19'3 Torque on a loop

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19.3 Torque on a loop

• Torque on a current loop
• Electric motors

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A force is produced by a current in a magnetic
field
But for a rotating motor we need a rotational
force
Torque Force x perpendicular distance
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Torque around a rotational axis
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Torque due to a current loop in a magnetic field
a
Top view
L
I
B
Side view
F
t
FBIL
a/2
a/2
F
?FaBILaBIA Aarea of loop
B
counterclockwise
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Torque depends on angle
B
F
tBILa
F
½a
F
Perpendicular distance ½asin?
tBILasin?
?
F
T is the angle between B and normal to the loop
F
t0
B
F
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Loop with N turns of wire with area A
Torque increases with N, B, I and A Torque is
maximum when ?90o,when the loop is parallel to
the field Torque is zero when ?0 when loop is
perpendicular to the field
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A 3A current loop with 100 turn and an area of
0.2 m2 makes an angle of 30o with a magnetic
field of 0.3T.a) Find the torque exerted on the
coil. b) What is the direction of rotation? c)
What happens if the current is reversed in the
coil?
30o
B
T60o
a)
counter clockwise direction
b)
the torque will have the same magnitude but in
the opposite (clockwise) direction,
c)
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Electric motor
A current loop in a magnetic field produces a
torque
Problem
A dc current does not produce complete rotation
B

• in opposite
• direction

?0
dc current only rotates coil until it is
perpendicular to the field
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Solution with direct current source is to use a
commutator.
Split-ring commutator reverses the current
direction when ?0.
Current reverses
continuous clockwise torque