Physics Department

In Defense of the Bohr Model

2012-02-09T23:37:00.000-08:00

Scientific American journalist Jennifer Ouellette writes about why we should hold on to the Bohr model of the atom despite it not actually being a real description of what an atom looks like...

One of the standout anecdotes in Carl Zimmer’s most excellent compilation, Science Ink (a.k.a. My Favorite Science Book of 2011 And Possibly Ever) occurs in the first few pages:

“A former student [physics major] got a tattoo of a cartoon atom on the back of one of his legs. He told me that the first day after he got it, he went to rugby practice, and was showing it to someone when one of the seniors on the team (also a physics major) walked by. The senior looked at it, and said, ‘Oh, please. The Bohr model?’ And walked off.”

Oh, snap! Guess that poor underclassman got told! And he must live with the shame of his naive physics knowledge on his skin permanently (barring modification or tattoo removal treatments). Welcome to Hipster Physics!
Seriously, though, this is not the first time a physicist has complained about the much-maligned Bohr model of the atom. It’s like a rite of passage, the day you learn that the eye-catching little diagram of a small nucleus orbited by electrons you see all around — from the logo of the US Atomic Energy Commission, to the scene changes in episodes of The Big Bang Theory — simply isn’t the most accurate model for the atom anymore among “serious” scientists (or science writers). And espousing it is grounds for mockery, usually in the form of polite snickers and chuckling condescension from those “in the know.”

Clearly I am not a hipster, because I love the Bohr model, and will staunchly defend its use — at least in popular physics books for general audiences, and introductory courses for undergraduates. Sure, it’s been superseded since Niels Bohr first proposed it in 1913, as our understanding of the quantum world has advanced. I’m not advocating its return to cutting-edge physics research. But when it comes to outreach, it’s the perfect entry-level model for atomic structure.
Let’s get into the Wayback machine and go back to the dawn of the 20th century, just after J.J. Thomson had discovered the electron, and proposed his “plum pudding” model of the atom (see below). Bear in mind that for centuries, physicists had fought the very idea of an atom, despite the fact that Democritus had been an “atomist” two thousand years earlier. (The Time Lord likes to point out that, in this respect, the chemists were way ahead of the physicists; they accepted the existence of the atom much earlier.)
Thomson initially called his mysterious little particles “corpuscles,” and suggested that they were the primary components of an atom: a collection of negatively charged “plums” immersed in a positively-charged “soup,” or “pudding.” But then, in 1909, Ernest Rutherford went and discovered the atomic nucleus via a classic scattering experiment involving gold foil. Such an effect (scattering of alpha particles) occurred because there was a hard, dense center to atomic structure.
Thomson’s plum pudding model was handily discarded, and in its stead, Rutherford proposed something more akin to the planets orbiting the sun in our solar system. The nucleus serves as a “sun” at the center, and is positively charged, while the electrons are “planets” and negatively charged, moving about the nucleus in circular orbits.

It was pretty close to the popular design we’re familiar with today, but it violated classical physics in a very important way: if the Rutherford model were correct, the electrons would emit radiation as they orbited, such that over time, the electrons would spiral inward and collapse into the nucleus. All atoms would be inherently unstable. Since they weren’t, obviously something else was going on.
Furthermore, the frequency of the radiation would increase as the electron spiraled inward, because the orbit would get smaller and the electron would move ever-faster. That just didn’t happen. And this model didn’t agree with electrical discharge experiments demonstrating that atoms only emit light (electromagnetic radiation) in discrete frequencies, leading to Max Planck proposing “quanta” in 1900, thereby launching a revolution in physics.

Phew! Clearly, Rutherford’s model needed to be brought in line with the nascent field of quantum mechanics before it could be truly viable. Enter a young Danish upstart named Niels Bohr, who’d come to Rutherford’s lab via a postdoc with Thomson after earning his PhD in physics from the University of Copenhagen. Bohr set about adapting Rutherford’s model to accommodate the need for discrete units of energy (the quanta).
The model he came up with is the one we know and love today (often termed the Rutherford-Bohr model), in which electrons move about the atomic nucleus in circular orbits, just as in Rutherford’s model. But those orbits have set discrete energies, and those energies are related to an orbit’s size: the lowest energy, or “ground state,” is associated with the smallest orbit. Whenever an electron changes speed or direction (according to the Bohr model), it emits radiation in the specific frequencies associated with particular orbitals.

