Category Archives: Physics

It's Like Building a Bridge

Posted on by

Uncertain Principles: Physics Is All About Analogies

I’ve got a little speech about this that I give when I talk about simple harmonic oscillators in the intro mechanics class, that I started giving because I got sick of the students giving me pitying looks when I went on about masses on springs. Because, really, who gives a damn about masses on springs?

Of course, any physicist knows that the reason we spend time talking about masses on springs is not because masses on springs are inherently fascinating, but because so many systems that are interesting can be made to look like masses on springs. That is, there is an analogy to be made between the behavior of a really simple system that we can solve exactly (the mass-on-a-spring problem) and much more complicated systems that we would really like to be able to solve exactly.

This goes along with Every Problem Looks Like a Nail, my link to brief comment on an xkcd cartoon: To first order, everything is an harmonic oscillator.

In science, we build models. Analogies are pre-fab models, based on an already-existing floor plan. Or, in the Feynman context of discussing magnets

I really can’t do a good job, any job, of explaining magnetic force in terms of something else that you’re more familiar with, because I don’t understand it in terms of anything else you’re more familiar with.

to put a concept in terms of concepts you do understand, and to use another analogy, it’s a bridge to a concept you understand.

Brother Maynard! Bring Up the Holy Hand Grenade!

Posted on by

Radioactive rabbit trapped at Hanford

Workers first found contaminated rabbit droppings last week in the 300 Area, said Todd Nelson, spokesman for Washington Closure Hanford, the Department of Energy contractor cleaning up Hanford.

Several rabbits were trapped and the one was found to be highly contaminated with radioactive cesium.

I wonder what superpowers (of a non-breeding nature) you would get if bitten by a radioactive rabbit?

What Quantum Mechanics is (and isn't) Good For

Posted on by

What Is Quantum Mechanics Good for?

Max Born said that by manipulating this wave function that Schrödinger developed, you could tell the probability of finding the electron at any point in space and time. From that, it turns out that the electron can only have certain discrete energies inside an atom. This had been discovered experimentally; this is the source of the famous line spectrum that atoms exhibit and that accounts for why neon lights are red whereas sodium streetlights have a yellow tinge. It has to do with the line spectra of their respective elements.

But to have an actual understanding of where these discrete energies come from—that electrons and atoms can only have certain energies and no other—is one of the most amazing things about quantum mechanics. It’s as though you are driving a car on a racetrack and you are only allowed to go in multiples of 10 miles per hour. When you take that and you bring many atoms together, all of those energies broaden out into a band of possible energies.

I like the point about how in basic discovery, nobody is thinking about applications down the line — Schrödinger didn’t have the diode laser in mind when he was developing the theory

If you went to Schrödinger in 1926 and said, “Nice equation, Erwin. What’s it good for?” He’s not going to say, “Well, if you want to store music in a compact digital format…”

via

Why so Blue?

Posted on by

Causes of Color

Why use a black body radiator as a standard, when no such thing exists?

It turns out that black body radiation provides us with a set of very precise working equations that relate the temperature of an object to the light it emits. Working from the ideal and using Planck”s law, we can predict the energy distribution across the spectrum for a given temperature. The total emitted power is calculated using the Stefan-Boltzmann law. The wavelength of the peak emission, and hence the color that dominates for this temperature, is provided by Wien’s displacement law. Knowing the ideal case allows us to predict or calculate actual values by correcting for the imperfections of actual hot objects.

h/t to ewmon

The Sadness is Nowhere Near Infinite

Posted on by

You need to a flashplayer enabled browser to view this YouTube video

Two things:

– a 500kg-ish pumpkin has passed its ability to stay spherical under its own weight. I imagine someone studies this kind of thing. Yes, they do.

– this is something that is dying to be shot in slow-motion

While I'm In Fist-Shaking Mode …

Posted on by

as I was yesterday, it’s time to stand on the porch and curse the Seebeck effect, which is the phenomenon behind thermoelectric currents. If you take two different conductors and make them touch each other, you get cooties a current if there is a temperature gradient present in the system. You can run this in the other direction and create a temperature difference, which is why thermoelectric coolers/heaters work; this is the Peltier effect and is the technology in most portable active coolers (if it plugs in and doesn’t have a compressor, then it’s probably a Peltier cooler).

Now, imagine you have a vacuum system running an experiment which is very sensitive to magnetic fields. Because you are trapping atoms, you have MOT coils dissipating a few Watts of power (while the trap is on) and you are heating your alkali metal reservoir to facilitate their introduction into the vacuum system. You will have a temperature gradient along your nicely conductive vacuum system. So if you have some dissimilar metals touching the vacuum system, somewhere, you’ll get a tiny amount of current flowing through it, creating a tiny magnetic field. But since such magnetic fields are phenomena non grata, having a thermoelectic connection — which you did not expect to have — is a bit of a pain.

Found it though. Something was inappropriately touching the vacuum system, so we had it arrested and now we make sure there’s a few mils of kapton between it and the vacuum chamber.

In Defense of Physics Pedantry

Posted on by

In yesterday’s Get a Grip, Drew asks a reasonable question in response to my pedantry about the use of terminology:

So when is the word ‘suck’ used appropriately (trying not to sound dirty, here)? Can’t there be a colloquial usage if we know what it actually means?

Used appropriately? Probably never, or rarely. And I have to admit, I can’t think of a situation (on short notice) where this conceptual mistake would cause a problem. We scientists can be rather anal meticulous about terminology, and there is a reason for it. Sloppy terminology might lead one to construct a flawed model of how things actually work, much like when one uses an analogy — there are always circumstances under which the explanation fails to hold. By using the proper terminology, the model is better and there are fewer circumstances under which it will fail.

 

This is not the only example of the sloppy language phenomenon. Others include heat and deceleration. Heat is probably the worst, and in no small part because physicists are sloppy in using it. To begin with, we present it in two different ways: as a process, by which energy is transferred because of a temperature difference, and also as the energy itself that is transferred. A problem arises when we use the two interchangeably. We then talk of heat flow or heat transfer, which is awkward if we are referring to a process. Beyond that, this reinforces the notion that heat (or, in general, energy) is a substance, as if you could have a little pile of heat somewhere, and heat transfer then invokes the image of pouring this substance from one container to another. The huge drawback here is that the misconception sidesteps thinking about the physical processes of conduction, convection and radiation. Heat (like work) isn’t something in a container, but we reinforce this error by using the term “heat capacity,” which tends to encourage this idea. All of this without even getting into the commonly-held misconception that infrared light and heat are the same thing.

 

Deceleration is an unnecessary term in physics, because acceleration is a vector, which just makes it a special case where the acceleration and velocity are in opposite directions. But some students have a hard time with the concept of vectors, and decoupling the terminology probably isn’t helpful, especially when you get into circular motion, where there’s an acceleration that doesn’t change the speed at all.

 

As I said at the outset, I can’t think of the pathway where using “suck” leads one onto the moors of misconception and into the bog of bafflement (not to mention possibly going over the cliffs of insanity), but I’m sure there is one. Because if there is one thing I learned while teaching, it’s not to underestimate the ways in which students will misunderstand concepts.

(edit: I don’t mean this last remark in a bad way — it’s not meant to disparage students. The issue is that if a teacher erroneously assume s/he understands what a student’s misconception is, the misdiagnosis is going to make it harder to fix the problem.)