Download The Teltron Tube File

Le Fevre High School
SACE Stage 2 Physics
Motion of Charged Particles in Electric Fields – Teltron Tube
AIM: :
To investigate the effect of electric and magnetic fields on electrons
Apparatus High voltage power supply, teltron electron deflection tube, 6 V DC continuously
variable power supply, ammeter, bar magnet.
Description and Explanation of Experiment:
The electron tube consists of 3 essential parts.
1. An electron gun consisting of a heated filament to produce electrons and anode (positive electrode)
maintained at a high potential with respect to the filament to accelerate electrons across to the
deflecting plates.
Top plate
connection
parallel plates
Filament
Filament lead
connections
Anode
connection
bottom plate
connection
2. A pair of parallel plates which produce a deflecting electric field.
Note that the plates are 5.2 cm apart and the electric field between them is given by
E
V
V
=
volt/metre
d 0.052
(Equation 1)
This equation assumes a uniform field.
3. A pair of coils outside the tube separated by a distance equal to their radius. Such coils are called
Helmholtz coils and produce a magnetic field that is nearly uniform in the region between them.
r
r
Coil Sockets
For these particular coils the magnetic field strength is given by
-3
B = (4.23 x 10 )(IB ) Tesla
(Equation 2)
Where IB is the Current flowing in the coils.
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NB The coils are connected to a low voltage power supply. (Tesla is the unit of magnetic field strength.)
Theory
A: Magnetic field only
Consider the motion of a beam of electrons in a magnetic field only. Moving electrons experience a force
given by:
F = q v B sin  and is at right angles to v and B in a direction given by the right hand rule.
F = e v B as sin = 1 in this case and q = e for electrons. Note electrons are negative in charge
F
v
(mutually perpendicular vectors)
B (at 90o and into the plane of the page)
This force causes v to change in direction without changing in magnitude and the force always
maintains a direction at right angles to v so the electrons are forced into a circular path with the
centripetal force being provided by the magnetic force.
ie.
F cent = F magnetic.
m v2
 e v BR
R
the
(Equation 3) where BR is the magnetic field required to force
electrons into a path of radius R
(This idea is used in the Velocity Selector - no longer required in the 1999 syllabus)
B: Electric field only
Consider the motion of electrons in an electric field only. The parallel plates produce a field (ie.
constant in magnitude and direction) so each electron experience a force which is constant in size and
direction given by:
F = eE
eV
d
(equation 4)
+
Force
v
This force causes the electrons to move in a parabolic path as it is constant in size and direction.
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Procedure:
1- Connect the circuit shown in the circuit diagram. Be careful not to knock the anode connection at the
side, as any damage is expensive to repair. Have the circuit checked before switching on.
EHT POWER SUPPLY
0
6
12
EHT
0
+
-
Notice that the parallel plates are both connected to the positive terminal of the E H. T. Under these
conditions there is no PD between the plates and hence no field. This is always done when no electric
field is required between the plates rather than just disconnecting them and leaving them “floating”,
because it prevents the build up of charge by collection of stray electrons and the consequent
generation of odd electric fields.
Later you will set up an electric field between the plates by removing one of the leads from the positive
terminal and moving it to the negative terminal Whenever any such changes are made switch of the
EHT and wait until the voltmeter reads zero.
2. When the circuit has been checked, switch on the EHT supply. Adjust the anode voltage to 3 000 Volt
(using the coarse and fine controls) and observe the path of the undeflected beam. Using the bar magnet
provided try observing the effect it has on the beam. Don’t knock the glass tube with the magnet)
3 Magnetic Deflection only.
Now Connect the Helmholtz coils to the 6 V DC Supply as shown. Have your circuit checked
Use a continuously variable 6 V DC power supply.
A
A
0V
Z
Z
Z
6 V DC
A
6V
I
A
A
Z
Step 1: Switch on the current (IB) to obtain a magnetic field between the coils.
Adjust the current in the coils to 0.1 A.
Q1. The magnetic field between the coils deflects the beam into a curved path. What is the shape of
this path?
Q2. The glowing filament produces a yellow ray of light through the slit of the anode. What is the
shape of the path of this light?
Q3. What can be concluded about the effects of magnetic fields on light waves?
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Step 2: Increase IB from 0.1 A to 0.3 A with Voltage = 3000 V.
Q4. .Describe and explain what happens to the radius of curvature of the paths when I B is increased
from 0.1A to 0.3 A with V = 3000 V. Note that the magnetic field is now stronger but the electrons
still have the same speed.
Step 3: Increase the anode voltage to 3500 V with IB = 0.1 A. Note that this time the magnetic field is the
same but the electrons are going faster.
Q5. Describe and explain what happens. Use the speed dependence of magnetic force (F = evB) in
your answer.
ie use
m v2
mv
 e v B R so R 
R
e BR
4. Electrostatic deflection only.
Now switch off the magnetic field and swap the lead connected to the bottom deflecting plate from the
positive terminal to the negative terminal of the E H T power supply. Switch off the EHT while you do
this. This sets up a potential difference between the plates and hence an electric field.
Observe the electric field deflects the beam into a curved path.
Q6. What is the direction of the deflection under the electrostatic deflection?
Q7. What is the shape of the path ?
Q8. What is the motion of electrons in an electric field similar to ?
What is common to the two situations ?
Increase the E H T voltage from 3000 V to 3500 V.
Q9. Describe and explain what happens to the trace on the screen when the EHT is raised from
3000 V to 3500 V
Q10. What is the shape of the path of the ray of light from the filament?
Hence what can you conclude about the effect of E fields on light waves
Q11. Is there any other evidence in this experiment for conclusions in Q3 and Q10? Describe and
explain the evidence.
Requirements
Full write up including answers to all questions. This is a SUMMATIVE piece.