general physics 2magnetism1 mass spectrometer used to identify atoms and their concentrations in a...
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General Physics 2 Magnetism 1
Mass Spectrometer
• Used to identify atoms and their concentrations in a sample of material.
• Between S1 and S2, molecules go straight if
• q E = q v B • or v = E/B
• After S2, B produces circular motion
• q v B’ = m v2/r• m = q B’ r/v = q B B’ r/E
• Found isotopes
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General Physics 2 Magnetism 2
E8 – Topics to cover
• Magnetic Force on a wire• Torque on a wire• Electric Motors - lab• Creating currents in moving loops• Eddy currents in moving conductors
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General Physics 2 Magnetism 3
Force on a wire
• Magnetic force of a straight wire segment
€
rF m,seg =
L
c
r I ×
r Β bar = L
r I ×
r B
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General Physics 2 Magnetism 4
Demo: Magnetic Force
• Interaction between B field and current
Moving charges are affected by B fields
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General Physics 2 Magnetism 5
Force on Current in B Field
Direction of F •given by Right Hand Rule
•fingers = I•bend fingers = B•thumb = F(for uniform field)
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General Physics 2 Magnetism 6
Think-Pair-Share
• (a) What is the magnitude of the force per meter of length on a straight wire carrying an 8.40-A current when perpendicular to a 0.90-T uniform magnetic field?
• (b) What if the angle between the wire and field is 45.00?
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General Physics 2 Magnetism 7
Torque on a wire
• A loop carrying a current will experience a torque which causes the loop to ROTATE!
• μ is the loop’s magnetic moment• Expresses the strength and south-to-north orientation of
the loop’s equivalent bar magnet
€
v rτ = rμ × r
Β bar
c=
r μ ×
r B
where magr μ ( ) = NAI
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General Physics 2 Magnetism 8
Torque on a Loop
• Forces on segments of a loop cause the loop to rotate• Simplify by replacing loop with magnet
• Fingers in direction of current, thumb represents NORTH pole of magnet
• Think about the loop as a magnet will rotate so that the north face of the magnet points in the direction of the magnetic field
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General Physics 2 Magnetism 9
Electric Motors - Torque
• In each orientation, which way will the loop rotate?
• Changing the direction of the current will keep the loop spinning
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Commutator
• To change the direction of the current and keep the loop spinning, a commutator is used
• http://www.walter-fendt.de/ph14e/electricmotor.htm
General Physics 2 Magnetism 10
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General Physics 2 Magnetism 11
Eddy currents in moving conductors
• You can create a current in a metal plate by moving the plate through a magnetic field.
• Current density will be constant in the region of the magnetic field
€
J = σ c
Fm
q=
σ cvBbar
c
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General Physics 2 Magnetism 12
Practice Problems
• Magnetism: Worksheets 3 and 5
• Finish before next class
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Group Problems
• E8B.1• E8B.4• E8S.5
General Physics 2 Magnetism 13