Friday, March 30, 2018

Revamping Introductory Physics Laboratory - Part 8

If you are not aware of my own pet project, this post will get you up to speed.

It has been a while since I updated this series, but better late than never. For this one, I'm going a bit against my own philosophy that physics just doesn't say what goes up must come down, but also when and where it comes down. For this exercise, I'm sticking with just the "what goes up must come down" part, i.e. only the qualitative aspect, not the quantitative aspect. But I have a good reason for it. It is because, in my experience, students often have a tough time understanding the concept itself, and I often found myself having to spend a considerable amount of time on this before I could proceed to the quantitative aspect of it. So in this exercise, the main idea is to make the students understand the concept and not worry about the "numbers" yet.

The topic of this exercise is Lenz's Law. This "lab" can actually be done either with real equipment in a laboratory setting, or done using a virtual setup. The PhET virtual setup for Faraday's Law is perfectly suited for this:

If you are doing this as a real experiment, you will need a solenoid, a bar magnet, a galvanometer with the "zero" position at the center of the scale, and some connecting wires. I've used a tightly-wound homemade solenoid, and it works fine. Note that you will need to know the direction of how the solenoid is wound (i.e. you need to be able to see the windings) so that you can figure out the sense of rotation of any current flow in the solenoid.

For this exercise, I will use the PhET simulation. I have used this as part of my in-class lecture on this topic, since every student in my class has a laptop or tablet and can access the PhET website during class. And yes, they were reminded to bring those devices to class for this topic.

Let's start with the aim of this exercise: It is to let students figure for themselves a "general rule" on when there is current in the circuit, and the direction of this current.

Keep in mind that the whole principle of this revamped lab idea is that the instruction is kept to a minimum, the students do not need to know the actual physics concept or principle, and we let them discover or learn about the phenomenon for themselves. So with that in mind, my only instruction to the students is this:

By using the magnet and moving it in and out of the solenoid, find a GENERAL RULE on when there is current in the circuit, and the direction of this current. Your rule must be able to tell  me that by doing an action, it will or will not result in a current in the circuit, and the direction that this current flows.

I give them a bit of a guideline, especially for those who are a bit clueless on what to do.

  • To simplify things, first figure out the direction of current flow in the solenoid if the galvanometer deflects to the right, and the direction of current flow if the galvanometer deflects to the left. Let's define the point of view by looking at the solenoid from the right, i.e. from where the magnet is in the figure above. The galvanometer will deflect to the right if current flows into it via the positive terminal, while it will deflect to the left if current flows into it via the negative terminal. So if one were to trace this path carefully, one can see that when viewed from the right, the current goes clockwise for positive deflection, and counter-clockwise for negative deflection. Armed with this info, we don't need to figure out anymore the sense of rotation of current in the solenoid, since by looking at the galvanometer deflection alone, we can tell the direction immediately.
  • Next, since we only care about when there is current in the circuit, and the direction of this current, and not its magnitude (yet), we can simplify the relevant magnetic field coming out of the bar magnet. For this exercise, we can just consider the magnetic field along the pole of the magnet, i.e. the direction of the magnetic field at the two pole ends. So a student must be given the information (if he/she doesn't know it already), that the magnetic field points straight out from the N-pole of the magnet, while at the other end, the magnetic field points straight in into the S-pole of the magnet. This is defined via convention.
  • A few students will simply not know what to do or how to start, so I give them a list of things for them to check out: (i) move the N-pole of the magnet into the solenoid while watching the galvanometer. (ii) stop moving the magnet and leave the N-pole inside the solenoid. (iii) move the N-pole of the magnet out of the solenoid while watching the galvanometer. (iv) repeat the same thing steps with the S-pole of the magnet.
  • Remind the students that they need to be able to describe clearly and succinctly a general rule for what they see. This means that they are required to convey, in writing, what they understand (communication skills). I tell the students that once they think they have written their general rule down, TEST it. See if their general rule explains everything that they observe in this exercise. For example, does their general rule explains why the galvanometer shows no deflection (i.e. no current) when the magnet is not moving inside the coil?
If this were done as a real experiment, I required the students to write exactly what they did and what they observed at every step.

What this exercise does is (i) to force students to think analytically on how to understand and make sense of what they observe and (ii) to get the students to communicate clearly what they understand in their heads into written form. Both of these are invaluable skills, and not just in physics. The second part is not as trivial as you think, because I find that a lot of students still have not mastered the art of conveying something in their heads via written communication (students who are not native English speakers will have a tougher time with this part so they may need extra assistance).

In my lessons, I introduce Lenz's law as the "qualitative" description, and then follow it up with Faraday's Law as the "quantitative" description of the same phenomenon. In addition to showing how Faraday's law "explains" Lenz's law, it will also allow you to explain why the galvanometer deflects with different amplitudes depending on how fast you move the magnet in or out of the solenoid. But this is one as part of the class lesson rather than as part of the lab exercise.

I toyed with the idea of putting the "answer" here (it is not as if one can't google for it), but I'm going to leave it out for now and let any interested party try it out. I will update this post at a future date to include my version of the "general rule" that reflects what Lenz's law says.


