SP211 Lab 6
e/me ratio of the electron
We will measure e/me by accelerating electrons across a potential to give them an initial speed,
then measuring properties of their uniform circular motion in a B-field.
Put your work/notes for this lab in your course notebook in some organized fashion.
1. The magnetic field produced by a Helmholtz coil (along its axis of symmetry) with N turns,
current i, radius R, a distance z from its center, is
B=
µ0 N iR2
3
.
2 (R2 + z 2 ) 2
Use this expression to calculate the B-field exactly half-way between the two Helmholtz coils, when
z = R/2 (this is the setup we will use).
2. The electrons are accelerated across a potential difference Vacc within the apparatus. Using the
conservation of mechanical energy, determine the speed of the electrons for both Vacc = 200 V and
Vacc = 300 V.
3. An electron moving perpendicular to a uniform B-field feels a force FB = evB. The direction of
the magnetic force is always ⊥ to the electron’s velocity, so this sets up uniform circular motion.
The magnetic force is thus a centripetal force in this situation, so we have
evB = me
v2
r
where r is the radius of the electron’s trajectory. Using this, derive an expression for the ratio e/me
in terms of v, B, and r.
4. Turn on the apparatus; DO NOT INTRODUCE A VOLTAGE OR CURRENT FOR AT LEAST
5 MINUTES. After 5 minutes, set the apparatus to Vacc = 200 V. Record the currents necessary
to bend the beam into diameters of 5 cm and 10 cm. For each measurement:
i. determine the magnitude of the B-field produced by the coils (note that the apparatus has
N = 132 turns), and
ii. extract a value for e/me using your result from step 3.
5. Repeat step 4 for Vacc = 300 V. Do you get a consistent result?