19.5 Magnetic field of a current loop

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19.5 Magnetic field of a current loop

• Current loop
• Solenoid
• Electromagnets

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Motivation. To construct electromagnets, i.e. a
device to convert current to magnetic field.
Some elements of design of electromagnets. The
magnetic field due to current through loops or
coils of wire.
single coil
solenoid multiple turns of wire
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Magnetic field in a current loop
B field- out of loop
I
I
Current loop B field is the sum of fields
Straight wire
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B field due to current loop
Side view
looks like a magnetic dipole
B out of loop
N
I
S
B into loop
A current loop creates a magnetic dipole
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Current loop acts as a magnetic dipole
Example- Electric motors
current loop
B
N
N
S
S
x
Magnetic Field
Current loop creates magnetic dipole
Magnetic dipole is rotated in the magnetic field
Torque results from rotation of a magnetic dipole
in a magnetic field.
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Solenoid
Current in a solenoid produces magnetic dipole
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B field in solenoid
High field inside solenoid Lower fields
outside Uniform relatively constant field in
central region
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B-field in center by Amperes Law
L
Nno. of turns in length L
Only segment 1 contributes because Bll?L for
other segments zero
i.e. 2 in the picture
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Magnetic field in a long solenoid
BmonI
at center
B at the ends is less
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An electromagnet with has 100 turns of wire wound
around an air core with length of 3.0 cm. If a
current of 20 A is passed through the wire, what
is the B field at center of the magnet.
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Magnetic materials
The permeability, m, may differ from mo if a
magnetic material, such as iron is contained in
the solenoid
material m/mo vacuum 1.000 air 13.6x10-7 ir
on 1000-3000
at B 0.3-1 T
High permeability higher B fields for the same
current in the solenoid
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Magnetic material
Solenoid
Iron core
Magnetic dipoles in iron are aligned by the B
field to produce a larger B field
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Iron core electromagnet
solenoid with many turns
iron-core
I
V
B?nI
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Loudspeaker
cylindrical permanent magnet
diaphram
N
S
N
S
electromagnet
flexible edge
oscillating voltage input