LAB #5:
e
of the Electron
m
Theory:
• What is Lorentz force? Definition and equation.
• Explain motion of a charged particle in a uniform magnetic field.
• Derive equation for calculating e/m as a function of accelerating potential and
magnetic field.
• How we calculate B for magnetic field produced by Helmholtz coils?
Experiment:
1. Determine the radius of the Helmholtz coil A, by measuring its diameter
Dc 10 times. Find average Dc and statistical uncertainty. From there
calculate A.
2. Keep constant current at (2±0.02)Amps
3. Adjust voltage to set electron beam diameter D to 11 different values
ranging from 5cm to 10 cm. Measure D on the glass rod and from there
find radius of the electron beam, R.
(V±2)Volts (D±0.25)cm
5
5.5
R(m)
R2(m2)
6
6.5
….
…..
9.5
10
Calculations:
1. Calculate B (using equation 5.11) with the uncertainty.
2. Calculate e/m (using equation 5.9) with the uncertainty using one data point from
the data table.
3. Plot ∆V vs. R2 using JagFit and find the slope.
4. From the slope of the line and equation 5.9 determine the e/m and its uncertainty.
5. Compare both e/m values with the accepted one of 1.75*1011C/kg