Diss the Bohr model all you like — that innovation snagged its creator the 1922 Nobel Prize in Physics. As Sheldon Cooper would say, “Bazinga!”
Yeah, okay, it’s not perfect. The biggest issue is that it violates the Uncertainty Principle (which wasn’t even formulated until 1927). Remember, the principle states that you can’t correctly pinpoint both a particle’s position and momentum (energy) at the same time, and in the Bohr model, you’ve got electrons with both known orbits and well-defined radii.
(There’s also other shortcomings related to predictions about the spectra of larger atoms and the relative intensities of spectral lines, yadda, yadda, yadda, but we’re focusing on the most major objections for the sake of simplicity. John and Jane Q. Public are not lying awake at night quibbling over the Zeeman effect.) And technically, the electrons don’t really “move” around the nucleus in orbits. Erwin Schroedinger (of the famous cat paradox) was the one who proved that electrons are really waves (although they show up as particles when you perform an experiment to determine its position), and those waves are stationary.

Sure, you can check to see where an electron is, but each time you do, it will show up in a different position — not because it’s moving, but because of the superposition of states. The electron doesn’t have a fixed position until you look at it and the wave function collapses. (However, if you make a ton of measurements and plot the various positions of the electron, eventually you’ll get a ghostly orbit-like pattern such as the one depicted above.)
That’s why Schroedinger’s atomic model dispenses with orbitals in favor of energy levels, which is what physicists really care about anyway. It still shares some similar concepts with the Bohr model. For instance, if an atom heats up (i.e., is energized), its electrons move to higher levels. As they cool and fall back to their normal ground state, the excess energy has to go somewhere, so it’s emitted as photons, which our eyes perceive as light. And those photons possess frequencies that match the change in energy levels, in keeping with earlier experiments.
Confused yet? No wonder! To understand why physicists discarded the Bohr model, you’ve got to delve into the mind-bending intricacies of quantum mechanics, and explain all kinds of things the average person likely has never encountered in any real depth: wave functions, uncertainty, superposition of states, spectral lines, and so on.

That’s why I prefer the Bohr model to introduce non-scientists to the basics of atomic structure. It gets across the basic concepts (discrete intervals and why light is emitted in specific units of frequency), and offers the neophyte a handy visualization via the analog of the atom as small-scale solar system. There’s plenty of opportunity to enhance someone’s understanding later as they progress in their basic physics knowledge — education happens in stages, not all at once. In fact, the Bohr model offers the perfect opening to talk about some of those more advanced ideas.

Source: Scientific American

Scale of the Universe

2012-02-09T09:39:00.001-08:00

Here's a link to one of the coolest animations I've seen on the scale of things in the universe, from quarks to people to the limits of the observable universe.

http://images.4channel.org/f/src/589217_scale_of_universe_enhanced.swf

Assume a frictionless vacuum....

2012-02-08T11:23:00.000-08:00

Static electricity in space

2012-02-08T11:08:00.000-08:00

Expedition 30 astronaut Don Pettit uses knitting needles and water droplets to demonstrate physics in space during his mission on the International Space Station. This is part of the first video in a series for a partnership between NASA and the American Physical Society to share unique videos from the International Space Station with students, educators and science fans from around the world.

Famous Irish Astronomer

2012-02-02T03:44:00.000-08:00

THE ASTRONOMER Jocelyn Bell-Burnell is one of Ireland’s most accomplished scientists. While still a research student she discovered pulsars and went on to become a distinguished scientist who made important astronomical discoveries.

She is a true hero of Irish science for her many accomplishments and for her ongoing contribution to a better public understanding of science. Her discovery of pulsars is one of the famous stories in science and it is also one of the most infamous.

Read the full article in the Irish Times here.

