A student lab in environmental physics by VjLbQ2V2

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									        A student lab in environmental physics version 191202


        Piet Blankert and Jan Mulder1

Department of Physics and Astronomy, Free University, De Boelelaan 1081,
1081 HV Amsterdam, The Netherlands

Abstract: In the framework of teaching environmental physics at the Free University
of Amsterdam (VUA) six experiments in environmental physics have been
developed. The experiments are incorporated in the student labs for physics majors.
Therefore, first an overview is given of the set-up and methodology of the student
labs in general. Next, the experiments are described. They are available on the web
for anybody to copy them including a typical ‘example experiment’ to test the
equipment and experimental procedure. For one of them, Laser Doppler
Anemometry, it is illustrated how the web looks like.


        1. Physics student labs at the VUA
In the undergraduate laboratories at the VUA, physics major students are prepared
for their master research work by 8 laboratory courses. Four short courses,
distributed over the years, train the students in writing a report, giving an oral
presentation, and the application of electronics, data-analysis and automation. The
four main courses concern experiments in physics and have as central teaching goal
the development of students' research skills. The aims of these courses, with an
emphasis on the first, (a), are:
a) To acquire insight into setting up, performing and interpreting an experiment, so
that students are able to follow a methodical approach to solve an experimental
problem.
b) To obtain knowledge about equipment and measuring methods.
c) To observe a number of physical phenomena and their relationships.

In all physics experiments students are trained to follow the steps of the research
cycle:
1. To translate the experimental problem into measurable quantities
2. To justify the choice of an experimental method
3. To execute measurements
4. To handle and analyse the experimental results
5. To draw conclusions


        1
           Piet Blankert (pietbl@nat.vu.nl) and Jan Mulder (janm@nat.vu.nl) are staff members of the VUA
students labs.


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Students’ research skills are gradually built up following two lines:
1- The practical instructions are not cook-book like. The courses have an open
character. In subsequent courses, the focus shifts to more of the aims (a), (b) and (c).
2- During the courses in the first two years the guidance is close. In the third year it
is more remote, in order to develop students’ independence in doing research.

Each experiment has three formal moments of guidance:
Exploratory discussion. Emphasis lies on guidance of the students. Students are
working on step 1 of the research cycle, mentioned above, while the tutor gives
feedback.
 Work plan discussion. Halfway through the research cycle (steps 2 and 3) the
student takes the initiative. The student shows how (s)he will set up the experiment,
and which choices are made.
Report discussion. In the report students show how they master also steps 4 and 5 of
the research cycle. The discussion is an evaluation of the whole experiment by
student and tutor together.

The role of the tutor develops from guide to sparring partner to evaluator.


       2. Environmental physics experiments
At the student’s lab the following environmental physics experiments were
developed, keeping in mind the character of environmental physics, discussed in the
preceding paper [1]:
    Determine the Hydraulic Conductivity of a sample
       Soil is imitated by a homogeneous sample of small spheres. For several diameters of these spheres and lengths
       of the samples the hydraulic conductivity is measured.
    Determine the Thermal Conductivity of Sand
       On the axis of a cylinder heat is supplied to a sand sample. The radial temperature profile is measured and the
       influence of the boundary conditions estimated.
    Heat transfer by Radiation and Convection
       The competition between these two ways of heat transfer is studied as a function of experimental parameters
       like pressure and colour of the heated surface.
      Laser Doppler Anemometry
       The velocity of a fluid is measured by means of the Laser Doppler method. In particular the radial velocity
       profile of the flow in a cylindrical tube can be determined.
      Radon in the Environment2
       The amount of radon gas exhaled by different materials can be determined by measuring the time-dependent
       radon daughter concentrations.
      Laser Remote Sensing
       The wavelength- and time-dependent fluorescence of the photosynthetic system of green plants can be
       monitored at a distance. This results in some basic information on fluorescence and on the process of, which
       could give some indication about the health of green plants.




       2
           The web version of this experiment is not completed yet


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       3. The experiments on the web
In order to make the information about the experiments available to others the
description of the experiments was put on the web [2]. The information is organised
in such a way that it should be possible to copy these experiments in any physics
department. Therefore, the relevant theory; a detailed description of the
experimental set-up, the measurement procedures, the signal processing and the
data-analysis are given.

Reproducing an example experiment give the opportunity to test the equipment at
other labs. Therefore, such an experiment is worked out in a ‘cook-book’ manner
for each of the environmental experiments. In this way they are different from the
usual, open set-up described above. To compensate, more emphasis than usual is
given to related experiments and to theoretical and experimental problems students
may encounter. By performing these experiments or tackling the problems, students
may develop their research skills and their creativity anyway.

