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19 Overview
• magnetic force fields & sources
• forces on charges & currents
• Homework:
• 3, 7, 10, 33, 45, 55, 75.
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Magnetic Field, B
• SI Unit: tesla, T = N/A·m
• Direction shown below
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Magnetic Poles
• Do ______ have _____ electric charge
• _______________ plates easily make
uniform electric fields.
• Uniform B field
difficult to achieve
3
Magnetic Force on Charge
• F = qvB sin θ
• ___________ points in
direction of force on a
________________
charge (Flat RHR)
• Negative charge feels
________________
directed force
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Properties of the Magnetic Force
• F = qvB sin θ
• Proportional to: charge, speed, field, &
sine of angle between v and B.
• perpendicular to both v and B.
• /
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Ex. An object with a charge of 2.0 mC is moving
perpendicular to a magnetic field of strength 1.0 T
at a speed of 1.0 x 103 m/s. a) Sketch the situation.
b) Find the magnetic force on the particle.
c) Indicate the direction of the force on your sketch.
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Applications of Magnetic Force
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electric motor
Mass spectrometer
magnetohydrodynamic propulsion
colliders
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Cyclotron Motion
• Magnetic force is
__________________
to velocity
• causes a free charge to
move in a circle
• Does no ___________
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Cyclotron Motion
• qvB = _____________
• radius of motion r = mv/qB
• period of motion = T
( vT = 2pr )
• frequency = 1/T
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Ex. A particle, m = 1gram, v = 10m/s, q = 1mC,
moves perpendicular to a uniform field B = 1.0 T
directed into the screen. Sketch the motion and
calculate its radius.
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Magnetic Force on a Current
Carrying Wire
• net magnetic force = net magnetic force on
all moving charges
•
qv
=
IL
• (C)(m/s) = (C/s)(m)
•
F = ILB sin θ
• //
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Force on a Current Carrying Wire
• F = ILB sin θ
• Direction is given by a
right hand rule
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Ex. A wire of length 1m is perpendicular to a uniform
magnetic field of strength 0.50 T. a) Sketch the situation.
b) Choose a direction for the current and indicate the
direction of the force on the wire for your choice. c) If the
magnitude of the force is 10 N, find the current in the wire.
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Electric Motors
The ILB force causes a motor coil to turn. Current
__________________ is changed by a split ring.
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Torque on Coil
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area, A number of turns, N
current in wires, I
in field, B
angle between coil normal & B: q.
torque = NIABsinq.
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torque example
• Tesla Motors Car produces
__________________ = 271 m·N
NIAB = 271 m·N.
• estimated reasonable? specs:
• with B = 0.3T: NIA = 904.
• with A = 0.05sq.m.: NI = 18,000.
• with I = 20A: N = 900turns
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Straight Wire & Loop
o I
B
2pr
Bcenter  N
o I
2R
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Solenoid
Bcenter
N
 o I
L
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Ferromagnetism
• Unmagnetized
• Induced
• Permanent
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Summary
•
•
•
•
•
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F = qvB sinq = ILB sinq (Flat RHR)
B fields around wires (Curled RHR)
B field inside a solenoid is nearly uniform
electric motors utilize B force
charge motion (v, I) causes B fields
//
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