Online Junior Science Quiz

2012-02-02T03:42:00.001-08:00

Revise your Junior Cert Science with these handy online quizzes.
http://www.sciencequiz.net/jcscience/index.html

Misconceptions in Electricity

2012-01-13T08:40:00.000-08:00

When electrical engineer William Beaty was working on the design of an electricity exhibit for the Boston Museum of Science, he decided to check out some elementary school science textbooks in search of good ways to communicate fundamental concepts on the subject.

Bad idea.

What he found was a morass of misconceptions, mistakes and misinformation in one text after another. Not one of the books, he found, even contained what he considered to be a valid definition of what electricity is, much less how it works. And he discovered something else: Even his own understanding of the subject, despite his years in the profession, was flawed; he was still the victim of deeply-help misconceptions that he had learned in grade school.

''The majority of my misconceptions had been specifically taught to me,'' he said. ''[They] were in my science textbooks long ago, and they were still in most modern textbooks.''

For a detailed look at misconceptions in physics and especially electricity, check out William's website here.

Found by Noel Cunningham

Teaching Waves with Google Earth

2012-01-12T12:18:00.000-08:00

Google Earth is a huge source of interesting illustrations of various natural phenomena. It can represent a valuable tool for science education, not only for teaching geography and geology, but also physics. This paper suggests ways that Google Earth can be used for introducing the physics of waves.

Click here to download the pdf.

Found by Noel Cunningham

Online Leaving Cert Physics Experiments

2012-01-12T12:06:00.001-08:00

This site has a great selection of java simulations for revising many of the experiments on the Leaving Cert physics course.

http://www.mathsphysics.com/Physics/applets.html

Found by Noel Cunningham

How to die when falling into lava

2011-12-30T05:17:00.000-08:00

So what will actually happen when the evil villain falls into that burning pit of lava? It turns out that everyone is wrong about how people die when falling into lava. To explain what really happens, here's Erik Klemetti from Wired Science.

“In that scene from Return of the King when Gollum falls into the pit of lava, would he have really just sunk into the lava like that?”

At first, it seemed like an easy question. Well, not so much easy as obvious: yes. However, the more I thought about it, the more I though that pretty much every scene I’ve ever noticed where somebody falls into lava and dies has got to be wrong.

Some are just straight out obvious to explain — like in Volcano when the subway maintenance director jumps from the subway car that is being inundated by lava after he saved the unconscious subway driver. The guy jumps from the subway, but not far enough to miss landing in maybe 6 inches of basaltic lava and he proceeds to more or less melt away into the lava like the Wicked Witch of the West. Not likely. Maybe some very severe burns, maybe lost feet (think Darth Vader), but no wholesale melting like that.

However, the death of Gollum at the end of Return of the King got me thinking. Gollum, if you remember, dove into the lava of Mount Doom after his precious ring was thrown in — he proceeds to sink into the lava (see below) and leaves the ring floating on the lava until it melts away. Guess what? Sinking into lava just will not happen if you’re a human (or remotely human). You’d need to be a Terminator to sink into molten rock/metal … and here’s why.

My precious. Wait, why I am sinking?

Molten lava is nothing like water. Sure, everyone thinks that liquid rock (magma) is going to behave like any other liquid (e.g., water), but there are some key physical properties that tell us it just isn’t the case. Let’s compare!

Water has a density of 1000 kg/m³ and a viscosity of 0.00089 Pa*s.
Lava has a density of 3100 kg/m³ and a viscosity of 100-1000 Pa*s.

Pa*s is the SI unit for viscosity — some people might be familiar with other viscosity measures like poise. Viscosity is, more or less, the resistance to flow, so if you throw something in a liquid, a low viscosity liquid (like water) will “get out the way” and you’ll sink faster relative to a high viscosity liquid (like cold corn syrup). The density of the liquid will also play a role in how quickly you might sink based on your own density. So, when we’re looking at water versus lava, lava is ~3.1 times the density and between ~100,000 to 1,100,000 times the viscosity. They are very different!

Luigi is with the cat on the notion of falling into lava.

