
unit length
每單位長度的線圈匝數
12
Application of electromagnet:
1. Electric bells (電鈴)
spring 簧片
push button 按鈕開關
battery 電池組
metal contact 金屬接點
electromagnet 電磁鐵
hammer 鐵錘
gong 銅鑼
13
The push button is pressed
按下按鈕開關
Electromagnet is switched
on.
電流通過電磁鐵
The spring is attracted by
the electromagnet.
電磁鐵吸引簧片
The contact is broken.
簧片與金屬接點斷開
The spring moves back
to original position.
簧片彈回原位
The contact is closed.
簧片和金屬接點再次接觸
The process repeats.
這個過程不斷重複
14
2.Telephones 電話
electromagnet
電磁鐵
Varying electric current
passes the
electromagnet
通過電磁鐵的電流不斷改
變
Varying magnetic field
is produced.
產生不斷改變 的磁場
metal plate
金屬片
The metal plate vibrates.
金屬片振動
Sound is produced
產生聲音
15
Magnetic force on a current
16
Fleming’s left-hand rule
弗林明左手定則
magnetic force(F)
磁力的方向
magnetic field(B)
磁場的方向
Current (i)
電流的方向
17
A current in a magnetic field:
置於磁場內的載電流導線會感受到磁力
F is increased when:
在下列情況下，磁力均
會增加：
S-pole
B
F
1. i is increased.
I
電流增加
2. B is increased.
磁場增強
3. length of the wire is
increased.
F = 0 when B // I
導線較長
18
Magnetic field at 900 with i, F is maximum
電流的方向與磁場的方向成直角，磁力
B // i, F = 0
當電流與磁場平行時，磁力等於零。
19
Moving-coil loudspeakers
動圈式揚聲器
paper cone(錐形紙盤)
Coil (線圈)
Magnet
磁鐵
varying I
不斷改變
的電流
Varying current
(a.c.)through the
coil.
不斷改變的電流
通過線圈
The coil experiences
varying magnetic
force.
線圈受不斷改變 的
磁力作用
Paper cone vibrates
producing sound
產生聲波
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coil moves inwards
and outwards.
線圈振動
S
N
S
The paper cone
vibrates.
使錐形紙盤振動
S
N
S
Sound is produced.
產生聲波
21
Turning effect of a current carrying coil
載電流線圈在磁場中的轉動效應
Downward force on AB
magnetic field
磁場
C
B
AB 邊 所受的力向下
upward force on AB
CD 邊 所受的力向上
force on AB = 0
BC 邊 沒有磁力作用
D
A
Current電流
When coil vertical, F = 0
It rotates due to inertia
Coil oscillates for d.c.
current
The coil rotates.
線圈轉動
22
The turning effect can be increased by:
以下方法可增加載電流線圈的轉動效應：
 increasing i (增加電流的大小)
 increasing B (增強磁場 )
 winding more turns (增加線圈的匝數)
 increasing the area of the coil
(增加線圈的面積)
23
Structure of moving-coil ammeter
(動圈式檢流計的結構)
Pointer(指針 )
scale
cylindrical
magnet
(弧形磁鐵)
I
Coil
(線圈)
N
S
soft iron
core
(軟鐵心)
hair spring
(游絲)
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1. The pointer stops when the magnetic turning effect
= restoring torque of the hair springs.
(線圈的轉動效應與游絲的反向轉動效應互相抵
消, 線圈停止轉動)
2. Deflection increases with i
3. (電流,偏轉)
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d.c motor (直流電動機 能沿單一方向持續轉動):
coil線圈
Commutator /
half-rings:
connected to
coil.
換向器:兩個
Magnet
磁鐵
+
連接線圈的半
圓環
axis of rotation
–
Carbon brush
電刷
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2 at vertical position...
在垂直的位置
1 At the start...
開始時
A
B
B
A
C
D
C
+
+
–
coil rotates as
current flows
(i from B to C).
線圈在通電後轉動
D
–
F = 0! The coil keeps
turning due to inertia.
 線圈因自己的慣性而保
持轉動。
27
half-rings and the
brushes interchanges.
3 After ½ cycle...
兩個半圓環都會與另一
個電刷接觸。
B
A
Current in the coil is
reversed (i from C to B).
C
D
+
-
改變電流的方向
Magnetic forces acting on
the coil is reversed too.
作用在線圈的磁力逆轉
The coil keeps on rotating
in the same direction.
線圈繼續沿單一方向轉動。
28
A practical motor (實際的電動機)
1. It contains several coils at an angle to each
other.(擁有多組線圈，以不同角度互相圍繞。)
2. wires are wound around a soft iron core.
magnetic field strength
線圈繞在軟鐵心上
磁場強度
3. Electromagnets are used instead of
permanent magnets.
magnetic field strength
使用電磁鐵取代永久磁鐵
磁場強度
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Electromagnetic induction:
Generation of electricity from magnetism.
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Experiment:
move up
move down
1. The galvanometer deflects to 2 sides.
2. A current flows through the wire called induced
current .
3. Wire at rest, i = 0.
4. wire is moved up and down repeatedly, a.c. results.
31
Induced emf / current can be produced by:
1. Cutting B field lines by a moving conductor or,
2. Changing B field inside a resting conductor.
B
increasing/decreasing
XXXXXXXX
XXXXXXXX
XXXXXXXX
XXXXXXXX
32
Fleming’s right - hand rule(弗林明右手定則):
motion of the
conductor(F)
magnetic
field(B)
induced current(i)
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e.g.
south pole
wire
induced
current
感生電流
a wire cuts through
the magnetic field
lines.
當導體切割磁場線
時...
Induced voltage/
induced current
便會產生感生電壓。
34
The direction of induced current can also be found
by: Lenz’s law
我們可利用下列方法找到感生電流的方向：楞次定律
An induced current flows in a direction so as to
oppose the change producing it.
感生電流總是跟產生它的改變抗衡。
N
S
Induced current i
opposes magnet’s
motion into coil.
感生電流 i 抗衡磁
棒進入線圈
35
S
N
Induced current i opposes magnet’s motion
moving away coil.
感生電流 i抗衡磁棒移離線圈。
36
e.g.
v
X
X
X
X
X
X
X
X
V/i
t
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induced voltage/ i
rate at which :(1)the
 conductor cuts through
the magnetic field.(2)B is
changing
導體切割磁場線的速率
induced voltage When(以下方法令感生電壓增加):
1. conductor or the magnet moving faster
導體或磁鐵的運動速率
2. length of the conductor in the magnetic field
導體在磁場中的長度
3. magnetic field changing more rapidly
磁場的改變速率
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Application of electromagnetic induction:
1. microphone
diaphragm
sound waves strike
the diaphragm.
S
diaphragm
vibrates.
coil vibrates.
N a.c.
S
sound
waves
movable coil
Varying induced current
flows in the coil.
magnet
The signal is converted
into sound by loudspeaker.
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Generators
coil
1. a.c. generators
B
I flows
from A to B
A
slip rings
carbon brush
40
1 When the coil is at vertical position...
B
C
A
 coil does not cut
the magnetic field
lines.
 No induced current.
D
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2 When the coil has rotated 1800
C
B
induced current
flows in the
coil(B to A).
D
A
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A
A
A
induced current
1
2
1
number of
revolution
The induced current =0 when the coil passes the
vertical position.
43
P.162
1-5
44
Simple d.c. generators
coil
similar to a.c.
generator
i flows clockwisely
commutator / half - ring.
45
Induced current / voltage in generators can be
increased by:
1.use a stronger magnet
2.wind more turns
3.use a soft iron core
4.Increase the coil area
5. coil rotating at a higher speed
46
Bicycle dynamo
driving
wheel
axle
soft iron
C-core
case
coil
(fixed)
permanent
magnet
to head lamp
wheel
rotates
magnet
spins
Current is
induced
The head
lamp lights up
47
The structure of a transformer
soft iron core
secondary coil
a.c.
source/VP
primary coil
a.c.
Output/ VS
48
Formula:
Vs Ns

