Wednesday, July 18, 2018

Khan Academy's Photoelectric Effect Video Lesson

A lot of people use Khan Academy's video lessons. I know that they are quite popular, and I often time get asked about some of the material in the video, both by my students and also in online discussions. Generally, I have no problems with their videos, but I often wonder who exactly design the content of the videos, because I often find subtle issues and problems. It is not unusual for me to find that they were inaccurate in some things, and these are usually not the type of errors that say, an expert in such subjects would make.

I was asked about this photoelectric effect lesson by someone about a month ago. I've seen it before but never paid much attention to it till now. And now I think I should have looked at it closer, because there are a couple of misleading and inaccurate information about this.

Here is the video:

First, let's tackled the title here, because it is perpetuating a misconception.

Photoelectric effect | Electronic structure of atoms
First of all, the photoelectric effect doesn't have anything to do with "structure of atoms". It has, however, something to do with the structure of the solid metal! The work function, for example, is not part of an atom's energy level. Rather, it is due to the combination of all the atoms of the metal, forming this BANDS of energy. Such bands do not occur in individual atoms. This is why metals have conduction band and atoms do not.

We need to get people to understand that solid state physics is not identical to atomic/molecular physics. When many atoms get together to form a solid, their behavior as a conglomerate is different than their behavior as individual atoms. For many practical purpose, the atoms lose their individuality and instead, form a collective property. This is the most important message that you can learn from this.

And now, the content of the video. I guess the video is trying to tackle a very narrow topic on how to use Einstein's equation, but they are very sloppy on the language that they use. First of all, if you don't know anything else, from the video, you'd get the impression that a photon is an ordinary type of "particle", much like an electron. The illustration of a photon reinforced this erroneous picture. So let's be clear here. A "photon" is not a typical "particle" that we think of. It isn't defined by its "size" or shape. Rather, it is an entity that carries a specific amount of energy and momentum (and angular momentum). That's almost all that we can say without getting into further complications of QED.

But the most serious inaccuracy in the video is when it tackled the energy needed to liberate an electron from the metal. This energy was labelled as E_0. This was then equate to the work function of the metal.

E_0 is equal to the work function of the metal ONLY for the most energetic photoelectrons. It is not the work function for all the other photoelectrons. Photoelectrons are emitted with a range of energies. This is because they came from conduction electrons that are at the Fermi energy or below it. If they came from the Fermi energy, then they only have to overcome the work function. These will correspond to the most energetic photoelectrons. However, if they come from below the Fermi energy, then they have to overcome not only the work function, but also the binding energy. So the kinetic energy of these photoelectrons are not as high as the most energetic ones. So their "E_0" is NOT equal to the work function.

This is why when we have students do the photoelectric effect experiments in General Physics courses, we ask them to find the stopping potential, which is the potential that will stop the most energetic photoelectrons from reaching the anode. Only the info given by these most energetic photoelectrons will give you directly the work function.

Certainly, I don't think that this will affect the viewers ability to use the Einstein equation, which was probably the main purpose of the video. But there is an opportunity here to not mislead the viewers and make the video tighter and more accurate. It also might save many of us from having to explain to other people when they tried to go into this deeper (especially students of physics). For a video that is viewed by such a wide audience, this is not the type of inaccuracies that I expect for them to have missed.



In this article, Ethan Siegel valiantly tried to explain, in simple language, what "multiverse" is within the astrophysical/cosmological context:

Inflation doesn't end everywhere at once, but rather in select, disconnected locations at any given time, while the space between those locations continues to inflate. There should be multiple, enormous regions of space where inflation ends and a hot Big Bang begins, but they can never encounter one another, as they're separated by regions of inflating space. Wherever inflation begins, it is all but guaranteed to continue for an eternity, at least in places.

Where inflation ends for us, we get a hot Big Bang. The part of the Universe we observe is just one part of this region where inflation ended, with more unobservable Universe beyond that. But there are countlessly many regions, all disconnected from one another, with the same exact story.

Unfortunately, as is the problem with String theory, none of these have testable prediction that can push it out of the realm of speculation and into being a true science.


