Online Animations of Time Evolving Physical Systemsby Dr. Alan ScottDepartment of PhysicsUniversity of Wisconsin-Stout

This animation applies many concepts. The most important being Newton's 2nd Law. Other concepts include weight and gravity, friction, tension, and inclined planes. The frames showing breaking the weight into its components,forceof friction, tension,and problemsolving technique can be used as transparencies to discuss these topics. The animation can be shown illustrating the subsequent movement based upon the given conditions. The students can be ask to calculate/predict the movement of the blocks in the exercise part of the animation. They can compare theirresults with how it does move. This diagram can also be used as a template where the instructor can change the parameters and have students determine thesystem's behavior.

The Bohr atom is asimplified view of the true nature of the atom. It's pedagogical strength isthat it can be easily conceptualized and that it correctly predicts the emission lines of the hydrogen atom. Thus, one could argue that the model has acertain degree of accuracy. A correct description involves solutions to the Shrodinger equation and describing the motion and position of the electron witha probability distribution. The first part of this animation presents thefundamentals of the Bohr theory. The exercise has the student calculating thewavelength of light emitted for a particular energy transition. Students canalso be questioned about any other transitions by modifying the"base" template.The presentation should include a discussion of energy diagrams, absorption,and emission. A common point of confusion is distinguishing this emissionspectrum from the Black-BodyCurve spectrum. The instructor should emphasize the latter is a continuousspectrum emitted from a solid (usually solid) at a given temperature.

The efficiency of heatengines are defined as the amount of usable energy that can be obtained byabsorbing heat (or energy) from a hot reservior, converting some of that heat to usable work, then discharging some remainder of heat into a cold reservior.The Carnot Cycle describes the maximum amount of work that can be extracted when operating a heat engine between to temperature reserviors. This animationshows this process in the physical and graphical sense. It also calculates the efficiency when operated between to temperature reserviors. This template can be used to assist students when given the conditions in the exercise partof the animation. And if the instructor wants to change the parameters, anothermore generic template can be used and modified.

This animation has threemain parts: (1) determining the radius of curvature for a charged particlemoving through a magnetic field, (2) knowing the direction the force is applied, and (3) calculating the velocity of the particle based upon atime-of-flight measurement. Four example trajectories are given in theanimation. The exercise asks the students to determine which detector will fire given a neutron,deuterium nucleus, and another electron trajectory. It can be pointed out by the instructor or deduced by the studentthat a neutron will produce no signal in any of the charged particledetectors. In theory, plastic scintillation detectors with photomultiplier tubes can detect neutrons if the neutrons have a direct collision withparticles in the detectors thus creating charged particle collision fragments.

The instructor can provideas much or as little of information he or she chooses (as long as the problemcan still be solved). The base template leaves blank most of the information for setting up a problem. So new problem scan be readily generated. Some curvatures might not hit one of the numbered detectors. In these cases, it is recommended to have the students place an X atthe location where the particle leaves the region of uniform magnetic field.This exercise also requires the student to be able to scale distances.

When a source ofsound is approaching or retreating from an observer (or listener) it'sfrequency, as heard by the observer, is shifted to a higher or lower value.This effect also occurs for light waves enabling radial velocities of distantstars to be determined. Other topics that can be discussed with the DopplerEffect are radar guns, bat echo-location, weather radar, etc. To really appreciate the Doppler Effect concept, one should also mention the role soundintensity plays with the human perception of frequency. This animation illustrates the behavior of the frequency and sound intensity "heard"by an observer near railroad tracks with a train traveling by. The sound waves being shown in the animation are not drawn to scale and are only anillustration. The graphs are mathematically correct given an arbitrary intensity, Io, and source frequency. The base templatecan be used by modifying the relative velocity, source frequency, soundintensity, and position of the observer to generate further questions orapplications of this effect. Observed frequency and intensity can be calculatedfor any given train position. For further information please refer to the journal article "Overcoming Naïve Mental Models in Explaining the DopplerShift: An Illusion Creates Confusion", American Journal of Physics, v65,July '97 (618-621).6. Multiple Slit Diffraction The pattern that light createson a screen due to diffraction through some aperture can be difficult tocomprehend. This animation should be presented only after students have been introduced to constructive and destructive interference of waves along with Huygen's Principle. The pattern that ultimately develops on a screen from lighthaving passed through N number of openings or slits, is a superposition of N numberof single slit diffraction patterns. The width of each slit, spacing betweeneach slit, and the number of slits is important.

The curve representing light and dark fringes is mathematically correct and has followed equation18.13 in the text "Vibrations and Waves in Physics" by Iain G. Main,2nd edition. The animation starts with single slit interferencepatterns and progresses into multiple slits. In the limit that N gets verylarge we have a diffraction grating. The equation dsin(q)=ml describes the appearance of constructive interference peaks for a diffraction grating. One can use the following template (or with less information) to see if students can effectively apply this equation. This is inaddition to the exercise given in the animation.