Monday, March 26, 2018

Newton's Gravitational Law Still Valid At Sub-Nanometer Scale

A new experiment using neutron scattering off noble gasses has shown no deviation from Newton's gravitational law at 0.1 nm scale.

The team fired pulses of neutrons at a chamber filled with either helium or xenon gas and monitored both the travel time of the neutrons through the gas and the neutrons’ scattering angles. From these measurements, they reconstructed the scattering process with the aid of simulations. They found that the scattering-angle distribution fit the predictions—based only on known laws of physics—for neutrons bouncing off gas nuclei. This result indicates that, within the sensitivity of the experiment, no unexplained force—be it modified gravity or another type of interaction—acts on length scales below 0.1 nm.

This one may not be as transparent, since it required quite a bit of reconstruction to simulate the interaction. So while the length scale being probed has improved considerably, I'm not so sure on how convincing this result is.

Still, where are those curled-up extra dimensions anyway?


Thursday, March 22, 2018

Fermilab Accelerator Complex

This is a neat animation video of the Fermilab Accelerator Complex as it is now, and all the various experiments and capabilities that it has.

Of course, the "big ring", which was the Tevatron, is no longer running now, and thus, no high-energy particle collider experiments being conducted anymore.


An Astrophysicist Describes Stephen Hawking's Last Paper

The astrophysicist in this case is, of course, Ethan Siegel, who I've cited here a few times.

In this article, he describes what Hawking's last paper is all about, if you want simple description of it. The link to the preprint (we'll update this post if and when it is published) is also given if you don't have it already.

Here is, in a nutshell, what they do. They create a (deformed) conformal field theory that is mathematically equivalent (or dual) to an eternally inflating spacetime, and investigate some mathematical properties of that field theory. They look, in particular, at where the border of a spacetime that inflates for an eternity (forward in time) versus one that doesn't, and choose that as the interesting problem to consider. They then look at the geometries that arise from this field theory, try to map that back onto our physically inflating Universe, and draw a conclusion from that. Based on what they find, they contend that the exit from inflation doesn't give you something eternally inflating into the future, with disconnected pockets where hot Big Bangs occur, but rather that the exit is finite and smooth. In other words, it gives you a single Universe, not a series of disconnected Universes embedded in a larger multiverse.

There! Do you even need to read the actual paper after that?


BTW, let's also give some love to his co-author, Thomas Hertog, who seems to be left out in many of this discussion and news articles.


Tuesday, March 20, 2018

Micro Fusion In Nanowires Array

This is a rather astounding result. The authors have managed to cause deuterons-deuterons fusion in an array of nanowires via igniting it using only joule-level pulsed laser[1], i.e. not using the huge, gigantic lasers such as that as the National Ignition Facility.

This is an open-access paper and you can get the full version at this link.

And no, before you jump all over this one and think that this is the next fusion power generator, you need to think again. The authors are touting this as a viable (and cheaper) ultra-fast pulsed neutron source, which can be useful in many applications and studies.


[1] A. Curtis et al., Nature Communnications DOI: 10.1038/s41467-018-03445-

Thursday, March 15, 2018

SQUID: History and Applications

No, this is not the squid that you eat. It is the Superconducting Quantum Interference Device, which is really a very clear application of quantum mechanics via the use of superconductors.

This is a lecture presented by UC-Berkeley's John Clarke at the 2018 APS March Meeting.


Wednesday, March 14, 2018

Stephen Hawking: 1942–2018

Of course, the biggest physics news of the day is the passing of Stephen Hawking at the age of 76.

Unfortunately, as popular as he is in the public arena, it also means that he left us without being awarded the highest prize in physics, which is the Nobel prize. This isn't unusual, especially for a theorist, because there are many theorists whose contribution became of utmost importance only many years later after they are gone.

Still, as a scientist who had attained a highly-unusual superstar status among the public, I will not be surprised if he has had a lasting impact of the field, and the perception of the field among the public and aspiring physicists.

RIP, Stephen.


Tuesday, March 13, 2018

Twin Paradox - The "Real" Explanation

So this thing doesn't seem to go away. The Twin Paradox is a common question that gets asked in class and online. And of course, the most common answer being given to explain away the paradox is that there is a broken symmetry between the two twins, and thus, they should not experience the same thing. But often, this involves one twin experiencing an acceleration/deceleration, which the other twin did not experience.

However, in this video, Don Lincoln tries to correct the explanation and argues that even without any acceleration/deceleration, one twin will STILL not have the same set of experience (he/she is in two different reference frame, while the "non-moving" twin stays in just one reference frame) when compared to the other twin, and thus, this broken symmetry resolves the twin paradox.

The math is simple algebra, but you do have to keep the notation straight, and the signs.


Teaching Intro Physics To Life Science Students

Teaching intro General Physics to Life Science/Bio students is something I do regularly. And it can be quite challenging because, in my case, calculus is not required and isn't used in the lesson. So there are many things that can't be easily derived from scratch.

