Feeds:
Posts
Comments

Posts Tagged ‘physics’

Ice is a very common solid here on Earth yet one of the most puzzling. Take the seemingly simple question, “Why is ice slippery?”, for example. Common wisdom says that when you step on an icy surface the pressure melts the ice a little bit to create a thin layer of water that acts as a lubricant. It’s due to the unique property of water: the solid form is less dense than the liquid form. We take it for granted that an ice cube will float in a glass of water, but for most material the solid form would sink to the bottom. And because ice has a lower density it is also true that the melting point of ice is depressed as pressure increases.

Ice has a hexagonal crystal structure, but what happens at the surface boundary to make ice slippery?

The theory goes that as you walk on ice the increase in pressure lowers the melting point of the top layer of the ice and it melts for a brief moment, then refreezes as you pass by. The problem with this explanation is that the effect is very small and would only reduce the melting point by a few hundredths of a degree at most. Yet ice is still slippery when its temperature is far below the melting point.

One possible explanation is that friction plays a part as well. The act of walking on ice creates friction which heats the ice to create a slippery surface. But the problem with this is that ice is still slippery regardless of whether or not you are moving. If you are standing still there is no friction, yet it’s still slippery.

A better theory is that ice has an intrinsic liquid layer. Water molecules at the surface remain unfrozen because there are no molecules above them to hold them in place. This was first proposed in 1859 by Michael Faraday who noticed that two ice cubes will fuse together if they are pressed against one another. Faraday’s explanation for this is that the liquid layers freeze when they are no longer at the surface. But even this theory is not quite correct.

In 1996, a team led by Gabor Somorjai, a professor of chemistry at Lawrence Berkeley Laboratory, bombarded the surface of ice with electrons. By observing how they bounced off they were able to make an amazing discovery: What actually makes the surface of ice slippery are rapidly vibrating water molecules. These “liquid-like” water molecules do not move from side to side—only up and down. This is an important distinction. If the atoms moved from side to side, the layer would actually become liquid, which is what happens when the temperature rises above 0° C. It turns out that it is this “liquid-like” layer that makes ice slippery.

1) True or false: Ice floats in water because it is more dense.

2) The best theory to explain why ice is slippery relies on _________.
a) friction b) pressure c) a liquid-like surface layer d) all of the above

3) True or false: An increase in pressure will reduce the melting point of ice.

4) Michael Faraday first demonstrated that ice has an intrinsic ____________.
a) liquid layer b) hexagonal structure c) chemical makeup d) cubic structure

5) Research at Berkeley reveals that ice has a surface layer that is _________.

Read Full Post »

Fusion’s Future

This week we will wrap up our series on nuclear fusion by taking a look at one possible scenario for the future of fusion and how it could eventually play a part in ending our reliance on oil and natural gas.

Even though nuclear fusion does not produce radioactive waste directly, it does produce neutron radiation which does require shielding. But some fusion reactions are aneutronic. One such reaction is a pure reaction of helium-3 where two He3 nuclei fuse to create one helium-4 nucleus and two protons. Since the protons are electrically charged, they can be contained by an electric or magnetic field. And it gets even better, because by containing these protons it would be possible to convert this energy directly into electricity, bypassing the need to heat water and create steam which runs through turbines which then powers electric generators.

The fusion reaction of two helium-3 atoms yields one helium-4 atom and two protons plus energy.

But there are some problems, the biggest of which is that helium-3 is virtually nonexistent here on Earth. Our closest source for helium-3 is on the Moon, which has sparked a new space race which may one day lead to mining on the Moon. China, Russia, India, Japan and Germany have all declared their intention to make it to the Moon with the intent of eventually mining helium-3 and bringing it back to use as fuel for fusion reactors here on Earth. NASA, too, is scheduled to be on the Moon by 2020 and to have a permanent base by 2024. And while NASA has not specifically came out and declared an intention to mine helium-3, it does have advocates of helium-3 mining in influential positions.

The other problem is the extreme temperature required in order to begin a fusion reaction of pure helium-3, which is estimated to be six time hotter than the interior of the Sun. The only research facility currently doing successful helium-3 fusion reactions is at the Fusion Technology Institute at the University of Wisconsin-Madison, where they have been able to confine the reaction with a technology known as inertial electrostatic confinement (IEC). The benefit of IEC is that it doesn’t need a massive confinement structure—their experiment is table-top sized. It is estimated that we are 50 years away from creating clean fusion energy, but the potential advantages are so great there is no doubt that research will continue and possibly one day in the not-to-distant future clean energy will be more than just a dream.

1) True or false: Helium-3 is abundant here on Earth.

2) Helium-3 has ____ neutrons in it nucleus.

a) 1  b) 2  c) 3  d) 4

3) True or false: In order to create a fusion reaction of pure helium-3, temperatures in excess of those at the interior of the Sun are needed.

4) The technology currently being used to contain helium-3 fusion reactions is called _________________ (IEC).

a) initial early consignment  b) inertial electrostatic confinement  c) inertial elementary confoundment  d) internal electrostatic confinement

5) The closest place to mine helium-3 is at __________.