It is a common misconception for students to think that the "shadowed" part of themoon is created by the Earth's shadow. This is only true during Lunar eclipses.This animation tries to eliminate this misconception and provide someunderstanding of when the Moon is visible from Earth. One good use of thisanimation is to show students the animation then present overhead transparenciesof each phase of the Moon. These transparencies are not labeled with respect to whether the Moon is visible or what phase it is in based upon the position ofthe flag on Earth. The students can be asked to provide this missing information. Here are some graphics for the Moon phases full,waninggibbous, waningquarter, waningcrescent, new,waxingcrescent, waxingquarter, waxinggibbous. The instructor can arbitrarily stick the "flag" where they want. The orientation (or rotation) of the Moon as seen from the Earth maynot be truly accurate. It is important to note the angle made between the Sun-Earth-Moon in conjunction with the phase of the Moon.

The earthquake animation illustrates how P and S wave fronts propagate away from an epicenter. The first earthquake wave presentation shows the wave fronts as expanding concentric circles and what effect they have on the seismographs. Illustrating thisexpanding wave front provides the students a concrete visual illustration on which a more abstract analysis can be built. An abstract analysis is requiredto determine the epicenter from just the seismograph readouts as given in the second earthquake wave readout.

A template that includes a US topographical map with seismograph readouts can be given tothe students as a "hands-on" activity using a compass and ruler. Here is a more generic template in case the instructor wishes to build their own seismograph readout. From thetime interval between the arrival of P and S waves, they can determine the distance from each seismograph station. Three stations are required to locatethe epicenter. The given seismograph readout can be used or another of the instructors choosing can be generated and implemented.

This animation illustrates three basic ways that an emf (or voltage) can be generated in a conducting coilby virtue of its interaction with an external magnetic field. Both the physical system and graphical results are shown. In the first case, a changing magneticfield strength produces an emf. This first case is a nice condition to apply Lenz's Law. One can determine that the induced current flows in the clockwise direction because the rate of change of magnetic field strength is out of the page. Thenext case changes the area of the coils. The right hand rule can be effectivelyapplied to determine the direction of the current. The last case involves the coils spinning in the magnetic field. One could apply Lenz's law in each of the cases to determine the direction of the induced emf (or current). In workingthe exercise, it is important to emphasize that maximum emf occurs when themagnetic flux (or number of field lines) through the coils is changing mostrapidly. Here is a still image onto which students can sketch or predict the shape of the graphs.

Each of Kepler's Laws are presented along with an illustration of their physical representation.Eventhough Haley's Comet is not a planet it does follow Kepler's Laws. It was chosen as part of the animation because of its large size and eccentricity. Theconcept of equal areas in equal amounts of time can be made more concrete by shading the area in question. In order to get a discernably sizable area whenthe Comet is at Aphelion (farthest distance), the animation compares it to thenumber of revolutions the Earth sweeps out in the same amount of time. The exercise requires algebra in applying Kepler's third law to compare the ratios of (T2/R3) for Earth and Mars. The units can be arbitrary (Earth years and Astronomical Units are convenient).

11. Laser

The Helium-Neon laser isused often in physics courses. So it seems appropriate to examine how thelasing process works for this particular type of laser. The He-Ne laser works by an interaction between these atoms and their respective excited states.  Thus, students will need to have seen energy level diagrams and be able to relate energy transitions to photon wavelengths.

A brief summary of thegeneral process involves:

(1) Helium atom is "pumped" into an excited state by an electricalpulse. This excited state is meta-stable and takes a long time to decay.

(2) A Helium atom collides with a Neon atom to transfer the Neon atom intoan excited state.

(3) The Neon atom decays with more structure in the energy levels. The Neon atom first decays into a meta-stable state which either spontaneously decays or is induced into emission of a photon by another photon of the same wavelength.This is the lasing transition. The Neon atom then returns to the ground state.The exercise has the student calculate the wavelength of the laser light using the given equations and the energy diagrams.

The ray colors presented in this animation are not to indicate that red, blue, and green light wavelengthsbehave significantly different. Yes, the index of refraction is technically afunction of the wavelength of light, but be careful the students do notincorrectly interpret the meaning of the colors in this animation. The different colors signify the three main rays used in geometrical optics for ray tracing.The first part of the animation illustrates how divergent and convergent lenses"bend" or refract light. It also shows how the rays diverge orconverge as the object is brought closer to both types of lenses. The last partillustrates a multiple lens system. Both the first multiple lens example and the second can be used as an exercise in ray tracing for the students. The presentation ofthis animation should include a discussion of virtual/real images and thelens/mirror equation.

