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Brown University
Physics Department
PHYS 0060
Experiment Outlines
EXPERIMENT OUTLINES
(SECTION 3)
Experiment Suggestions for Physics 0060
The following pages contain “area” experiments lists, a format that is an attempt to keep
pace with changes in course emphasis and texts, as well as the changes in apparatus, that
take place quite often. In suggested experiments for Physics 0060 there are six areas, but
of course only four units will be done by any group. The area(s) appropriate for each unit
will be identified in orientation sessions.
The lists of experiments give only the briefest indication of the experiments. For details,
the reference binders must be examined.
Area 6– 1 – Heat and Thermodynamics
The Equivalence of Heat and Work (0070): A small copper drum is heated by friction,
the mechanical work (lifting a weight) that friction turns to heat establishes the (joule)
work associated with the calorie (heat). The same object, heated by a current at known
voltage, similarly establishes the electrical work in a fixed time.
Heat-Conduction through a Rod (0088): When one end of a rod is immersed in ice
water, the temperature change at the other end, recorded over time, would lead to the
thermal conductivity of the material. Controlled thermal isolation is critical.
R-Value of Insulating Sheets (0090): The experiment was reported in AJP, in an article
that nicely shows the equivalence of heat and electrical conduction has not been done
here. With dry-ice on one side of a slab of heat insulator, the temperature change on the
other side measures the insulation properties. The article defines “R-value”, a measure
for heat insulators. Again, controlling heat flow is critical.
Area 6– 2 – Charge, Force and Static Fields
Two-Dimensional Field Maps (0010): Two-dimensional representations of static fields
are generated by placing conductors of choice on high resistance paper. Conductors of
various shapes can be made from conducting tape or from metal cuts outs. When the
conductors are set to potentials by low-voltage supplies, the potentials everywhere on the
paper can be measured, showing the patterns of the equipotentials, from which the
associated field lines are obtained. (Silver ink, formerly used to draw electrodes, has
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Experiment Outlines
been replaced by the prefabricated metal conductors and the conducting tape.) See also
Electrostatic Effects, 0060.
Millikan Oil Drop Experiment (0030): The “oil drops” of this classical experiment
have become tiny (600 micron) latex droplets that are sprayed into a small chamber. A
vertical electric field moves these particles, when they ionize, upward or downward,
while gravity also acts on them. Information collected by observing several droplets
shows that ionization does not produce a continuum of charge, but instead single and
multiple values of a fundamental charge unit (the electron charge).
Electrostatic Force Balance (0040): (Caution: High Voltage) When a voltage is applied
across two parallel plates, equal and opposite charges are induced which cause the plates
to attract each other. By fixing one plate and measuring the force on the other (e.g.
suspended by a spring), the electrostatic force or related quantities follows.
Electrostatic Effects (0060): Various electrostatic phenomena are studied, using, for
example an electrometer and Faraday pail. Se also Two-Dimensional Field Maps, 0010
for overlapping field mapping portion of this write up – use copper tape electrodes.
Perrin Tube and Electrostatics (0064)): (Caution: High Voltage) This type of tube
generates an electron beam that can be deflected magnetically into a Faraday cage. A
very high impedance measurement (an electrometer) measures the build-up of negative
charge on the cage. Similar measurements with spherical conductors and metal plates
using the electrometer show the sometimes unfamiliar aspects of electrostatics.
Coulomb Balance (0070): (Caution: High Voltage) This experiment allows one to
measure the force between two charged objects as a function of their charge and
separation.
Note: Delicate (difficult) experiment
Area 6–3 – Zero and Low Frequency Currents
This area includes a large range of phenomena related to the production and use of
current flow. The underlying theory also covers a wide range, and you should scan the
entire list of suggestions below (the list order has nothing to do with the topic schedule of
the lectures). Select according to your own background and interest as well as the course
schedule.
