A Student Lab in Environmental Physics

A student lab in environmental physics version 191202

Piet Blankert and Jan Mulder[1]

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

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 tutortogether.

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 Environment[2]
The amount of radon gas exhaled by different materials can be determinedby 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.

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 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] captions (die moeten op een apart toegevoegd vel)

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.

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 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.

[1] Piet Blankert () and Jan Mulder () are staff members of the VUA students labs.

[2] The web version of this experiment is not completed yet