SP212 Lab: Six Magnetic Force
Version: February 24, 2015
PHYSICS II
LAB 6
SP212
MagneticForcePre‐LabAssignment
HomeworkProblem
Electronsofchargeeandmassmareacceleratedthroughapotential
differenceVaccel,andfiredfromanelectrongun,asshowninthedetail
includedinthefigure.Thegunfirestheelectronsintoauniform
magneticfieldB.Theelectronsleavetheelectrongunwithkinetic
energyeVaccel,andtheirvelocityvectorastheyleavethegunis
perpendiculartoBsothattheelectronstravelinacircleofradiusr,as
showninthefigure.Theacceleratingvoltage,magneticfieldstrength,
andorbitalradiuscanallbemeasured.Inclass,welearnedthat
conservationofmechanicalenergygives½mv2=eVaccel,andthat
Newton’ssecondlawforuniformcircularmotiongivesmv=eBr.
1. Usethesetwoequationstocalculatetheelectron’scharge‐to‐mass
ratio,e/m,intermsofthethreemeasuredquantitiesVaccel,B,andr,
andshowallyourwork.
Page 1 of 2 SP212 Lab: Six Magnetic Force
Version: February 24, 2015
2. IftheB‐fieldeqnis B  8.05 x104
T
* i ,andthe Vaccel  300V fillinthe
A
missingdatainthefollowingtable.
KineticEnergyofelectroninjoules
=
Electron’sSpeedinmeters/sec
=
Helmholtz HhCoil
Orbit
B‐field
Diameter (Hh)Coil
(T)
current
(m)
(A)
Magnitude
ofForce
dueto
magnetism
(N)
Magnitude Ratioof
e/m
of
centripetal (C/kg)
Force(N)
0.09
1.62
0.08
1.80
0.07
2.04
0.06
2.39
Averagee/m =
3. Inyourownwords,howdoesyourcalculatedvalueofe/mcompare
tothetruevalue?Whatarepossiblereasonsfordisagreement?
4. Plotyourforceduetomagnetismvsthecentripetalforce.What
shouldanidealplotlooklike?Howdoesyourscompare?Turnin
plot.
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