General Physics 2Magnetism1 Mass Spectrometer Used to identify atoms and their concentrations in a sample of material. Between S1 and S2, molecules go.

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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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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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

General Physics 2 Magnetism 4

Demo: Magnetic Force

• Interaction between B field and current

Moving charges are affected by B fields

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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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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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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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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Electric Motors - Torque

• In each orientation, which way will the loop rotate?

• Changing the direction of the current will keep the loop spinning

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

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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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Practice Problems

• Magnetism: Worksheets 3 and 5

• Finish before next class

Group Problems

• E8B.1• E8B.4• E8S.5

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