Do you suppose throwing yourself into lava would have the same effect as falling into a lake? Probably not. The average human has a density of ~1010 kg/m³, so a little bit more dense than water. That is why we can sometimes float in water and sometimes sink in water. You can control your buoyancy (to an extent) by how much air you hold and whether you choose to swim in fresh water or salt water (which has a density of 1027 kg/m³, so you’re more likely to float). If you are less than one-third the density of basalt (and you are, admit it), it is going to be next to impossible to sink into that liquid.

You can try the experiment at home (without grievous bodily harm). Take your favorite motor oil (I prefer 5W30) at room temperature and fill a small pail. Motor oil at room temperature should have a density of ~920 kg/m³ and viscosity of ~1 Pa-s – this will be your lava. Cut a little fellow out of styrofoam. It has a density of ~300 kg/m³, so it is roughly 1/3 the density of the oil. Now, position your Styroguy on the edge and push him in. Does he sink instantly into the oil? No! So, neither should you in you fall into lava. Now, Stryoguy didn’t get the full effect by then proceeding into bursting into flames, which would be your bonus for falling into lava — remember, most of the red-hot lava pictures in movies like likely basaltic lava at ~1,100 to 1,200°C (for comparison, your oven on broil is ~275°C). However, if you’re already in a position to fall into lava, you had it coming.

Just like if you fell into lava, Stryoguy floats on oil when you push him in.

So, there you have it. The next time you see someone in a movie fall into a cauldron of roiling lava and they sink into it, remember that it can never happen. The sacrifice of styroguy will be remembered.

Source: Wired Science

Careers in Physics

2011-12-30T05:03:00.000-08:00

Ever wonder where a career in physics will take you?

Check out the Institute of Physics website here as they look at a day in the life of 28 different physics graduates. For more information download the full pdf here.

Surface Tension Exhibit

2011-12-21T11:16:00.000-08:00

The current exhibit in the Trinity Science Gallery is SURFACE TENSION, it closes on 20.1.12 so there's still a few weeks to see it before it closes.

The future of water is the subject of tension. Water is both disposable and sacred, a muse for artists and a necessity for life – a source of healing and of conflict. The Earth has abundant water, but only a very small proportion is available for human use. How should this be managed and sustained, and what would a water-scarce future look like?

SURFACE TENSION brings together work by artists, designers, engineers and scientists to explore the future of water, playing on its physical properties, its role in politics and economics, and ways in which it may be harnessed, cleaned, and distributed.

Birth of a Star

2011-12-21T05:35:00.000-08:00

NASA’s Hubble Space Telescope has spotted a young star undergoing violent birth. The star, named S106 IR, has a mass of about 15 times that of our sun and lies approximately 2,000 light-years away in the constellation Cygnus, the Swan. Formed from a cloud of gas and dust with more than 25,000 times the sun’s mass, the star is just about to mature and settle down to what astronomers call the main sequence portion of its life, where it will glow steadily like our sun.

But before it grows up, the star is releasing a fierce torrent of ultraviolet radiation, heating up the surrounding cloud to temperatures greater than 18,000 degrees Fahrenheit. This causes the hydrogen gas to glow blue. The cooler, red dust lane in the center partially hides the star from view but it can still be seen shining near the lower part of the image.

Most young stars blast tons of energy and dust, creating gigantic butterfly-wing lobes on their sides like the ones seen here. Within about a million years, the object will end this forceful stage and become a giant blue star, shining brightly as it burns hydrogen into helium.

While its birth has clearly been fierce, the end of its life will be marked by an even more violent explosion: a supernova that will outshine galaxies.
Source: Wired Science

Ultimate Graphing Challenge

2011-12-21T05:23:00.000-08:00

Check out www.theuniverseandmore.com for a fantastic graphing challenge game.

Found by Noel Cunningham

Electrostatics Presentation

2009-04-03T04:00:00.000-07:00

Class notes on Electrostatics

Great Physics Applets and Animations

2008-12-13T06:57:00.000-08:00

These will complement your studies in Physics.

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Flash/#nuclear

2008-12-11T05:28:00.000-08:00

Please visit the following site for some interesting Physics applets: http://jersey.uoregon.edu/