Abstract
(10 pts.) Provide an abstract for this experiment here.
The purpose of this experiment was to measure the e/m ratio of an electron, the ratio of
an electron's charge to its mass, and compare it to the currently accepted value of e/m.
A
DC power supply was connected to an ammeter, Helmholtz Coils, and a voltmeter.
The
accelerating voltage exiting the Helmholtz Coils was set to 50.3 volts in the first part of
the experiment and 85.0 volts in the second part.
Varying currents were run through the
Helmholtz Coils in order to create a magnetic field that deflected the electron beam 0.5,
1.0, 1.25, 1.5, 1.75,
and 2.0 cm from its originally vertical path.
Using graphing
techniques, we were able to calculate an e/m of 2.01*10^11 +/ 0.04 C/kg for the low
voltage and 1.7*10^11 +/ 0.05 C/kg for the higher voltage.
The weighted average was
then calculated to be 1.86*10^11 +/ 0.03 C/kg.
These values are not consistent with the
currently accepted value of 1.758820*10^11 C/kg.
The discrepancy between the values
is most likely due to inaccuracy in measuring the diameter of the electron beam because
of the large margin of error in visually determining the distance that the beam was
deflected.
Data, Results, Graphs
1. Graph used to determine e/m.
Numerical Results
1. Theoretical value: e/m = ______ ± _____ C/kg
As indicated in the lab manual, the currently accepted value for e/m is 1.758820*10^11
C/kg.
2. Value from graph using lower voltage (~50V): e/m = ______ ± _____ C/kg
The e/m value can be obtained using equation (10) and the slope of the graph of B^2 vs.
1/(d^2 +a^2).
The slope of the graph equals 2*10^9 T^2*m.
Equation (10) tells us that
e/m = 8V/slope.
Replacing V with the voltage 50.3 +/ 0.5 volts and putting in the slope,
we get 2.01*10^11 C/kg.
The uncertainty is calculated from the uncertainty in V and the uncertainty of the slope.
The equations determining the uncertainty can be seen in the attached sample calculations
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 Fall '08
 WALKER
 Charge, Current, Mass, Power, Magnetic Field, Helmholtz coils, e/m

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