Transcript Forces on Charges

Forces on Charges
Spinning Around Again
Here we are again:
 Today we will have a discussion about charge motion in
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magnetic fields and so some observations of wires in
magnetic fields.
There is A new WebAssign (Aren’t you excited??) posted.
I am still working on the gradebook so you can see your
grades all together.
Friday will be a Quiz on this week’s work. Maybe last week’s
too!
Next week more of the same.
Last Session:
 F=Bqv Sin(B,V)
 Right hand rule for direction.
A LONG, STRAIGHT WIRE
o I
B
2 r
VECTOR!
o  4 10 7 T  m A
permeability of
free space
What is magnetic field at X??
B
X

o I
B
2 r
450
1 Ampere
(4 107 T  m A)  (1Amp)
B
2  (1meter )
Current OUT of Paper
B  2 107 T
DIRECTION? “NW”
REMEMBER:
The effect of the sign of a moving
Positive and negative
charges will feel opposite
effects from a magnetic
field.
The Velocity Selector
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Magnetism
Let’s Look at the effect of crossed E and B Fields:
x x x B
E
x x x
v
q,m
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•
Magnetism
What is the relation between the intensities of the electric and
magnetic fields for the particle to move in a straight line ?.
x x x B
E
x x x
v
q• m
FE = q E and FB = q v B
If FE = FB the particle will move
following a straight line trajectory
qE=qvB
v=E/B
FB FE
•
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Magnetism
A charged particle is injected into a Magnetic Field.
Can the field do WORK on the particle?
A
B
C
D
YES
NO
It depends on the direction
the particle is moving in
with respect to the field.
I want to go back on Spring
Break!
The Motion of a Charged Particle in a Magnetic Field
The electrical force can do work on a
charged particle.
Parabola
The magnetic force cannot do work on a
charged particle.
The Motion of a Charged Particle in a Magnetic Field
The magnetic force always remains
perpendicular to the velocity and is
directed toward the center of the
circular path.
v2
Fc  m
r
v2
qvB  m
r
mv
r
qB
21.4 The Mass Spectrometer
mv mv
r

qB eB
1
2
mv 2  qV
1
2
mv 2  eV
magnitude of
electron charge
 er 2  2
 B
m  
 2V 
KE=PE
Mass Spectrometer
Smaller Mass
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Magnetism
Kratos Profile HV-3 Gas Chromatograph & Direct Probe Mass
Spectrometer
Description
Medium resolution double focusing (E/B) magnetic sector mass spectrometer with gas chromatograph and direct
probe inlets; electron impact and chemical ionization sources.
Figure 3.5.1 Chromatogram at target concentration.
1 = benzalazine, 2 AcPP.
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Magnetism
EXAMPLLE: Molecular structure and mass
spectrum of 1-acetyl-4-(2-pyridyl)piperazine. The
mass spectrum was obtained with a Perkin-Elmer
ion trap detector.
An Example
A beam of electrons whose kinetic energy is K emerges from a thin-foil
“window” at the end of an accelerator tube. There is a metal plate a distance d
from this window and perpendicular to the direction of the emerging beam. Show
that we can prevent the beam from hitting the plate if we apply a uniform
magnetic field B such that
2mK
B
2 2
ed
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Magnetism
Problem Continued
From Before
r
mv
r
qB
1 2
2K
K  mv so v 
2
m
m 2K
2mK
r

d
2 2
eB m
e B
Solve for B :
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2mK
B
e2d 2
Magnetism