Read Full Post »

Cold Fusion made the cover of
Time magazine’s May 1989 issue.

In the spring of 1989, Stanley Pons and Martin Fleishmann, in partnership with the University of Utah, claimed that they were able to create a fusion reaction in a test tube at room temperature. By the first week of May, 1989, the news had made the cover of Business Week, Time and Newsweek—the first time the same story had been on the cover of all three magazines since the Kennedy assassination. Clearly this was big news. There was a tremendous desire for this to be true: The Chernobyl disaster was still fresh in people’s minds having occurred a year previously, and the day after the press conference the Exxon Valdez oil spill occurred. People wanted to believe in the possibility of a cheap and abundant source of energy that didn’t create greenhouse gasses or radioactive waste.

But before we get into the specifics of what happened, lets review what nuclear fusion is and how it works. Fusion is when two smaller atoms fuse together to make a larger atom. The most common form is hydrogen fusion, where two hydrogen atoms combine to form a helium atom, releasing massive amounts of energy in the process. This is what occurs in our Sun and in most stars. The benefit of fusion compared to its nuclear cousin, fission, is that it does not create any radioactive waste products. The downside is that it is much more difficult to create and contain. Hydrogen fusion itself can happen in several different ways, depending on the various isotopes involved. 99.98% of hydrogen has a nucleus with only a single proton (1H). Deuterium (2H) is a stable isotope that contains one proton and one neutron in its nucleus and tritium (3H) is an unstable isotope that contains one proton and two neutrons. The process that fuels our sun combines deuterium and tritium to make helium.

Deuterium fusion: two deuterium atom fuse to create helium 4, releasing energy in the process, which then either ejects a proton to create tritium, or ejects a neutron to create helium 3, releasing more energy in either case. (Neutrons are shown in blue and protons are shown in red.)

The nuclear process that was put forth as the mechanism for cold fusion used deuterium only, produced by running an electric current through heavy water (D2O). This causes the water molecules to split, liberating deuterium. The electrode in their experiment was made of palladium, which would then absorb the deuterium gas. Palladium has the uncommon ability to absorb up to 900 times its own volume of hydrogen at room temperature. The claim was that as the palladium absorbed the heavy hydrogen, its temperature shot up, and according to Fleishmann and Pons, created a net surplus of energy. So much energy, in fact, that during one experiment the water burned a hole in the beaker, the lab table it was sitting on top of and the floor below it.

Sounds great, right? Energy so cheap it wouldn’t even be worth metering. But shortly after their press conference, problems began to develop. The first problem was that they were not able to propose a mechanism for how fusion could occur at room temperature. In order to fuse atoms, one must overcome the substantial electrostatic force when you try and bring two positively-charged nuclei close enough together so that the strong nuclear force will be able to act upon them. Normally it would require temperatures in excess of 120 million °C. To say that the deuterium gas is compressed 900 times by a palladium electrode isn’t going to even get you close. Consider, for example, the planet Jupiter which has a layer of hydrogen near its core that is believed to be in excess of 30,000 °C and pressure tens of thousands of times greater than normal atmospheric pressure on Earth, yet this is not enough to fuse hydrogen which is why the gas giant is a planet and not a star.

Another problem was that their experiment was set up to measure heat and not the by-products of nuclear fusion, specifically neutrons, gamma rays, helium and tritium. In the following weeks, several labs were announcing that they could reproduce parts of the experiment, but not all. The Department of Energy could not reproduce the results. And despite all the claims, in the end there was never any solid evidence for the production of helium, gamma rays or neutrons. A few labs found small amounts of tritium, but not enough to be more than what could be explained by contamination. These labs were in the business of measuring nuclear products and thus had plenty of sources of possible contamination—something very difficult to avoid when you are looking for such low levels in the first place.

It turns out that data had been altered before going to congress to appeal for funding. A graph had been fudged to match expected results. By May 1989, the gig was up. The American Physical Society held a session on cold fusion, at which many reports of failed experiments were heard. At the session’s end, eight of the nine leading speakers said they considered the initial Pons and Fleischmann claim dead. Steven Koonin of Caltech called the Utah report a result of “the incompetence and delusion of Pons and Fleischmann” which was met with applause. Physics Today, in a 2005 report, stated that new reports of cold fusion were still no more convincing than 15 years previous. Its a good reminder that we must always rely on the scientific method to arrive at the truth.

1) True or false: Palladium is able to absorb up to 800 times its own volume of hydrogen.

2) Which of the following is not a by-product of fusion?
a) heat  b) neutrons  c) helium  d) heavy water

3) _________ is a stable isotope of hydrogen.
a) protium  b) deuterium  c) tritium  d) helium

4) In order to fuse two positively-charged nuclei, you must overcome a substantial _______ force.
a) strong  b) electrostatic  c) gravitational  d) nuclear5) What does science rely on to arrive at the truth?

Read Full Post »