Vp N p
turn ratio
where Vp = input voltage
Vs = output voltage
Np = number of turns in the primary coil
Ns = number of turns in the secondary coil
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input voltage < output voltage
called step-up transformer.
input voltage > output voltage
called step-down transformer.
50
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For ideal transformer,
power input  power output
I p  Vp  Is  Vs
Ip
Vs

I s Vp
where Ip = input current
Is = output current
Vs Ns

Vp N p
Ip
Ns

Is N p
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
Main energy loss in transformers
1. Energy loss in copper windings (use thicker
wires)
2. Energy loss in magnetizing and
demagnetizing the core(use soft iron core)
3. Energy loss due to eddy current(use a
laminated core)
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Efficiency of transformers
output power
Efficiency 
 100 %
input power
54
P.157
1-4
P.163
6,7
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Example 5 Efficiency of a transformer
A transformer is used to operate a 12 V, 30 W lamp
from a 220 V mains supply.
I
220 V a.c.
R
lamp
12 V, 30 W
(a) Find the efficiency of the
transformer if a current of
0.15 A flows through the
primary coil (= ip).
Efficiency 
output power
x100 %
input power
Input Power= VI = 220x 0.15 =33W
Efficiency 
30 W
33W
 90.9%
X100%
56
(b) Instead of using a transformer, the
I
220 V a.c.
R
lamp
12 V, 30 W
lamp could be operated by
connecting it in series with a
suitable resistor to a 220 V mains
supply. Find the resistance of the
resistor and the efficiency of this
circuit.
P 30 W
 2.5 A
Current I  
12 V
V
Voltage across R  220 V  12 V  208 V
V
208 V

R
 83.2 
I
2.5 A
Input power = VI = 220 x 2.5 = 550W
30 W
Efficiency 
 100%  5.45 %
550 W
57
Transmission of electricity
Electricity companies supply electricity in the
form of alternating current (can be stepped
up/down).
Power transmission is at high voltage
58
Power distribution system
400 kV
23 kV
132 kV
factory
office
power
station step-up
transformer
11 kV
step-down
33 kV
transformer
step-down
transformer
home
220 V
59
e.g.
If 10 kW of electrical power is to be transmitted from power
station to consumers by using overhead cables (R = 3 Ohms) at:
1. 220V
2. 400 kV
2. P = Vi
1. P = Vi
i = 10000W/220V
i = 10000W/400 000V
i = 45.45A
i = 0.025 A
Power loss =
i2
R=
(45.45)2(3)
= 6198 W
Power loss = i2 R =( 0.025)2(3)
= 0.002 W
60
Alternative method:
P= 10000 W
R3
220V
R = 0, 0V
1. P = Vi
i = 10000W/220V
i = 45.45A
p.d. across the cable = iR = 45.45(3) = 136.35 V
p.d. at consumer = 220 – 136.35 = 83.65 V
Power supply to consumer = 83.65(45.45) = 3801.8925 W
Power loss at cables = 10000 – 3801.8925 = 6198 W (= i2 R =
(45.45)2(3) = 6198 W)
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Class work :
P.163
6, 7, 8, 9, 10, 11, 12.
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