I've resolved, a long time ago, that the approach to teaching such a class has to be different than the approach to teaching the calculus-based class, which is often populated by physics, chemistry, and engineering majors. In my experience, the average math skill is lower in the non-calc-based general physics class, which isn't too surprising. But more challenging than that, there is less of an interest and inclination towards the physics subject from such students. Most, if not all, of the Life Science/Bio students are in the class because they have to, and some even have an active dislike of the subject matter.

So it is definitely a challenge to not only convey the material in an understandable manner, but also to perk up their interest in the material so that they will do well in the course. It is why I tend to read papers like this one, which studied the correlation between life science students' interest, attitudes, and performance in a general physics class.[1] In particular, I'm always interested in using examples from biology/medicine to illustrate the particular physics topics that we cover in a lecture. As concluded in this paper, tailoring the subject matter to overlap with what the students are majoring in can affect not only the interest in the subject, but also their performance. This is a no-brainer for many of us, but this paper clearly shows the correlation.

BTW, it helps if the text being used is also geared towards the life science students.  The one that I had used before is "College Physics" by Giambattista, Richardson, and Richardson. I like the part where at the beginning of each chapter, it lists out some of the relevant applications in biology, medicine, etc. I just wish that the text has more examples from such areas, and more homework exercises in those areas, the way the paper described the examples and problems that were used in the course.


[1] C.H. Crouch et al. Phys. Rev. Phys. Educ. v.14, 010111 (2018).

Friday, March 09, 2018

Fusion Power Is 15 Years Away?

This news article is reporting that "MIT scientists" is predicting that we will have nuclear fusion power in 15 years time.

The project, a collaboration between scientists at MIT and a private company, will take a radically different approach to other efforts to transform fusion from an expensive science experiment into a viable commercial energy source. The team intend to use a new class of high-temperature superconductors they predict will allow them to create the world’s first fusion reactor that produces more energy than needs to be put in to get the fusion reaction going.

Bob Mumgaard, CEO of the private company Commonwealth Fusion Systems, which has attracted $50 million in support of this effort from the Italian energy company Eni, said: “The aspiration is to have a working power plant in time to combat climate change. We think we have the science, speed and scale to put carbon-free fusion power on the grid in 15 years.”

Interestingly, there was no direct quote from any MIT scientists here who is working on the project. The article quoted MIT's vice-president for research, but she's not working on this project.

So essentially, it appears that no one from MIT is making this claim, but everyone else on the peripheral is.

Let's mark this and check back in 15 years. Still, I will not be holding my breath.


Wednesday, March 07, 2018

Seeing Anyons With STM?

This is a very intriguing theoretical paper that proposes the detection of anyon using STM (you get free access to the actual paper from the website). The detection involves the measurement of the local density of states (LDOS), and then counting the resonance "rings". This is shown in Fig. 1 and 2 of the paper.[1]

This is quite a fascinating idea, because to get these fractional effects, one has to have a 2D confinement of the charges involved.

Now it becomes a race in seeing who might be able to produce such an experiment to detect these rings. STMs are pretty common, but it is now a matter of having the suitable material to see this.


[1] Z. Papic et. al. PRX v.8, 011037 (2018).

Tuesday, March 06, 2018

Magnon Transistors

A number of papers appear almost simultaneously on the invention of "magnon transistors". Instead of a transistor that directs the direction of electronic current flow, these are transistor that direct magnetic spin current flow, i.e. magnon flow.

Magnonic devices run exclusively on spin currents. (Spintronic devices, another electronics alternative, include both charge and spin currents.) To picture a magnon, imagine a row of spins pointing up, representing a magnetic material, and then imagine briefly flipping the spin at one end. This motion leads to a propagating wave that moves through the material as each spin influences its neighbor. Magnons can travel quickly and efficiently over long distances—up to about a centimeter in the best materials—without significantly losing energy or heating up the material, a feat not possible for electrons. But before building fast and efficient magnonic circuits, researchers need components that can regulate magnon currents.

I know I have been repeating this over and over again, but this is another example where basic research in condensed matter/solid state physics is now finding application in modern electronics.


Thursday, March 01, 2018

Thermal Footprints of Early Stars

Imagine being able to detect signals coming from the first stars formed in our universe, almost 180 million years after the Big Bang. This is why this astounding feat has been receiving popular media coverage.

A new paper published in Nature this week reports on the measurements of thermal radiation from such events.

A long-standing theory that still awaits testing predicts that absorption of UV radiation from early stars by nearby clouds of hydrogen could have driven TS back down to TG, but not lower. In other words, the cosmic dawn would make the gas seem colder when observed at radio frequencies. This would create an absorption feature in the spectrum of the background radiation left over from the Big Bang.

Bowman et al. now report the possible detection of just such an absorption signal. The authors measured TS , averaged over much of the sky and over a contiguous range of radio frequencies; each frequency provides a window on a different time in the Universe’s past. The measurement is very difficult because it must be performed using an extremely well-calibrated VHF radio antenna and receiver, to enable the weak cosmological signal to be separated from much stronger celestial signals and from those within the electronics systems of the apparatus used. 

For those of you who are not familiar with science, when you read the link, please read how the experimenters made the effort to ensure that their results are not due to their experimental technique or instrumentation.