Physics 2102 Spring 2002 Lecture 8
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Transcript Physics 2102 Spring 2002 Lecture 8
Physics 2102
Aurora Borealis
Jonathan Dowling
Lecture 20: WED 04 MAR
Magnetic fields
Ch.28.9-10
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“I’ll be back….
Principle behind electric motors.
Torque on a Current Loop:
Rectangular coil: A=ab, current = i
Net force on current loop = 0
But: Net torque is NOT zero!
F1 F3 iaB
F F1 sin( )
Torque Fb iabBsin( )
For a coil with N turns,
= N I A B sin,
where A is the area of coil
Magnetic Dipole Moment
We just showed: = NiABsin
N = number of turns in coil
A=area of coil.
Define: magnetic dipole moment
( NiA)nˆ
, nˆ
Right hand rule:
curl fingers in direction
of current;
thumb points along
B
As in the case of electric dipoles, magnetic dipoles tend to align
with the magnetic field.
Electric vs. Magnetic Dipoles
( NiA)nˆ
+Q
p=Qa
-Q
QE
r r
E p E
r r
UE p E
r
QE
r
B Br
r
UB B
r
r