Spinning Around Again

Forces on Charges

Here we are again:Today we will have a discussion about charge

motion in 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

rIB o

2

AmT104 7 o

permeability of free space

VECTOR!

Current OUT of Paper

450

1 Ampere

1 MeterX

What is magnetic field at X??

rIB o

2

7

7

(4 10 T m A) (1 )2 (1 )

2 10

AmpBmeter

B T

DIRECTION? “NW”

B

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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Let’s Look at the effect of crossedcrossed E and B Fields:

xx xx

xx

•q , m

B

v

E

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What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?.

•FEFB

FE = q E and FB = q v B

If FE = FB the particle will movefollowing a straight line trajectory

q E = q v B

v = E / B

xx xx

xx

•q m

B

v

EB

E

A charged particle is injected into a Magnetic Field.Can the field do WORK on the particle?

A YESB NOC It depends on the direction

the particle is moving inwith respect to the field.

D I want to go back on SpringBreak!

The Motion of a Charged Particle in a Magnetic Field

The electrical force can do work on acharged particle.

The magnetic force cannot do work on acharged particle.

Parabola

The Motion of a Charged Particle in a Magnetic Field

The magnetic force always remainsperpendicular to the velocity and is directed toward the center of the circular path.

rvmFc

2

rvmqvB

2

qBmvr

21.4 The Mass Spectrometer

eBmv

qBmvr

212

212

mv qV

mv eV

magnitude ofelectron charge

KE=PE

22

2B

Verm

Mass Spectrometer

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Smaller Mass

http://upload.wikimedia.org/wikipedia/commons/b/b8/Mass_spectrometer_schematics.png

Kratos Profile HV-3 Gas Chromatograph & Direct Probe Mass SpectrometerDescriptionMedium resolution double focusing (E/B) magnetic sector mass spectrometer with gas chromatograph and direct probe inlets; electron impact and chemical ionization sources.

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Figure 3.5.1  Chromatogram at target concentration. 1  = benzalazine, 2

AcPP.

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

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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

22

2demKB

Problem Continued

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r

22

22

2

e2mKB

:Bfor Solvee22

eBm

2 so 21

qBmvr

Before From

d

dBmK

mKr

mKvmvK