Download Giessler/Crookes Tube and Cathode Ray

Electron Lesson
Development of Ideas in Physical Science
Fall 2005
Professor Etkina
Joseph Santonacita
Jeffrey Goett
Giessler/Crookes Tube Activity
Directions: Record all observations for each experiment and reason through a possible explanation for
the phenomenon observed in the tubes.
Observational Experiment #1
This observational experiment was made in a bent Geissler tube. The lower case “g” is where the most intense
green glow occurs. The intense area of green glow is located directly across the cathode of the Geissler tubes.
Where does the glow appear to originate from? Examine the picture(s) below.
 Green intense green glow
Observational Experiment #2
This observational experiment is made with number Geissler tubes all with different cathodes. The cathode
materials are as follows, copper, lead tin copper, iron, zinc and cadium. Every time the green reflection is
observed in the tube. What is a possible explanation for what is observed?
Cathode
Observational Experiment #3
This observational experiment is made in a Geissler tube with a Maltese cross in the middle of the tube. The
picture below shows what is made after the cathode rays hit the cross and the back of the Geissler tube. What
are the possible explanations for what was observed?
If possible conduct the simulation online here
http://micro.magnet.fsu.edu/electromag/java/crookestube/
Or examine the pictures below.
Observational Experiment #4
Predict what will happen as a magnetic field approaches an electromagnetic wave. Examine the interactions of
electromagnetic waves and magnetic fields. Observe what happens when a magnet is placed near the path of a
laser pointer.
Testing Experiment #5
Utilizing the information from Experiments #1, #2 and #3 make a prediction about the nature of the cathode
rays observed. Examine and observe what happens to the cathode rays when a bar magnet is placed near the
cathode rays. Does this experiment disprove or prove anything about the prediction made in experiment #4?
Revise or restate your prediction if necessary.
S
Bar Magnet
N
S
Examine the magnetic field produced by a bar magnet. What does this information tell us about the magnetic
field entering the cathode’s path?
S
N
By examining the direction of the magnetic field and deflection, what is a possible explanation
for the direction in which the cathode rays bend?
Testing Experiment #6
Predict what will happen to the cathode rays when they enter an electric field. For this particular device, when
the DC voltage is set to 0 to -50 V the bottom plates are positively charged and the upper plates are negatively
charged. The plates switch their charge when the voltage is placed from 0V to +50V. Examine the cathode
rays interactions with an electric field. Adjust the potential difference between the parallel plates; observe what
happens as the rays pass through the plates. How do these observations compare with your predictions?
Parallel Plates
Analysis
How can you explain what cathode rays are? Are these cathode rays light? Justify why/why not?
Year / Name
1855 / Johann Heinrich
Wilhelm Geissler
What did the person do?
Provided innovations a mercury pumping system for
the vacuum pump to create a better vacuum. Melted
electrodes of glass tubes then filled the tubes with
gas at very low pressure.
Lowing the pressure the darker the tube and the
longer the glow. A magnetic field influenced the
glow.
Element of Scientific Exploration
1869 / Johann Wilhelm
Hittorf
Glow is cut off when any solid body interferes with
the path. The L shaped tube suggested the cathode
rays propagate in a straight line. Bent by a magnet.
Observation
1870s / Eugene Goldstein
Coined the phrase “cathode rays” Established the
rays were emitted perpendicularly from the cathode,
cathode rays were emitted in spite of the material of
the cathode.
First to propose that since negative particles would
be deflected the same way the cathode rays were, he
suggested the cathode rays were negatively charged.
Observation
Showed rays were being repelled from the cathode,
(rather than attracted to the anode) did not seem to be
affected by an electric field, were affected by by a
magnetic field, demonstrated a shadow with a
maltese cross, predicted these particles were
negatively charged.
Noticed that the illumination of the walls and the
light emitted by them should display the Doppler
effect. It did not.
Observation/Explanation/Prediction
1884 / Arthur Schuster
The visible cathode rays came from the interaction
with the gas inside. Calculated the maximum and
minimum limits for q/m
Observation/Explanation/Prediction/
Testing experiment
Hertz
Did not observe the cathode rays passing through an
electric field to have any influence on their motion,
primarily because the field was weak.
Prediction/Testing Experiment
Phillip Lenard
Observation
1895-1896 / Wilhelm
Roentgen
Projected cathode rays out of a tube through a piece
of metal (1 cm, gold) which proved the rays were
something much smaller than the size of an atom
In his discovery of X-Rays found out the air is
ionized as these mysterious rays pass through it.
Jean Perrin
Found cathode rays charged an electroscope
Observation /
Explanation/Prediction
1858 / Julius Plucker
1871 / Cromwell
Fleetwood Varley
1879 / William Crookes
1880 / Peter Gutheir Tait
Observation
Observation/explanation/Prediction
Prediction
Observation /
Explanation/Prediction /
Testing
Year / Name
1855 / Johann Heinrich
Wilhelm Geissler
1858 / Julius Plucker
1869 / Johann Wilhelm
Hittorf
1870s / Eugene Goldstein
1871 / Cromwell Fleetwood
Varley
1879 / William Crookes
1880 / Peter Gutheir Tait
1884 / Arthur Schuster
Hertz
Phillip Lenard
1895-1896 / Wilhelm
Roentgen
Jean Perrin
What did the person do?
Element of Scientific
Exploration
Roentgen & The X-Ray
Above (left) A picture of Roentgen. Above (right
and bottom) The Physical Institute and laboratory room in which Roentgen first noted and investigated Xrays.
Top Left: The famous radiograph made by Roentgen on Dec. 22, 1895, and sent to physicist Franz Exner in Vienna. This is
traditionally known as "the first X-ray picture" and "the radiograph of Mrs. Roentgen's hand.
Top Right: Radiograph of the hand of Albert von Kolliker, made at the conclusion of Roentgen's lecture and demonstration at the
Wurzburg Physical-Medical Society on Jan. 23, 1896.
Bottom Left: One of a series of experimental radiographs made by Roentgen to determine the ability of the rays to penetrate various
solids.
Pictures: Otto Walkhoff (1860-1934), a dentist in Braunschweig, Germany, produced the first dental radiographs in January 1896,
less than two weeks after Roentgen's announcement of his discovery. Walkhoff placed a small glass photographic plate coated with
rubber dam in his own mouth, then seated himself for a twenty-five minute exposure to the rays. Although the resulting shadows were
less than adequate for any diagnostic purposes, the possible applications of the technique were clear. The other photo is radiograph is
attribute to Walkhoff, possibly of a patient.
Exploring Cathode Ray’s Particle Nature by Measuring its Mass to Charge Ratio (Based upon
Thomson’s Experiments)
Through a series of experiments, we have determined that Cathode Rays are composed of particles, not waves.
The next step is to figure out whether the particles are molecules, atoms, or subatomic particles. To help, let’s
figure out a way to measure the ratio of the particle’s mass to its charge.
Observations: We pass a cathode ray through a region between two parallel plates. In this region, there are
two fields, an electric field pointing downward and a magnetic field pointing into the page. For a certain
magnitude of the electric and magnetic fields, the ray passes straight through the region as illustrated in Figure
A. If we increase the electric field relative to the magnetic field, the ray bends upward, as illustrated in Figure
B. Likewise, if we increase the magnetic field relative to the electric field, then the ray bends downward as
illustrated in Figure C.
+
q
+

