2. §11.1法拉第电磁感应定律
Faraday’s Discovery and the Law of Induction
Faraday’s law has far-reaching technological applications. It
lies behind our entire system of electrical power generation
and play a role in most of the electronic devices we use.
阿喇果 Dominique Francois Jean Arago 英国格林尼治
1822 1824
亨利 J. Henry
1829
法拉第 M. Faraday
1831.08.29
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3. Four Experiments 四个著名实验
磁棒 电流计线圈 CAI
感应电流
通电线圈
当磁棒或通电线圈相对于另一线圈运动时
⑴相对运动
⑵磁场发生变化
接通与断开线圈不动
相对运动被排除掉
磁场发生变化
这样的认识是否完全正确?
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4. The change of the magnetic field磁场发生变化
Changing the area of the loop
Unchanging the magnetic field
CAI
uniform magnetic field
change the magnetic flux through the loop
穿过回路的磁通量发生了变化
∫∫ ⋅=
A
B
AdB
rr
Φ
Suppose a loop enclosing an area A is placed in a
magnetic field . Then the magnetic flux through
the loop is
B
r
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5. The change of the magnetic flux through any surface
bounded by a loop causes a induction current in that
loop. 感应电流
electromotive force emf
∫∫
+
−
+
−
⋅=⋅= ldEld
q
F
k
k
rrr
r
ε
∫ ⋅=
l k ldE
rr
ε
R
internal circuit
external
circuit
+
感应电流的出现表明存在着某种推动电流的电动势
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6. 1832年
感应电动势 induction emf .
在相同条件下,不同金属导体回路中产生的感应电流与导体
的导电能力成正比。
The change of the magnetic flux through any surface
bounded by a loop causes an induction emf around
that loop,
穿过回路所包围面积的磁通量发生变化时,回路中产
生感应电动势
The induction emf leads to an induction current
科拉顿 Jean-Daniel Colladon
1823年 电流计 小磁针的偏转 退出返回
7. 韦伯 W. Weber诺埃曼 F. Neumann 1845年
The magnitude of the emf induced in a conducting loop is equal
to the rate at which the magnetic flux ΦB through that loop
changes with time.
ε
CAI
t
B
d
dΦ
ε ∝
t
k B
d
dΦ
ε −=
∫ ⋅=
L
k ldE
rr
ε
t
B
d
dΦ
ε −=SIin
∫∫ ⋅=
S
B SdB
rr
Φ
∫∫∫ ⋅−=⋅
SL
k SdB
t
ldE
rrrr
d
d
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11. Lenz’s law and the direction of induced current
The direction of the induced current is given by Lenz’s
law:
The induced current is in a direction such that the
magnetic field it creates tends to oppose the change that
created it
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13. ∫∫∫ ⋅−=⋅
SL
k SdB
t
ldE
rrrr
d
d
(1)磁场 不随时间变化,闭合导体回路的整体或
局 部在运动
B
r
Motional emf动生电动势
(2)磁场 随时间变化,闭合导体回路的任一部分
都不动
B
r
Induced emf感生电动势
磁场 随时间变化,闭合导体回路也有运动B
r
动生电动势 + 感生电动势
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