Magnetic Force on Conductors (0500 and 0055): The force between two wires that
results when current is present in each is measurable when the current is large and/or
multiple parallel wire configurations are used. Two sets of apparatus are available. The
balance is set up by fixing one group of parallel wires, and measuring the force on a
second group e.g. suspended by a spring (0050 or torsion wire (0055). The
“magnetostatic” force or related quantities follow.
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RC Studies - Relaxation Oscillator (0160): A small sealed tube of neon gas will break
down under a voltage that is allowed to build across it. When the buildup is controlled by
an RC circuit, the buildup time is predictable, and the subsequent discharge cuts itself off,
restarting the buildup. As a result, a free-running oscillator can be made, its cycle time
controlled by the RC circuit.
RCL Studies - Impedance Bridge (0170): The impedance bridge (see manual appendix)
is extremely useful in measuring a circuit, or equivalent circuit, of any network of passive
(R, C, or L) components and analyzing it into an equivalent resistive and reactive
element. The frequency at which the element is measured is supplied externally, and a
very wide range can be used. One possible approach, after a suitable exercise with
known inpedances, is measurement on less obvious elements, e.g. microphones and
speakers, over a frequency range.
Circuit studies (0180): The series of writeups grouped in this file formed a connected
sequence in basic DC and AC circuits. They were designed to be coherent in themselves,
independent of whether supporting lectures have not yet reached (or have skipped) the
topics involved. Thus, some of the writeups may be more detailed than you need, but
they are a rich source of circuit-study ideas, Experiments described range from simple
studies of resistive (DC) circuits to sophisticated measurements of AC resonance
phenomena in circuits containing R, L and C.
Charge/Mass Apparatus [Teltron] (0190): A special electron tube that makes an
electron beam visible over an area of several centimeters is the principal feature of this
apparatus. The beam can be deflected magnetically in the field of surrounding Helmholtz
Coils, and electrically by means of plates within the tube. With this apparatus, the ratio
of the electron's charge to its mass can be determined by more than one method, or
conversely using e/m as known, the electric and magnetic field configurations can be
studied.
DC Power Supply (0200): One of the most frequently used laboratory devices, and a
basic component of countless electronic devices, is the power supply section that
converts commercial 60-cycle AC to direct current. Tracing in detail the construction of
such a supply spans a large range of basic electrical circuit theory. The AC power passes
through a transformer to a rectifier section, and the pulsating, but now unidirectional
output passes through resistive and reactive (capacitive and/or inductive) elements to
reduce the “ripple” to a satisfactory level. Correspondingly, by construction and
measurement of a DC supply from components, the corresponding theory is explored.
Make-your-own Coils, Probes Motors (210) (May not be available): This suggestion is
keyed to the do-it-yourself type. As an example of what is envisaged, a few years ago a
large class was presented with ten feet of coated wire, two small ceramic magnets, and a
paper clip. From these materials a contest in building the fastest battery-operated motor
developed. For the less competitive, the “reverse motor” or generator is equally
challenging. Its cousin, a “search coil” that measures the strength of a magnetic field
when rotated in it, would be especially useful in this lab.
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Area 6– 4 – High Frequency Fields and Stuff
The subject matter of this area builds to a large extent on that of the previous area. The
electric and magnetic effects are intrinsically combined in equations of a propagating
electromagnetic field. The difference in apparatus between a microwave receiver and a
radio receiver, for example, reflects the difference in wavelength of the radiation. There
is a large variety of experimental and practical electronic techniques for propagating,
detecting and amplifying electromagnetic waves over their range.
Selections in this area range from the straightforward to the difficult, and your grasp of
the course material in electricity and magnetism so far should influence your selection.
Be aware of the intrinsic limits of some “apparatus”. In particular, a digital circuitry
study is more of an exercise in the use of logic chips; the large amount of physics that
goes into an analog or logic circuit is permanently encapsulated and not accessible for
experimenting.
