Transcript Document

Physics 121: Electricity &
Magnetism – Lecture 10
Carsten Denker
NJIT Physics Department
Center for Solar–Terrestrial Research
Magnetic Field Due to a Current
dB 
0 ids  r
4 r 3
Biot–Savart’s law
0  4 107 Tm/A
B
Permeability constant
0i
4 R
Circular arc
March 28, 2007
0 i
2 R
Long
straight wire
B
Semi–infinite
straight wire
0 i
B
4 R
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Force Between two Parallel Currents
To find the force on a current–
carrying wire due to a second
current–carrying wire, first find the
field due to the second wire at the
site of the first wire. Then find the
force on the first wire due to that
field.
Parallel currents attract, and anti–
parallel currents repel.
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Ampere’s Law
 B  ds   i
0 enc
Ampere’s law
Curl your right hand around the Amperian loop, with the finger pointing
in the direction of integration. A current through the loop in the general
direction of your outstretched thumb is assigned a plus sign, and a
current generally in the opposite direction is assigned a minus sign.
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Center for Solar-Terrestrial Research
Solenoids and Toroids
Ideal solenoid
B  0in
Toroid
0iN 1
B
2 r
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Current–Carrying Coil as a
Magnetic Dipole
Current–carrying coil
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Bz 
0 
2 z 3
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