Physics 2102 Spring 2002 Lecture 8

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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   Fb  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

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