Physics 2225: e/m Experiment
Purpose
 Determine the charge to mass ratio (e/m) of an electron.
 Design your own experimental procedures. Decide
•
•
•
•
•
what measurements to take,
the range of values you will measure,
how to obtain the uncertainties of measured and computed quantities,
how to analyze the data graphically, and
what error propagation procedures to perform.
 Gain experience in writing a report in the form of a scientific paper.
Physics 2225: e/m Experiment
Background
1892: Hendrik Lorentz introduces the force acting on a charged particle
moving through a magnetic field (the Lorentz force, or magnetic force):
Fm  q (v  B)
The cross product means:
1) Only the components of the vectors v and B that are perpendicular
to each other contribute to the force.
2) The force is perpendicular to both the vector v and the vector B.
Physics 2225: e/m Experiment
Fm
B
(points into the page)
q
v
The charge here is assumed to be positive.
For negative charges the force would be downwards.
Physics 2225: e/m Experiment
circular path
r
v
Fm
B
Fm
q
(points into the
page)
v
The force continually changes the direction of the velocity, but not it’s
magnitude.
The direction of the force changes also and it’s magnitude also remains
the same.
Physics 2225: e/m Experiment
For Circular Motion (Radius r)
Fm  macentripetal
v2
m
r
Derive expression for e/m(v,B,r)=…..
Physics 2225: e/m Experiment
Getting the electron up to speed …and knowing what that speed is.
Speed increases as electrons approach plate
filament
Ifilament
filament
Voltage
(6V)
Some electrons miss the plate and fly
through the hole in the plate.
 electron beam comes out of hole.
electrons
E
v
v
v
Many electrons return
because they hit the plate.
(Note: return path of electrons
is opposite to conventional
current.)
accelerating
voltage
(150V-300V)
The filament current heats up the filament  electrons get enough energy to exit the
filament.
Electrons get accelerated by the force in the electric field (created by the
accelerating voltage).
Physics 2225: e/m Experiment
Getting the electron up to speed …and knowing what that
speed is.
electron has potential energy = eVacc
and kinetic energy = 0
electron has potential energy = 0
and kinetic energy = ½ mv2
v
v
v
Vacc
Conservation of energy:
1 2
mv  eVacc
2
Physics 2225: e/m Experiment
Generating the Magnetic Field
A pair of Helmholtz coils
a
B
I
B
I
N o I
3
5 2
  a
 4
I
Magnetic field in the
center of the Helmholtz
coil pair.
I
N turns of wire
in each coil.
Physics 2225: e/m Experiment
Power Supplies
Before you switch on the power supply:
• Identify all components according and verify that all connections are as
shown in the lab manual.
• Identify on the high voltage power supply the dial that controls the
accelerating voltage: Turn that dial all the way counterclockwise into the
zero position.
• Identify on the low voltage power supply the two dials that control the
current through the Helmholtz coil: Turn both dials (voltage control and
current control) counterclockwise into the zero position.
• Identify on the low voltage power supply the heater voltage dial. This is a
step dial. Verify that it is set to 6 Volts.
Physics 2225: e/m Experiment
High Voltage Supplies
Older model
Newer model
Physics 2225: e/m Experiment
Low Voltage Power Supplies
Older model
Newer model
Heater voltage:
must be dialed
to 6V
Helmholtz coil
current and
voltage
regulation
Physics 2225: e/m Experiment
Switching on the Power Supplies
Now switch on both power supplies. Check and make sure that the
Helmholtz coil current is zero and the accelerating voltage is also zero.
Look at the tube: You should see the filament glow orange/yellow.
Carefully dial up the accelerating voltage on the high voltage power supply
to about 200V (never exceed 300V).
Regulating the current in the Helmholtz coil:
There are 3 knobs that control voltage and current through the Helmholtz
coil:
1) Current adjustment knob for Helmholtz coils on the e/m apparatus.
2) Current adjustment knob on low voltage power supply.
3) Voltage adjustment knob on low voltage power supply.
 Next page: how to use these three knobs.
Physics 2225: e/m Experiment
Regulating the Current in the Helmholtz Coil:
First turn DC VOLTAGE
ADJUST to zero (all the
way counterclockwise)
Then turn DC CURRENT
ADJUST all the way
clockwise (max. current).
Then turn current adjust knob
On the e/m apparatus
all the way clockwise
(max. current).
The current meter should STILL READ ZERO.
Physics 2225: e/m Experiment
Regulating the Current in the Helmholtz Coil:
Carefully turn DC VOLTAGE
ADJUST clockwise.
Watch the current meter!
When current meter reaches
2 Amps: Stop!!
Leave at this setting!!
From now on only regulate
the current with this knob on
the e/m apparatus
Totally clockwise = 2A
Totally counterclockwise = 0A
Physics 2225: e/m Experiment
Switching Off Power Supplies
Before you switch off the power supply:
• Identify on the high voltage power supply the dial that controls the
accelerating voltage: Turn that dial all the way counterclockwise into
the zero position.
• Identify on the low voltage power supply the two dials that control
the current through the Helmholtz coil: Turn both dials (voltage control
and current control) counterclockwise into the zero position.
Now switch off.
Physics 2225: e/m Experiment
Hints
When measuring the radius of the electron’s circular path, take
advantage of the mirror-scale behind the tube:
- Move your head to the left or right until the mirror image of the
beam overlaps the actual electron beam. Then read off the
scale.
- Repeat the procedure for the left side.
- Determine diameter from the two values (radius is half of that).
Read off right side
Read off left side
Physics 2225: e/m Experiment
Important for the Longevity of Apparatus
• Never exceed 300V on the accelerating voltage. Always monitor
this voltage on the attached volt meter when changing that
voltage.
• Never exceed 2A on the Helmholtz coil current. Always monitor
this current with the attached current meter when changing that
current.
• Always dial down accelerating voltage and Helmholtz current to
zero before switching off the power supplies.
Physics 2225: e/m Experiment
Lab Report
 Lab report must be written in the form of a scientific paper
(see example in the back of the lab manual).
 Each person must write their own lab report
(no group report for this experiment).
 You have 1 week to complete and submit the lab report.
 Late lab reports will get a deduction of 5% per day late
(e.g., if your report is 3 days late you will only get credit for
85% of the score that you would have gotten if you submitted
on time).