Thursday, August 21, 2008

Oral Comprehensive Exams




So today was the Physics Oral Comprehensive exam. *Bleah*

I was supposed to give a 20 minute presentation on the research I did last semester, then open it up for questions. The presentation is to 3 members of the faculty.

20 minute talk 2 hour block of time. - It's a TRAP. I know it's a trap. It has to be a trap right?

Oh boy. It was an ambush alright.

I give the presentation, we talk a bit. They ask a question about holography (WTF?)

Then it goes most seriously pear shaped.

Tell us about the uncertainty principle:
Ok, well the uncertainty principle can be formulated several ways, the traditional the one that is used is that the uncertainty in position of a particle and the uncertainty in it's momentum must be on the order of h-bar over two or greater.

White it on the board:
Mark writes delta x delta y ~ h-bar/2

What do the symbols mean.
Mark explains

But what is delta x?:
It's the uncertainty in the position. Look if you measure the location of something, you can only measure it with arbitrary precision, what the uncertainty principle says is that no mater precise you measure x, it is impossible to measure p to a precision greater than that given by this relation. We can term it in terms of energy and time, so say for example we know how much energy a process takes, like say an electronic transition in an atom, then we can determine how long the process takes.

No, go back to the first one, what is delta x?:
Sir, I have no clue what you are asking.

You know there is a way to prove that relationship?:
I'm sure there is.

We then discussed other variables.

We then had a fine performance of Mark forgetting the Schroedinger Equation, and trying to derive the kinetic energy of a particle in the forbidden region past a barrier. Oh, and doing simple derivatives wrong.

I think I gained bonus points by explaining that if you took the barrier to a finite width, then that was the criterion for quantum tunneling, I then pushed on and gave the example of real world tunneling. Take two copper wires, expose them to air, they are no longer copper, they have a surface of copper oxide (which is an insulator). Put them together and current flows anyway. The electrons tunnel right through the barrier.

They changed the subject to asking me how a micro-channel plate works. I explain how it is manufactured, and how it works like a photomultiplier tube. I go on to talk about the way that the MCP is used in night vision goggles, proximity focus etc.

Discuss the vacuum levels achieved in the devices. They don't believe me. We discuss how said levels are achieved, they still don't believe me.

We discuss the energy band diagram of GaAs doped with Zinc. I draw in the acceptor levels and the fermi level. Dr. Bauer asks why the doping is so heavy, I eplain that it increases the photoeficiency.

He objects that it increases the bandgap:
yes sir, it does. But it also makes more electrons available for transitions, thus increasing the photoeficiency.

He does not appear to believe me, even though this is a fundamental property of the Fermi Level and every single theory of how electrons behave in materials.

I say look, as soon as you excite an electron to the conduction band, there is enough voltage here that the work function really doesn't matter. Further, just as in your lab, the surface is Ceasiated to reduce the electron affinity...

Ja, Ja, but...Dr. Bauer says that what he really wants to know is, when an electron strikes the surface, how do the other electrons come out:
Um, you mean the spectrum?

Yes:
Well, I don't know, I would presume that it is an Auger spectrum. (pronounced Au-je) Mark draws spectrum on board. Even if it was, there would be no way to measure it, as there is enough voltage here that the the original energy levels won't last long.

What do you know about Auger electrons:
Not much, I'm not an electron microscopist.

Auger Electrons are more than that:
Not Much. There is a strong spike at the original electron energy level, and a broad range of electrons of lower energy. The spike is from inelastic collisions and the rest is from multiple collisions.

Lets talk about statistical mechanics. What is the speed of the atoms in the atmosphere in this room:
*Blink* ... I have no idea...

If you had taken my 541 class you would know:
Well, in my class we spent most of our time doing Fermi and Bose-Einstein statistics.
I'm not used to thinking about large things like molecules.

Large? Molecules?:
Well, I'm used to dealing with photons and electrons, so yeah, molecules are large.

Do you know the speed of sound:
Think pause. You're going to hate this but, a bit over 1000 feet per second. At standard temperature and pressure. Mark waits while the board converts feet to meters in their heads. They say close enough.

I ask, did you say velocity or average velocity?

Very good. Well how would you find out?:
Well I would have to know the partition function.

Do you know kT at room temperature:
Sure it's 30 milli-electon volts... pause.. oh, well average energy is 3/2 kT. So Energy is 1/2 m v squared. So I can find it, I would need to know the mass of Nitrogen though. *I look around the room, no periodic table* uh oh.

Did I mention it's a trap?

We, go ahead and draw the periodic table:
*blink* *incredulous stare*

Oh just the first three rows:
Mark starts drawing, they comment it's a little odd, I say I left out the D-series and the actinides and lanthanides, I finally erase it and add a big gap for the D-series so that it looks more normal, I write in H, He, Li. I say Carbon has 4 P-shell electrons so it has to go here.

you put carbon in the wrong place:
What, oh, No carbon has four valance electrons, not P-shell electrons so it goes here, silicon goes here. I start drawing in roman numerals above the columns.

No go back and put in nitrogen:
I add nitrogen oxygen, fluorine. Start trying to figure out where neon goes.

no go back, what is the mass of nitrogen:
Well, Carbon is 12, so Nitrogen has to be 14. Call it one more proton and one more neutron, you can't do that in this part of the table. *points to lower portion* but up here it's safe enough to just figure one each.

That's why we asked:
Um, right. So Nitrogen is diatomic so N2 is 28 grams per Mol.

How much is that:
I write 28/6.022 x10^23.

So, to solve this I'd have to convert electron-volts to Joules, convert this to kilograms.

Never mind, do you know the Fermi energy of electrons in Copper:
*blink* Measured from the vacumm level?

No the Fermi energy:
Yes, but where are you measureing from? *Mark draws energy diagram on board. From here or here?

It's 7ev. Do you know how fast that is:
It's roughly a theth the speed of light.

Why do you say that?
Well I seem to recall that currents flow at about that velocity.

Well, if room temp is 35 milli-ev, then you can see it's rather fast:
Well yes?

They say something noncomitical about converting the energy to that of a photon:
Mark writes 1240 ev.nm on the board. This is one of my favorite numbers. Divide by 7 so. Call it roughly 180 nanometers. (I am not about to do long division on the board)

Pretty much at that point the ordeal ends as we have run out the clock.

*ARGH*

BTW, did I mention, "It's a TRAP"

2 comments:

Brigid said...

Well, I'm feeling particularly stupid now. . .:-)

Mark said...

;-) well I wasn't feeling to smart yesterday either.

did I mention, "It's a Trap"?