In the teacher section information is given about the way a specific experiment fits
in the VUA curriculum, how students should be guided and what problems the
students may encounter with hints of their solution. Furthermore information is
given about the maintenance, the required software and the total costs to set-up the
experiment. Finally, a short list of relevant literature is given.


       4. Example: Laser Doppler Anemometry

One of the aims of environmental physics is to stimulate technologies that
reduce pollution and noise or increase fuel efficiencies. To this end one often
needs to measure flows of liquid or air without interfering with the flow. Laser
Doppler Anemometry (LDA) is one of the techniques available, where the word
anemometry originates from the Greek ‘anemo’= wind.

In this section we discuss a few elements of the web experiment [2]. Fig. 1 (left)
shows the experimental set-up, where the flow of water through a straight, round
tube is investigated. A laser beam is led through a beam splitter BS and a mirror M,
resulting in two parallel beams, which are focussed by a lens L1 into a small part of
the tube, down to 0.1 mm. The interference of the two beams gives rise to a zebra
striped pattern, like a pedestrian crossover. The position of this zebra may be shifted
perpendicular to the flow by adjusting the position of the tube. The laser beams will
pass the flow and are blocked by a diaphragm so that only the scattered light will
pass two lenses and end up in a photo detector PD.

The fluid will contain small particles or bubbles, some of which will cross the zebra.
In a relativistic analysis one describes the scattering of the light by these particles as


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a Doppler effect, hence the name LDA. Classically it is particles crossing the zebra
will scatter light when on a bright stripe and not scatter if the pass a black stripe,
thus modulating the signal. The modulation frequency will be proportional to the
velocity of the particles. A typical example for a laminar flow is shown in Fig. 1
(right).

By moving the measurement volume across the flow the radial velocity profile can
be measured. For low Reynolds numbers the students will find a parabolic velocity
distribution (Fig. 2 left). Next they may calculate the total flow in L hr-1 and
compare it with the reading of a flow meter, resulting in a calibration plot (Fig 2,
right). On the web all the details of the example experiment at our students’ lab are
given, so figures 2 may act as a check for staff or students trying to reproduce our
results.


      References
[1] Egbert Boeker, Rienk van Grondelle and Piet Blankert, Environmental Physics
as a teaching concept, European Journal of Physics---------
[2] www.nat.vu.nl/envphysexp




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Figure captions (die moeten op een apart toegevoegd vel)

                                                   Intensity / [V]
                                                         2

     M         L1             D L2      L3    PD
                                                         0

BS
                                                      -2


                                                      -4
         Laser               Flow tube

                                                      -6
                                                     0       1       2       3      4         5    6
                                                                     time [sec]



Figure 1. On the left the experimental set up is displayed with a laser, a beam splitter
BS, a mirror M and a lens L1, which gives a zebra striped pattern in a tiny space of
the flow tube. Particles crossing the zebra modulate the signal in the photodiode PD
on the right of the tube as is shown in the figure on the right.

Jan,Piet, Links Fig 3.1 zonder het word ‘laser beam’ en zonder de legenda. Het
woord ‘Laser’ kan onder rechts van de laser worden gezet, dat spaart breedte. Aan
de rechterkant zou ik het eerste gedeelte van Fig 4.2 nemen, want dat is wat de
studenten zullen zien. Het woord ‘Signal intensity’ kan misschien rechts van de as
komen evenals de getallen –6, -4 enz. Ook dat spaart breedte. En de kromme niet in
geel maar in zwart-wit.

                   -1                                                -1
  U(r) / [mh ]                                       meter / [Lh ]
                                                    40
 40                                                 35

 30                                                 30
                                                    25
 20
                                                    20

 10                                                 15
                                                    10
  0
                                                     5
-10                                                  0
              -3
     -20x10         -10       0         10   20              0   10            20        30       40
                                                                                    -1
                                  -3
                          r / [ 10 m]                                     flow / [Lh ]



Figure 2. On the left the measured velocity U ( r )of the flow is shown as a function
of the distance r to the centre of the flow. From this parabolic velocity profile the


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total volume flow in L hour-1 passing a cross section is calculated. This may be
compared with the flow meter measured by a flow meter. By changing the input
flow one obtains the data of a calibration plot, shown on the right.

Jan, Piet. Links Fig 5.2 maar enigszins gecomprimeerd, eventueel het deel tussen –5
mm en + 15 mm. De variabel langs de verticale as weer rechts van de as brengen.
Aan de rechterkant Fig 5.1, ook gecomprimeerd, of alleen het deel tussen 5 en 25. Ik
ben me er van bewust dat de volgorde van de figuren op het web anders is, maar
zoals ik het deed is het wat makkelijker kort uit te leggen. Let er op dat ik de
variabelen langs de assen wat heb vereenvoudigd.




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