Electron Charge-to-Mass Ratio
Record Data: Alternate Method
11
012-14265A
Record Data: Alternate Method
•
Alternatively, the PASCO 850 Universal Interface (UI-5000) can be used with the High Current Sensor (PS-2193) and the
PASCO Capstone Software (UI-5400) to measure and record the voltage and current.
Table 1.1: Data
Analysis of e/m Measurements
The magnetic field,
B
, generated in a pair of Helmholtz coils is proportional to the current,
I
H
, passing through a single coil.
The constant of proportionality, k, can be determined from the coil radius,
R
, and the number of turns,
N
, on the coil.
With this expression for
B
, the initial formula for e/m,
becomes:
R = 158 mm and N = 130 turns per coil
U = Acceleration Voltage
R = Radius of the Helmholtz coils
N = Number of turns on each coil
0
= Permeability constant (4
x 10
-7
)
I
H
= Current through the Helmholtz coils
r = Radius of the electron beam (closed circle)
•
Accepted value of the charge-to-mass ratio, e/m, is 1.76 x 10
11
C/kg
Deflections of Electrons in an Electric Field
You can use the deflection plates to demonstrate how the electron beam is deflected in an electric field.
Trial
U (V)
I
H
(A)
r (mm)
e/m (C/kg)
Percent Error
1
2
3
4
5
B
4
5
---
3
2
---
0
NI
H
R
-----------------------------
=
e
m
-----
2U
B
2
r
2
------------
=
e
m
-----
2U
5
4
---
3
R
2
N
0
I
H
r
2
---------------------------
=
IMPORTANT:
Do not leave the electron beam on for long periods of time in this mode. The beam will
eventually wear through the glass bulb of the e/m Tube.