Tuesday, July 17, 2018

94 Aluminum Pie Pans On A Van de Graaf

What happens when you put 94 aluminum pie pans on a Van de Graaf? Sometime you do things just because it is darn fun!

Now let's see if you can offer your own explanation for this silly thing! :) Happy 10th Anniversary on YouTube, Frostbite Theater!


Monday, July 16, 2018

Neutrinos Come Knocking For Astronomy

I feel as if these are the golden years for astronomy and astrophysics.

First there was the discovery of gravitational waves. Then a major astronomical event occurred, and we were able to detect it using the "old" standard technique via EM radiation, and via the detection of gravitational waves from it. So now astronomy has two different types of "messengers" to tell us about such events.

Well now, make way for a third messenger, and that is ubiquitous neutrinos. Two papers published in Science last week detected neutrinos (along with the accompanying EM radiation) from a "blazer". The neutrino detection part was made predominantly at IceCube detector located in the Antarctica.

Both papers are available as open access here and here. A summary of this discovery can be found at PhysicsWorld (may require free registration).


Friday, July 13, 2018

The Most Significant Genius

No, not Einstein, or Feynman, or Newton. Fermilab's Don Lincoln celebrates the hugely-important contribution of Emmy Noether.

I have highlighted this genius previously, especially in connection to her insight relating symmetry to conservation laws (read here, here, and here).


Wednesday, July 11, 2018

First Human Scanned By Spectral X-Ray Scanner

Chalk this up to an application of high-energy physics in the medical diagnostic field. The first human has been scanned by a new type of x-ray scanner (registration required to read article at this moment).

The MARS scanner uses Medipix3 technology developed at CERN to produce multi-energy images with high spatial resolution and low noise. Medipix is a family of read-out chips originally developed for the Large Hadron Collider and modified for medical applications.

The Medipix3 detector measures the energy of each X-ray photon as it is detected. This spectral information is used to produce 3D images that show the individual constituents of the imaged tissue, providing significantly improved diagnostic information.

I'll repeat this, maybe to those not in the choir, that many of the esoteric experiments that you think have no relevance to your everyday lives, may turn out to be the ones that might save your lives, or the lives of your loved ones, down the road. So think about this when you talk to your elected political representatives when it comes to funding basic science.


Thursday, July 05, 2018

Einstein Is Right Again!

... or rather, General Relativity passed another test.

This is on the heels of the first ever verification of GR at the galactic scale. This time it is a test of GR's strong equivalence principle involving a neutron star and two white dwarfs (no, not the kind from that Snow White movie).[1]

Archibald and colleagues’ study breaks new ground because the gravitational energy inside a neutron star can account for as much as 20% of the body’s mass. The authors’ results therefore imply that the accelerations of gravitational energy and matter differ by no more than a few parts per 105 — a tenfold improvement over the bound from lunar laser ranging.

More importantly, the authors have provided what is known as a strong-field test of general relativity. Unlike the Solar System, for which Einstein’s theory predicts only small deviations from Newton’s theory of gravity, the motion of a neutron star in a gravitational field invokes full general relativity in all its complex glory. Einstein’s theory passes this strong-field test with flying colours.

The more they test it, the more convincing it becomes.


[1] A.M. Archibald et al., Nature, 559p73 (2018).

Tuesday, July 03, 2018

What Type of Physicist Are You?

... leader, successor, or toiler?

A new bibliometric study has found that authors can be roughly grouped into three categories: lead scientists who are already prominent in their fields, successors who are early career scientists, and toilers, which are those who do a lot of the dirty work but aren't going anywhere.

When looking at the citation data for mathematicians, psychologists and physicists, the authors identified three broad clusters that are “loosely based” on how the citations per year changes over time. Leaders tend to be experienced scientists who are widely recognized in their fields, which results in an annual citation increase. The successors tend to be early-career scientists who have had a surge in their citations in recent years. Toilers, meanwhile, may have a high citation count, but this stays mostly constant and may even drop slightly.

Not sure of the significance of this study, but hey, it's another criteria to classify people!