Area 6–5 – Light Wave Behavior
Velocity of Light (Two Methods) (0260):
1) Light is reflected from a rapidly rotating mirror to a distant mirror. When the light
returns, the rotating mirror has turned through a small angle, and the displacement
of the light beam back at the source area is one of the critical determinations. The
other is the exact angular speed of the motor. From the basic handout description,
several variations are possible, such as a laser source, extended light paths, or
generating pulses for a scope or frequency counter in order to determine the angular
velocity of the rotating mirror.
2) A modulated laser is used; the phase differences in light beams originating in the
same source and traveling different distances is measured on the oscilloscope, and
the velocity of light is calculated from this. Note: The rotating mirror method is
rather difficult. We suggest you do Method 2 first.
For the most part, the common behavior of the systems described below is the production
of alternate high and low intensity regions through the interference and diffraction of
light beams from different parts of an original source. The spacing of these fringes
depends both on the wavelength of the source radiation, and the geometry of the system
that selects and recombines radiation from the slits or analagous boundaries.
Filtered white light offers the possibility of comparing the interference and diffraction, in
the same system, of light with different dominant wavelengths. A mercury vapor lamp
provides a strong source of mixed yellow, green and blue light (5791, 5461 and 4358
Angstroms respectively), any one of which can be selected by the appropriate filter. A
hydrogen discharge tube is a strong source of red light (6600 Angstroms), and so is the
helium-neon laser (6328 Angstroms).
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Caution: The mercury lamp is intense and produces a harmful level of ultraviolet
radiation. Never look directly into it, even from a distance.
Caution: Any laser beam is potentially harmful and must never be allowed to enter the
eye directly. If the laser is to be used, all shiny surfaces along the path area must be
covered up before it is turned on (towel holders, water faucets, etc.), so one does not
inadvertently look into a reflected laser beam.
Caution: The hydrogen discharge tube is powered by 5000 volts. Keep well away from
all metal parts of the supply and connecting wires.
Michelson or Fabry-Perot Interferometry (0330): An interferometer is a table with
precisely machined movable parts, made to split a light beam. One or both of the
resulting beams undergoes a process capable of changing the “optical path” and the
beams are then recombined. Possible changes are seen in the interference patterns that
would occur. The extreme sensitivity of an interferometer provides for precise testing,
for example, of optical surfaces as a practical case, or the supposed change in the speed
of light as it moves through an “ether” – the historical Michelson-Morley experiment
that established the constancy of that speed. Several experiments are suggested in the
reference 0330.
Diffraction by Slits – Laser Source (0350): The diffraction of light passing near the
edge of a slit depends on its wavelength, and so is much more pronounced when a laser is
used, but these experiments could be done with filtered white light. Various single and
multiple slits, an optical table (Pasco), a photometer, and a linear translator with optic
probe attached, are the main components for a precise determination of the diffraction
effects. A diffraction grating is equivalent to many thousands of slits. Interference
between light diffracted from these slits, where the light is from a single (therefore
coherent) source is constructive or destructive depending on the angle of observation. As
a result, the location of maxima in the interference pattern can be related to the
wavelength of the incident light. The clearest determination is done using laser light, but
white light and filters, though more difficult of measurement, offer a variety of
wavelengths.
Optical Polarization (0370): The orientation of the electric and magnetic vectors
associated with an electromagnetic wave are the subject of polarization studies.
Crystalline substances, sensitive to the polarization of incident radiation, are the basis for
producing and analyzing polarized light, as described in the Pasco optics manual extract.
Two- and Multiple-Slit Interference – Mercury Source (0340): A pattern of slits and
multiple slits is used in conjunction with a mercury lamp. The latter has three strong
separate lines, each of which can be selected by proper choice of a filter. Measurements
are very precise with the associated apparatus, a spectrometer table. A similar apparatus
is used in one option of the next unit, to determine wavelengths, and eventually atomic
energy levels.
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Microwave Interference and Diffraction (0380): All the interference and diffraction
effects of light waves are duplicated by electromagnetic waves in the microwave region
with wavelengths from one to fifty cm. Here, of course, the slits and similar parameters
are of the same order of magnitude as the microwaves. A small microwave generator of
approximately 10–cm. waves and a detector with micrommeter readout are the basis of
the laboratory apparatus, with, of course, large screens and slits. Look especially at the
Cenco description microwave studies “Microwave Optics Demonstration Apparatus” in
the 0380 booklet. The handout 0380 is a brief summary of feasible experiments.