13. Planck Curve

All solid objects emit electromagnetic radiation. The Planck Curve (or Black Body Radiation Curve) describes the intensity of the radiation as a function of the wavelength. Thisanimation can be presented in conjunction with a discussion of Stefan's lawwhich describes the area under the curve or the total radiant energy per timeemitted. The exercise has the students using Wien'slaw to determine the temperature of an object based upon the peak intensityof radiation. The curve is mathematically accurate and follows equation 3.26 in the text "Modern Physics" by Kenneth Krane. It is important thatstudents are aware of the scaling on the vertical axis. The relative size ofthe curves are drastically different at 1000oC as compared to 10000oC.

This is an animated depiction of how a cave-in progresses. Every soil cave-in is unique but theredoes exist some general features that most possess. The purpose of this animation is to illustrate the more important features and present some basic facts. Excavation safety issues revolve around psychology (or the "CowboyEffect"), economics, and soil behavior. The psychology part can be verycomplicated. Supervisor/worker relations (which may include job security),interpersonal interactions, and a false sense that soil isn't dangerous or feeling that only "wimps" get hurt with soil. Economics is always anissue in our free-enterprise system. The temptation to cut back on safetyprocedures to save time and/or money is a serious issue. And, of course, thebehavior of soil is important. What conditions cause soil to move? An excavation with vertical walls sometimes produce signs that soil is notin equilibrium and is trying to move into a state of equilibrium. Such a signmay be cracks or crevasses developing in the ground near the edge of theexcavation. This is a sure sign that the soil is unstable and is prone tomovement. Here is a still image to point out hazards. A discussion of proper sloping or shoring either beforeor after this animation shown is recommended.

Soil does not behave like aspring (i.e. it does not follow Hooke's Law). Once compressed it rebounds onlyslightly upon un-loading. This animation is appropriate for discussing Over-consolidated and Normally consolidated soil. The compression index can becalculated from this type of laboratory test. Here is a still image for discussing important aspects.

How long it takes soil to settle a certain amount for given conditions is an important topic in soil mechanics. Plotting deformation versus time provides us information about the duration of settlement. This animation can be presented during a discussion about total settlement, drainage, pressure, theory (differential equation),etc. Here is a still image for discussing important aspects.

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This animation demonstrates how solids deform when subjected to stress. It can be presented during adiscussion of Young's Modulus. This still image (L) can be utilized as atransparency for relating theory with the physical conditions.

This illustration is toshow the physical meaning of an energy versus distance graph for objects underthe influence of a central force proportional to 1/r2.

19. CAT Scan

The complicated subject of Computer Axial Tomography has been simplified to two dimensions with this animation. This two-dimensional analysis makes the concept for tractable andprovides an excellent way to evaluate students understanding of the concept.Students can be ask to determine the shape of the object that is hidden during the scan or the instructor can design their own scanned object with detector signals from this template.

20. Relativity

This animation illustrates the fact that simultaneous events in one frame of reference are not simultaneous in another. The instructor should recommend that students pay close attention to when the animation is moving at normal speed and when itmoves in "slow motion". Light travels extremely fast! And this is not evident from the animation.

A discussion should follow that tries to answer the question "who has the 'correct' view?" Theanswer is that they are all correct. Simultaneity is relative to ones referenceframe.

This animation demonstrate the unusual behavior of "entangled wave functions" in therealm of quantum mechanics. It has profound influence on scientific philosophy and the present scientific "worldview".

(*The following animations were primarily designed for an introductoryAstronomy course.)

Stars have a life cycle.They are born, have mid-life, and die. The Hertzsprung-Russel diagram iscentral to understanding the life cycle of stars. This animation shows the physical state of a star (similar in size to our Sun) together with itsposition on the H-R diagram as it lives out its life.

23. Tidal Forces

This animation exaggerates the size of the Earth's hydrosphere for illustrative purposes. To fully appreciate tidal forces, one should start with simple conditions (i.e. just the Earth) and progressively add influences that effect the Earth's tidal cycles(i.e. Moon's gravitational pull and the Earth's rotation). The instructor could follow this animation up with a discussion of how the Sun influences the tides (spring and neap tides).

What would it be like to take a journey to the center of the Sun? The journey is a bit fanciful and imaginary but the conditions recorded bythe instruments are realistic. While inside the Sun, the behavior of theparticles at the sub-atomic scale is animated. A common misconception is thatnuclear reactions are happening throughout the Sun. In fact, they are only (or most frequently) happening near the center.

Both heliocentric and geocentric worldviews are capable of reproducing the behavior of planetary retrograde motion. So why was Copernicus an ardent supporter of one but not the other? Such a question can encourage thoughtful analysis about retrograde motion and the meaning of science and scientific models in general.

This model was consideredto be the correct explanation of planetary retrograde motion for manycenturies. It was also a less contentious philosophy or explanation in the eyesof most religions during this period of history.