# DEMONSTRATIONS

This page contains links to demonstrations that illustrate concepts covered in 131-132-133. Make sure to see the list of sites at the end where you can find more virtual demos.

Most of these demonstrations require that Java be enabled on your browser.

 Bright Yellow indicates the demos appropriate for this week.

## 132

Coulomb's law

good
This should be pretty easy. Test yourself to be sure.
Unknown charge

good
This is harder. Try it.
Force on charges

good
Determine the sign of the unknown charges. Hint: move some of the charges away from the others to simplify the problem. You are working with a square-law force.
Charge in a field

very
good
See the motion of a particle in a combined electric and magnetric field. You control all the conditions.
(Turn the magnetic field off, at least at first. A good combination to start with is: y=50, Vx=15, Ez=10 and all other quantities zero. You should be able to predict the shape of the trajectory before you try it.)
E-field plotter 1

very
good
Plots field lines and equipotentials for an arbitrary charge distribution of your choice. You can also let your system evolve with time. Start with only "electric field lines" selected. Then add equipotentials when you're ready.
E-field plotter 2

ok
Hit the "Start SimPhysics" button to get going. This one has many, many extra features, but I like the one above better. One nice addition is a cursor that shows the E-field direction and magnitude at any point. You might want to start by selecting each charge, then use the "particle info" entry under the "particle" menu to increase the charge to 2.
Field lines and equipotentials

good
Change the sign of the charge on the right. Understand?
Change the ratio of the charges. Understand?
This is important!
Potential

very
good
Click on the appropriate link to select Simulation A, B or C, then hit "play". A is pretty tame but make sure you understand it.

You should be able to answer these questions:
All sims: Are there charges present? Where?
Sim A: Which way is the electric field pointing?
Sim C: Note the motion of the charge when the simulation started. Replay it if needed. Now place the charge near C. Will it move toward or away from C?
Sim B: Same as above. The potential lines are a little off near the charges so I put this one last. The simulation is good enough, though.
Lightbulb test

good
A simple test of your understanding of series and parallel resistors and power.
Kirchhoff's Rules
1 2 3 4

good
Four examples of the use of Kirchhoff's Rules. This is from the Virtual Labs
& Simulations
Electricity site.
Circuit builder

good
You can build and test any kind of circuit discussed in class using this. Here is an example that uses the circuit builder to illustrate RC circuits.
RC Circuits

good
Practice with a virtual circuit.
RC Circuits

excellent
Shows where the current and energy go in an RC circuit with a switch.
Digital circuit simulator

good
Java Logic Circuit Simulator Applet. We don't cover digital electronics, but here is something you can play with if you know a little about logic circuits. All of the basic gate types are available.
Charges in a magnetic field

good
Make sure you can control/predict whether the particle goes clockwise or counter clockwise.
Charge in combined E and B fields

excellent
Rotate the coordinate system with your mouse so you can see the 3-D motion.
Deduce the current

good
Use a compass to deduce the current direction through two wires. If you can do this, you understand magnetic field generation by a straight wire. Important!
Biot-Savart Law

ok
Practice with the elements that make up the Biot & Savart Law.
Drag the wire

good
RLC circuit

ok
A key example involving the three basic linear circuit elements: resistor, capacitor, inductor.

## 133

Spring & mass

good
Control your own spring and block. Set k=10 and start the block in motion by dragging it and releasing. A test: can you identify the meaning of the red arrow without reading through the description?
Spring & mass and
Pendulum

good
Gives you control over all parameters and allows you to monitor the position, velocity, acceleration, energy, etc.
Transverse and Longitudinal Wave

good
Make sure to read the directions below the demo.
Longitudinal Wave  good Longitudinal waves are harder to visualize. This demo helps connect particle motion (displacement, velocity, acceleration) to the wave motion. It's simple enough. Just make sure you understand the graphs you see.
Various Kinds of Waves

good
Make sure to look at the longitudinal wave. Try and focus on an individual particle in the wave. Compare it's motion to that of the wave. They're related, but they're different.
Superposition Using Pulses

good
I think it's easier to understand superposition when pulses are used, but is this simulation correct? Try this: Invert one of the pulses by left clicking below it. Watch. There will be a moment when the superposed wave (shown in black) is completely flat. Is energy conserved?
Superposition Principle for Waves

very
good
Superpose two arbitrary sinusoidal waves to get a standing wave. MAKE SURE TO READ THE INSTRUCTIONS. Is this method of getting a standing wave at all related to the class demo showing a standing wave using a rope tied at one end?
Superposition Principle for Waves II  ok Allows you to superpose two waves with full control over amplitude, direction, phase, etc. You can use this to demonstrate beats, standing waves and all other major phenomena discussed. The phase control is a little buggy, unfortunately.
Beats

good
Change the frequency and phase difference between two waves.
Various Kinds of Interference

good
The basic categories of wave interference discussed and illustrated!
Sonic boom good First, run the simulation and observe the Doppler effect. From its point of view, the wave emitter is running at a single frequency, but what to observers think that are: (a) behind, (b) ahead, (c) below the emitter. [Part (c) is the trickiest case.]

Second, note that initially the source speed is slower than the wave speed. Think of the source as a jet plane now. Increase the source speed by dragging its arrow until it matches the wave speed. Something important changes. Why do people talk about shock waves for this situation? Now, how can that explain a sonic boom? This is tricky. For an answer, try the next demo...
The location of a supersonic airplane

good
Generating a sonic boom.

