Reading Quiz 17 Key

Reading Quiz 18 Key

Due before class Monday, Aug 6th

Physics 106, Summer 2012


Reading: Chapter 27 & 28 (all)

Did you complete the reading assignment?

Yes
No

Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?
Yes
No

KEY CONCEPTS:

1) All particles express wave-like behaviors; however, this phenomenon is only observable on a very small scale.

2) The energy of light is determined by its frequency, not its intensity.

3) Specific pairs of physical properties cannot be known simultaneously.

Quantum physics has quite possibly the most intimidating reputation of any subject. Professor Farnsworth explains the basics of quantum physics in 16 seconds to help you realize it is not as intimidating as you may think: http://www.youtube.com/watch?v=XqcaaUtPdAo

That description really is not all that far off. The basic idea of quantum physics is that there are no absolute certainties, only probabilities. In other words, because of the nearly infinite reactions going on between the smallest particles, we can never be 100% sure about our predictions on anything (position, momentum, energy, etc.). This does not mean that crazy things will happen for no reason (just that we do not always know what the reason is), and we can predict most things within very small ranges of error, but there is always the EXTREMELY SMALL chance that something we do not expect could happen. These predictions that we make do not use the classical model of physics which you have experienced and practiced with, but need to use an entirely different model, the quantum model. This new model is very different and seems very strange compared to classical physics; even Albert Einstein who was known for reshaping classical physics referred to quantum as "spooky."

Exercise 1: Wave-Particle Duality

The theory of quantum physics is based on wave-particle duality. The following link teaches a principle you have already learned, the double-slit experiment, but puts it into the world of quantum:

http://www.youtube.com/watch?v=DfPeprQ7oGc

What would you see if you shot large particles through a double-slit setup?

Two straight lines of particles spaced just like the slits are spaced.
A wave-like interference pattern with the brightest ridge at the center.

What would you see if you shot tiny particles through a double-slit setup?

Two straight lines of particles spaced just like the slits are spaced.
A wave-like interference pattern with the brightest ridge at the center.

Scientists can observe tiny particles using filters that will collect electrons going through them. How would putting an electron filter next to one of the two slits affect the behavior of the electrons?

Exercise 2: The Photoelectric Effect

The photoelectric effect is the effect that light has on the electrons of a metal plate when the light is shined on the plate. Go to the following applet: http://phet.colorado.edu/sims/photoelectric/photoelectric_en.jnlp

Click the 'Electron energy vs light frequency' box on the right hand side. Now increase and decrease the intensity of light. What affect does increasing the intensity of light have on the energy of electrons being knocked off of the plate?

Increases the energy
Decreases the energy
Does not affect the energy

Now increase and decrease the frequency of light. What affect does increasing the frequency of light have on the energy of the electrons being knocked off the plate?

Increases the energy
Decreases the energy
Does not affect the energy

It is common knowledge that the Sun's ultraviolet light is what causes sunburns. The most intense light emitted by the sun is visible light (between blue and green), so why are we most worried about the UV light?

Why will you be more badly burnt by the sun at midday then at sunset?

Exercise 3: The Heisenberg Uncertainty Principle

"The only true wisdom is in knowing you know nothing."- Socrates

The most famous pairing of physical quantities in the uncertainty principle is the pairing of position and momentum. Go to the following link to see how these pairs interact (click on the 'Load Applet' button after going to the site: http://eve.physics.ox.ac.uk/Personal/artur/Keble/Quanta/Applets/quantum/heisenbergmain.html

This applet can get very confusing very quickly if you don't know what you are looking at. First, change the 'center' scroll bar to 0.0 and keep it there. The only two graphs you need to worry about are the top left and top right graphs. The top left graph represents where the particle is, the higher the amplitude at that point, the more likely it is to be in that position. On the other hand, the more spread out the graph is, the more possibilities there are for the particle to be located somewhere else. The top right graph is similar, but for momentum rather than position. The skinnier the peak in the top right graph, the more you know about the particle's momentum.

Change the 'width' scroll bar to the left to decrease how well you know the momentum of the particle. Do you know more or less about the position of the particle now?

More
Less

If you knew the exact position of a particle, what would you know about its motion?

Joke of the Quiz: Chuck Norris is the only human being to display the Heisenberg uncertainty principle you can never know both exactly where and how quickly he will roundhouse-kick you in the face.


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