Quantifying forces on the Crookes Radiometer
Jose A. Lopez, Aaron Lopez, Lt. Rebecca Marvin, and Dr. Andres Larraza
Department of Physics, Naval Postgraduate School, Monterey, California 93943
Crookes Radiometer, or light mill, is a system of
rotating vanes set in a vacuum. One side of the vanes is
white and the opposite side is black. When light shines on
the surface of the vanes, they rotate toward the lighter
surface. The purpose of the experiment was to quantify
the two main forces behind this motion. Photo paper with
a black image printed on one half was used because it
rotates at a sufficiently measurable speed.
Different
geometries of radiometers were explored to identify
promising shapes. Radiometers in the size range of 8 x 16
mm with vertically oriented vanes were found to rotate the
fastest. The forces causing the rotation were modeled
using equations describing the effects. The experimental
approach taken to measure the accelerating forces was to
attach a vane to a sensitive scale and collect data from
inside the vacuum chamber. The actual measurement of
the forces failed. Alternate forms of measuring the forces
will have to be developed to explore the Crookes
Radiometer as a practical energy source.
Figure 1. A Crookes
Radiometer inside a vacuum
chamber. This commercial
model became our Control
system.
Different models of radiometers were made by gluing two
sheets of photo paper, with the wax and plastic still
attached on the paper. Glue was used to paste the glossy
sides together with the non-wax side facing outwards.
Exposed sides were either black or white.
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8x16
100
Figure 3a. Two sheets of
photo paper pasted
together; the bottom side is
identical to the top side.
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It was concluded that a radiometer with 8 x16 mm vanes
oriented vertically will rotate the fastest in a high vacuum
environment. Out-gassing is an issue that must be
addressed in order to be able to construct faster rotating
apparatus in the future. Solving the out-gassing problem
will allow for the construction of radiometers that will
maintain consistent velocities through repeated use. The
forces were not calculated directly through an experiment.
They were calculated indirectly by obtaining the
radiometer’s RPM and using equations to calculate the
forces. Once the micro strain gauges are obtained,
calculating the forces may be accomplished
experimentally.
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0
0
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40
60
80
100
120
140
Pressure(mTorr)
Figure 3b. This is the
typical setup. A vane
on top of a spindle,
which will go on top of
a needle inside the
vacuum chamber.
Literature cited
Test Run #3
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8x32
120
8x16
100
80
Results
It was determined that an 8 x16 mm radiometer vane
system was optimum for that particular experimental
setup. The experiment encountered problems when
vacuum chamber pressure would not drop below 100
mTorr. This was of great concern since it normally
reached pressures far below that in a matter of minutes.
Brush, S. G. and Everitt, C. W. F. 1969. “Maxwell, Osborne Reynolds,
and the Radiometer.” Historical Studies in the Physical Sciences
1:105-125. University of California Press, Berkeley, CA.
Woodruff, Arthur E. Oct. 1968. “The Radiometer and How it Does
Not Work.” The Physics Teacher 358-363. American Association
of Physics Teachers, College Park, MD.
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0
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80
100
120
140
Pressure(mTorr)
Figure 4. The scale will
measure the mass change
from the system at rest
and once it is altered by
the force of the gas acting
on it. Through the mass
difference we will be able
to calculate the force.
Figure 2. Radiometer made of Teflon resting on top of steel
needle.
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Conclusions
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Materials and setup
• 8 standard photo paper vanes glued together to make 4
thick vanes.
• 1glass spindle (extracted from a commercial radiometer)
• Iron needle on which the radiometers rest upon when
placed in the vacuum chamber.
•High Speed recording camera
•Vacuum Chamber
•Nylon washers
8x32
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A test setup has four systems in the vacuum chamber at
once, two of which are controls; their speeds are measured
by analyzing video frames. The next step was to calculate
which design and size would give the greater rotational
speed. The faster the rotation, the more accurate the
calculation of forces and less the significance of error.
(Refer to figure 5-6)
Materials for the project were fairly inexpensive:
The critical pressure decrease malfunction was resolved.
The force scale was found to be inadequate. The plastic
which the scale was made of, as well as the lubricants
used to keep it useable, outgassed in great amounts
causing the scale to become unstable at the low vacuum
pressures.
Test Run #2
RPM
A
Procedures
RPM
Abstract
Acknowledgments
Figures 5-6. The 8x16mm radiometer was the fastest
rotator as seen from the graphs. From run #2 and run
#3 it can be seen that there was a decrease in the
speed, perhaps due to the outgassing of the glue in the
vanes.
Micro strain gauges were ordered to increase precision.
Other experiments were conducted to identify faster
radiometers. A teflon spindle was constructed (Figure 2),
but it was too massive for the forces the vanes produced.
The original glass spindles proved to be the better choice;
by removing a metal ring from them and replacing it with
nylon washers, a re-usable and lighter spindle was created.
This spindle also allowed for better precision when the
vanes were mounted. In all experiments, the greatest
RPM was obtained using an 8x16 mm radiometer.
Figure 7.
Spreadsheet
with the data
of the fastest
rotating
radiometer
Professor Andres Larraza, Lt.
Rebecca Marvin, Steven Jacobs,
Jeff Catterlin, Professor Sam
Barone, Aaron Lopez, and Alison
Kerr of NPS.
Andy Newton, Joe Welch, Kelly
Locke, Cassandra Martin, and Ana
Hernandez of Hartnell College.
Funding for this project was
provided
by
a
Title
V
Strengthening Transfer Programs
grant.
For further
information
If you have any questions or comments
please
contact
jalopes@nps.edu,
alopez@nps.edu or larraza@nps.edu for
more information on this and related
projects.