+
+
+
+
q
v
m

+
-
-
-


+
+
+
+
q
-
-
+
-
-


+
+
+
E
B
E
-
+
v
m
B
E
-
+
v
m
B
-
+
-
-
-

Figure C
-
-
Figure
B
Figure
A
a) Model: Devise an explanation for your observations. Include diagrams and equations where appropriate.
b) Application: You observe the trajectory given in figure A when |E|= 465 N/C and |B|=1.55*10-4 T. How
fast are the particles that makeup the ray moving?
Next, we turn off the electric field so that only a magnetic field, |B|=1.55*10-4 T, acts on the particle. We
observe the trajectory given in the figure below.
5 cm
q
v
m
1.2 cm

c) Model: Devise an explanation for this motion. Give appropriate free body diagrams, equations,
etc. Is the shape of the trajectory parabolic or circular?
d) Application: Apply this model to calculate the ratio of the mass of the particle to its charge. Are the
cathode rays composed of negatively charged Copper atoms that have a mass to charge ratio 1.659*10-7 kg/C,
negatively charged proton-like particles with a mass to charge ratio of 1.044*10-8 kg/C, ionized Helium with a
mass to charge ratio of 4.15*10-8 kg/C, or electrons with a ratio mass to charge ratio of 5.692*10-12 kg/C?
Determining the fundamental electric charge (Based on Millikan’s procedure)
The simulation is either on the laptop’s desktop or at
http://www.physics.rutgers.edu/~jgoett/millikan/millikan.html
Observation: Open up the simulation and run the simulation multiple times, adjusting the magnitude of the
electric field and observing the resulting motion of the oil droplets.
Model: Devise a model to explain the motion of the droplets. Include sketches, diagrams, and equations
Application: Devise a procedure to measure the charge on a droplet. Record relevant data and calculate its
charge.
If we assume each droplet only contains a few extra electrons, we can calculate the charge of an electron.
Analyze the data and determine this charge.
In real life, one does not know the mass of the droplets, cannot begin with the droplets at rest, and cannot
conduct the experiment in a vacuum (why?).
Model: Using fluid dynamics, scientists have found the force of air on a sphere of radius a to have a magnitude
F  6ua v
It acts in the direction opposite the sphere’s motion. F is the force of the air on the droplet, u is the viscosity of
air, 1.73*10-5 N-s/m2, and v is the velocity of the droplet.
Devise a model of a droplet’s motion as it falls.

Application: Devise a method to measure the size of the droplet. Derive any equation needed to calculate it
and specify what data needs to be recorded. (The density of the oil is effectively 710 kg/m3).
Model: Now, let’s determine the charge on a particle. Use Stokes’ Law to model the motion of a particle in an
electric field. Devise a procedure for measuring the charge on a droplet. Derive any equation needed to
calculate it and specify the data that needs to be recorded.
Sample data
In an experiment, you measure the downwards v1, the free-fall velocity, and the upwards velocity of the droplet
with an electric field on it, v2, for different droplets. From this data determine the charge of an electron.
E (V/m)
Time (sec) to fall 2
mm under only
gravity
Time (sec) to fall 2
mm under gravity
and electric force
2000
139.9
213.2
2100
131.6
749.1
1900
93.9
491.4
q (C)
Millikan Oil Drop Apparatus