Download Ch33 The Magnetic Field

講者： 許永昌 老師
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Contents
 (起)Discovering magnetism
 Compass needle : a probe of Magnetic field
 (承)Moving charge is the source of magnetic field
 (轉)The magnetic force on a moving charge 
current-carrying wires.
 Force and Torque on Current loops
 (合)Microscopic discussion of Magnetic properties of
matter.
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(起)Discovering magnetism (請預讀
P998~P1000)
1.
Definition
2.
Interaction
3. No
magnetic
monopole
4. Differ
from
electricity
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Compass needle : a probe of
(請預讀P1000~P1002)
 Oersted’s discovery:
 In the midst of a
1819.
in
You should know and be used
to these notations.
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(承)Moving charge is the source of
magnetic field (請預讀P1000~P1011)
 Biot-Savart Law (1875):
 Permeability constant:

m0=4p10-7 Tm/A
 SI Unit of magnetic field: T (tesla)
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Moving charges  Current (請預讀
P1005~P1006)
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1
 v
Ds
Dt
 Dt: the time period used for a charge
carrier moved from surface 1 to 2.
 Dq:
 The net charge inside the segment.
 =the net charge pass through surface 2
during Dt.
m0 I Ds  rˆ

2
4p r



Dq 
I

Dt 
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Current Loop (請預讀P1007~P1012)
 A current loop is as a magnetic dipole.
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Ampere’s Law & Solenoid (請預讀
P1012~P1017)
 Ampere’s Law:
Example:
 B dr  m I
0 through
 Solenoid:
BL  m0 NI
 B  m0 nI .
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(轉)The magnetic force on a
moving charge (請預讀P1018~P1026)
 Lorentz force:
Fon q  qE  qv  B.
 We get
FB on wire  I l  B.
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Force and Torque on Current
loops(請預讀P1026~P1028)
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  Fi
 0,
i 1
 will be independent of the choice of origin.
   IA  B  m  B.
m : magnetic dipole moment = IA.
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(合)Microscopic discussion of
Magnetic properties of matter (請預讀
P1028~P1031)
 Candidates:
 Electron orbits

They will tend to cancel each other.
 Electron has


Mass: interact with FG.
Charge: interact with E.
: interact with B.
 Ferromagnetic material:
 The spins are aligned to be organized into magnetic
domain.
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