Charge-to-mass ratio of electron/proton

by jdc22
Tags: chargetomass, electron or proton, ratio
jdc22 is offline
Feb23-10, 07:15 PM
P: 1
1. The problem statement, all variables and given/known data
The Helmholtz coil current = 2.45 A, the magnetic field = 1.91 x 10^(-3) T, the accelerating voltage = 295 V, and the beam radius = .03m.

a) Calculate the e/m ratio of an electron. Show that e/m = 2V / rB

b) During the initial use of this experiment, physicists also knew the charge-to-mass ratio of a hydrogen ion, approximately 9.58 x 10^7 Coulombs/kg. If you had little knowledge of atomic structure, what two interpretations could you place on the differences between the two ratios?

2. Relevant equations
PE = Ve
KE = 1/2mv
Fmag = evB
Fcent = mv/r

3. The attempt at a solution
a) Ve = 1/2mv
v = √(2Ve / m) ; v = 2Ve/m
Fmag = Fcent
evB = mv/r
r = mv/Be

I can't seem to get past this part of the derivation. I've tried subbing in √(2Ve / m) into r = mv/Be but it turns out too messy.

b) I'm not too sure about the second part at all.. in what kind of context should I be thinking about the ratios?
Phys.Org News Partner Science news on
Review: With Galaxy S5, Samsung proves less can be more
Making graphene in your kitchen
Study casts doubt on climate benefit of biofuels from corn residue

Register to reply

Related Discussions
Electron Charge to Mass Ratio Introductory Physics Homework 7
Basic physics in remote universe (proton/electron mass ratio) Cosmology 1
[SOLVED] Electromagnetism, calculate electron charge/mass ratio Introductory Physics Homework 4
Proton electron mass ratio General Physics 3
charge to mass ratio of an electron shot through a cathode-ray oscilloscope Introductory Physics Homework 2