Transcript PHY 231 Lecture 29 (Fall 2006)
Physics 213 General Physics
Lecture 9
Exam 1: Raw Scores 35 30 25 20 15 10 5 0 0 20 40 60 Raw score 80 100
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Exam 1: % Scores Without Curve 35 30 25 20 15 10 5 0 0 20 40 60 80 100 % score without curve 120 140
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Exam 1: % Scores With Curve 35 30 25 20 15 10 5 0 0 20 40 60 80 100 % score with curve 120 140
N
1
N
2 90 25 100 %
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Last Meeting: Magnetism & Magnetic Field Today: Lorentz Force and Torque
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Determining the Direction of Force, the Right Hand Rule Point your fingers in the direction of
v
Curl the fingers in the direction of the magnetic field,
B
Your thumb points in the
F
a positive charge If the charge is negative, the force is opposite that determined by the right hand rule
┴ ┴ 9
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Circular Orbit of Charged Particle in a Magnetic Field r mv qB
Particle Moving in an External Magnetic Field If the particle’s velocity is not perpendicular to the field, the path followed by the particle is a spiral The spiral path is called a helix
Force on a Wire
B is into the page The current is up the page The force is to the left 13
Force on a Wire (cont.)
B is into the page The current is down the page The force is to the right 14
Force on a Wire
The magnetic force is exerted on each moving charge in the wire The total force is the sum of all the magnetic forces on all the individual charges producing the current F = B I ℓ sin θ of I
B
The direction is found by the right hand rule, placing your fingers in the direction of I instead of
v
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How is force determined?
Force on each charge carrier.
F i
qv d B
sin Total number of charge carriers.
N
nAl
Total Force
F total
nAqv d Bl
sin
IBl
sin In lecture 5 we derived
I
nAqv d
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Demo Wire in Magnetic Field
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┴
θ
20 θ
Torque on a Current Loop
Applies to any shape loop N is the number of turns in the coil Torque has a maximum value of NBIA
When
= 90°
Torque is zero when the field is perpendicular to the plane of the loop 21
Motors
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┴ μ =
θ μ
μ 23 θ
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μ
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μ
B sin
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