Area 6–6 – Quantized Energy States
This area is concerned with the characteristics of the elementary units of matter and
energy, and their interplay. In the suggested experiments the real objective is to observe
ways in which these entities behave; to focus the observations, a measurement of some
specific property is made in each case.
The experiments are duplications of classic determinations of fundamental constants.
They utilize commercial equipment designed for the specific purpose, allow little scope
for variations in technique, and require little or no debugging. However, since they are
relatively inexpensive facsimiles of the original research apparatus, and because your
time is constrained, the results could be dismaying unless you use all the experimental
sense you should have acquired by now. Make dry runs, do a preliminary analysis, notice
which experimental numbers (voltages, currents, etc.) are particularly sensitive. Then,
whenever possible, modify the technique, and the meters (and so on) so that you have the
best readings you can make. Make your final determinations using techniques that give
you the firmest estimates of possible error you can deduce. Finally, in your report, use all
this error information in evaluating your results.
Photoelectric Law (0390): A photoelectric cell is connected to a voltage source and
illuminated by monochromatic light selected from the mercury lamp output with a filter.
For each of the available wavelengths, the current of photoelectrons is measured with an
ultrasensitive ammeter as a function of applied voltage, to determine the maximum
photoelectron energy which is thus studied as a function of the illuminating wavelength.
In addition to verifying the theoretical relationship, the graph gives the value of Planck's
constant (strictly, h/e) and an estimate of the photoelectric work function, from its slope
and intercept respectively.
Mercury Excitation [Frank-Hertz] (0410): In this experiment the first excited state in
mercury measured by finding the energy at which electrons, traversing mercury vapor in
a special tube, are scattered inelastically. The energy is determined by the grid voltage,
and a value at which a large number of electrons, released from a cathode, collide
inelastically with the atoms is seen as a drop in the electron current to the plate of the
tube. In fact, several dips are generally seen, at equal voltage spacings, corresponding to
electrons that lose all their energy by collision somewhere within the tube, regain energy
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as they are again accelerated, and lose it again in a second collision and so on. These
additional excitations, besides pinpointing the level better, serve to define the cathode
work function. Variations are discussed in the 0410 guide. The equipment for Helium
excitation (see below) is somewhat easier to use.
Hydrogen Spectroscopy [Discharge Tube] (0420): A spectrometer table, similar to that
used for prism optics measurements and diffraction grating determinations of
wavelength, is here applied to analyze the light from a discharge tube. This light results
when the hydrogen gas in the tube is excited to breakdown by the voltage across the tube.
The wavelength spectrum is discrete, and the actual values test the predictions of Bohr
theory (the Balmer series) and determine the value of the Rydberg constant.
Helium Excitation (0433): This experiment has the same objectives as the H 2 (FrankHertz) Excitation described above. However, the gas is different, and therefore so are the
levels. The tube is quite different, runs at room temperature, and determines changes in
beam current by monitoring a fraction of the beam in a ring surrounding the electron
beam. While the tube needs to be run at such voltages that the “recurrent” spectra occur
here, there is one interesting feature. The tube can be run in a mode that displays the
excitation spectrum on a scope, allowing you to vary some parameters to optimize the
data (and the display) and see the results directly.
Electron diffraction (0440): (Caution: High Voltage) The wave character of electrons
(de Broglie waves) is uniquely demonstrated by the diffraction patterns observable when
electron beams interact with crystal lattices, and the phenomenon is an important
analytical tool in the study of crystal structures. A commercial apparatus is available
(Teltrontube) which brings electron diffraction within the scope of the elementary
laboratory; equipment and procedures for producing optical analogs to electrondiffraction patterns are also available and described in the reference documents.
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Versions
131125 – Deivid Ribeiro updated header and footer.
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