You'll have to think a little on this one. Read the instructions at the bottom so you under the controls first. Start with subsonic flight (ratio = 0.5). Make sure you understand the numbers that show up. Right-click to stop and restart the demo so you can see what is going on. At each point in its path (the white dots) the jet emits a spherical wave that expands in all directions. Think of the jet as a moving point source. The green and yellow lines just show the part of the wave emitted from a given white dot that is going in the right direction to reach the observer. Parts of the wave going in some other direction won't be heard and don't concern us here.

Now, switch to supersonic flight (ratio = 4.0).Note the change in the numbers that show up at the top. Waves emitted later reach the observer first!
Does the black line in this demo correspond to anything in the previous demo?
Why do the green and yellow lines come together in the funny way that they do?
The volume is related to the number of lines reaching the observer.

Try other ratios.
Decibels

good
Uses your computer's audio system to illustrate the effect of changing the sound level by 3,6,10 and 20 dB.
Propogation of Electromagnetic Waves

good
Very simple, but helps with picturing how E, B, and the direction fit together. Picturing EM waves is trickier than you might think. Question: the graphs of E and B descibe how the fields change in time, but where? That is, fields are being plotted for what region of space: on the x-axis only, or off of it or some other axis?
Wave propagation, reflection/refraction
using Huygen's Principle

good
Pause and continue the demo so you can follow everything that happens. Yes, it's true. Simple reflection and refraction is a more complicated process than you might at first think.
Wave propagation using Huygen's Principle II very
good
Another one, but more in the style of a tutorial. Hit "Start Simulation" to start the demo. After each simulation, hit "Next Step".
Ripple Tank  excellent This is one of the best of this type that I've seen. Two windows will come up. One contains the applet and the other contains directions. It's amazing what you can do. Note that if you click in the tank, you set off ripples as if you had dropped a pebble in it.
Reflection and Refraction

good
Sends a beam through an interface under your control. Illustrates reflection, refraction, total internal reflection.
A fish's view

ok
Fish have rights too, I suppose. Try making the object very wide and placing directly over the fish.
Physics of rainbows

ok
Also illustrates the idea of dispersion: different colors have different indices of refraction.Can a single raindrop produce a rainbow, even if very weak, for an observer on the ground?
Color  ok So what is the difference between mixing light and mixing paint?
Thin Lens

good
Try this! Automatic ray tracing, using a bundle of rays or the three special rays, as you adjust the object. Place the object on the focal plane and to either side of it.
Thin Lens II good The previous one is better, but here's another that provides automatic ray tracing using the three special rays we've discussed in class as you adjust the object.
Lens Problem #1
Lens Problem #2

good
Use these to test your understanding of lenses. See if you can find a position for the lenses that makes the answer to the question obvious.
If you can't solve these, see me!
Don't forget to hit "Start" at the bottom of the page.
Two-Slit Interference - Young's Experiment

excellent
Shows the wave train from each slit and how the interferences changes across the viewing screen. (Requires a flash player.)
Single Slit Diffraction
and
Double Slit Diffraction

good
If you're just starting to learn the subject, this illustrates the basic phenomena.
If the exam is near, you're in trouble if you can't predict what the result of adjusting the controls will be!
Interference of waves from two point sources  good Place the two point sources on top of each other and look at the result. Then move one of them sideways in small steps and observe the change. You should be able to explain what you see. What happens when the two sources are 1/2 a wavelength apart? How about a full wavelength?
Interference of waves from two point sources II

ok
Similar to the above. You can use this to explain what you saw in the previous one. However, it's harder to interpret until you get the hang of it. Here the essential point: if you see a black line on top of a white line, there's desctructive interference. If you see two lines intersection that have the same color, there's constructive interference. Also, try using "No pattern" at first.
The following demos cover aspects of modern physics: Special Relativity and Quantum Mechanics.
They're, perhaps necessarily, a little harder to understand but worth the effort.
Michelson-Morley Experiment

good
Illustrates what is expected from the experiment if light actually travels in a medium.
Space-Time Lab

ok
Shows you moving rods, clocks and more from the point of view of an outside observer and observer moving with the rod or clock. Make sure to set the velocity slider at the top. Also, try out the JAVA TA if you'd like a discussion.
Young's Experiment With Electrons  good A simulation of what you would see. This is important. Each individual electron impacts and registers as a single dot. Let it run for awhile. The problem is the pattern formed after many electrons have hit.....
Finally, notice how the graph isn't smooth. That "noise" is a fundamental part of how our world works.
Energy Well  good Gives you practice working with energy wells, without having to worry about quantum mechanics. It uses a cart with a pair of magnets on it and allows you to place magnets outside the car.
Quantum States in a Potential Well  good Drag the corners of the potential well and note how the states change. Both their position and number are important. Does a potential well have to have at least one bound state? Is it possible for one to have infinitely many states?
Quantum States in a Potential Well II  very
good
Similar to the previous one, but shows the wave function too. Select "show eigenstates" and "single well" to start off with.
Probability Illustrator  good Draw an arbitrary wavefunction using the mouse and then see what probability distribution results. You can also measure the probability using the green arrow heads at the bottom.  You should definitely know how to get a probability of 1 and what that means. If you don't, ask me.