Magnetic Force
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Transcript of Magnetic Force
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Magnetic Force
Physics 102Professor Lee
CarknerLecture 18
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PAL #17 Magnetic Field
Direction electron is fired into magnetic field that points north if it is deflected up
Force equation: F = qvB sin = sin-1 [(1.7X10-14)/((1.6X10-19)(3X105)
(0.5))] = v vector points 45 west of north, which is
pointed northwest, so electron was fired from southeast
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Electron in B Field
vB
North
West
South
East
From right hand rule:
B is north and force is up so v is from west
(reversed to east for electron)
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A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce the maximum deflection in the right direction?
A) RightB) LeftC) UpD) DownE) Right at you
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A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce the maximum deflection in the up direction?
A) RightB) LeftC) UpD) DownE) Right at you
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A beam of electrons is pointing right at you. What direction would a magnetic field have to have to produce no deflection?
A) RightB) LeftC) UpD) DownE) Right at you
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Electric and Magnetic Force How do the electric and magnetic forces differ?
Dependences Magnetic force depends on v and , as well as B and q
Vector
Force vector does change for a magnetic field, since as the particle is deflected the v vector changes
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Particle Motion A particle moving freely in a magnetic field will
have one of three paths, depending on Straight line
Circle
Helix
This assumes a uniform field that the particle
does not escape from
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Circular Motion
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Circular Motion If the particle moves at right angles to the field the
force vector will cause the path to bend
The particle will move in a circle How big is the circle?
Magnetic force is F = Centripetal force is F =
We can combine to getr = mv/qB
Radius of orbit of charged particle in a uniform magnetic field
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Circle Properties
Circle radius is inversely proportional to q and B
r is directly proportional to v and m
Can use this idea to make mass spectrometer
Send mixed atoms through the B field they will
come out separated by mass
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Today’s PAL
How long would it take an electron to complete one circular orbit around a 1 G magnetic field?
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Helical Motion If the initial velocity is not completely perpendicular
to the field, instead of a circle you get a spiral or helix Charged particles will spiral around magnetic field
lines
For example, if the lines begin and end at a pole
Examples: Gyrosynchrotron radio emission from planets and stars
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Helical Motion
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Solar Wind Particles in Earth’s Magnetic Field
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Magnetic Field and Current Since a current is moving charge, a magnet
will produce a force on a wire with a current flowing through it
So qv = IL, thus:F = BIL sin
We can use the right hand rule to get the
direction of the force Use the direction of the current instead of v
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Force on a Wire
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Force on a Loop of Wire
Consider a loop of wire placed so that it is lined up with a magnetic field Two sides will have forces at right angles
to the loop, but in opposite directions The loop will experience a torque
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Loop of Current
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Torque on Loop For a loop of width w and height h, force is F = BIL sin
for each long side Since = 90 and L = h,
The torque is the force times the moment arm
(distance to the center), which is w/2 Total torque = but hw is the area of the loop, A
If the field is at angle to the loop then
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Torque on Loop
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General Loops If there are multiple loops (N), the torque is the sum of
each = IBAN sin
A loop placed along a magnetic field will try to align such that the field goes straight through it
Can harness the spin to do work Called a motor
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Next Time
Read 20.7-20.8 Homework: Ch 20, P 4, 17, 38, 49 Exam #2 Friday