# Forces on Charges - UCF Physics

Forces on Charges Spinning Around Again 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 (Arent 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 weeks work.

Maybe last weeks 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 1

M er t e B 450 o I

2 r 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 7 Magnetism

Lets Look at the effect of crossed E and B Fields: x x x B E x x x v q,m 8 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 9 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 r mv mv 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 14 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. 17 Figure 3.5.1 Chromatogram at target concentration. 1 = benzalazine, 2 AcPP. 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 20 Magnetism Problem Continued r From Before mv r

qB 1 2 2K K mv so v 2 m m 2K 2mK r 2 2 d eB m e B Solve for B :

21 2mK B 2 2 ed Magnetism

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