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Talking about science
Go with the flow
What physics tells us about the energy crunch
Arthur Thurnau Professor of Physics and Astronomy
July 15, 2008
Everyone is talking about energy. Gas has risen above $4 a gallon. Profligate fossil fuel use drives climate change, the great generational concern of today's Michigan students. Should we build more nuclear power plants, trust in hydrogen, strive for clean coal, harvest the wind, go solar, or (gasp) conserve? Election year dynamics raise the volume still further. How is one to make sense of it all?
The first reflex of a physicist is to start with the basics, so let's try that. Energy is not a thing, a substance we can load into a tanker and ship to New York. It is, instead, a property which things possess. It's more like color or mass. You can't have a jug of green or a box of grams. Greenness and mass are properties possessed by things. Energy is like this too.
The concept of energy was introduced in physics by Thomas Young in 1804 when he wrote:
the name energy may be applied, with great propriety, to the product of the mass or weight of a body, into the square of the number expressing its velocity
I love the aside, "with great propriety." That's the language you use when trying to introduce an idea missed by the mighty Newton. Notice that Young doesn't tell us what energy is, but only how to determine how much there is.
Since Young's time we have found that energy appears in many guises, each of which is accompanied (with great propriety) by a way to calculate it. Young's energy of motion we call kinetic energy. Today we also speak of thermal energy, gravitational energy, chemical energy, radiant energy, and many others. Each "form" is just another piece of the accounting, another rule for calculating how much energy is present.
Surprisingly, we can't really say what energy is. Richard Feynman, one of the twentieth century's great physicists was clear about this in his famous Lectures on Physics: "we have no knowledge of what energy is." But we can tell you how much there is.
Physics has something more to offer; a pair of essential rules about how energy behaves. The first is simple; energy is conserved. It can never be either created or destroyed. Energy may move around, passing from one object to another. It may also change from one form to another. But whenever something happens in the universe, the total energy present before and after is precisely the same.
So why are we scrambling for renewable energy? If energy is conserved, isn't it all renewable? Why not just feed it back in, using it over and over? The explanation lies in the second fundamental rule about how energy behaves. Energy possesses an inexorable desire to spread out. This is personifying things a bit, and I don't mean to suggest that energy (whatever it is) actually wants to spread out. But it sure acts like that, and a couple examples should help clarify what I mean.
Put a hot cup of coffee, loaded with thermal energy, on the counter and you know what will happen. It will cool off. The thermal energy initially concentrated in it will flow out into the room until everything has an equal share, until everything is the same temperature. Your car roaring down the street is loaded with Young's kinetic energy. Slam on the brakes and you'll convert that energy of motion into thermal energy in your smoking brake pads. Perhaps this is too hidden to be convincing, so just imagine stopping the car by ramming your shoes into the ground like Fred Flintstone; very bad for the shoes. The car's energy is converted to heat which, like that in your coffee, leaks out into the environment.
This irresistible tendency to spread is the real challenge with energy. An energy resource for us is something in which energy has been temporarily stored. It sits there, ready and even eager to spread out. When we "use" energy, we free it up, allowing it to flow out from a concentrated source, passing along the way through systems of our choice. Oil, for example, locks up energy in the chemical bonds of its hydrocarbons. Put it in the right conditions (mix it with oxygen and fire off your spark plug) and it will release this locked up energy, allowing it to continue to spread.
Solar, wind, biomass, and hydro power all harness the flow of the sun's billion-billion Watts of energy.
Our technology takes many forms: engines, electric motors, light bulbs, and computers. All are just clever systems for harnessing energy as it flows outward and getting it to do what we want. Unfortunately this energy, after passing through, spreads out still further into the environment, as it must always do. The energy is all still there, but once spread out it is largely lost to us; we can't bring it together again for our convenient use. We can and should make our technology better, much more efficient, so that more of the energy that flows through is diverted for our purposes. But we can't escape the fact that energy is always diffusing, spreading until it becomes inaccessible.
Like energy conservation, this second principle is very deep, and impossible to evade. When we use energy, we are harnessing an inevitable flow, putting it to use as it passes through. The fossil fuels we clamor for are materials in which this irresistible flow has been long stalled, useful because they provide access to energy accumulated over many millions of years. Nuclear fuel (heavy elements like Uranium) is fundamentally the same. We've been drawing on this rich account of stored up energy for almost two centuries. As it slowly runs dry, we need to look to other ways of catching energy along its path from concentrated to diffuse.
Renewable energy resources are those in which we utilize energy more directly in its flow. Solar, wind, biomass, and hydro all involve catching energy on its current pass through the Earth. Each has the same ultimate source: radiant energy, racing away from the Sun, which pours down on the Earth. The billion-billion Watts of power the Sun provides to the Earth can be grabbed directly, or first harvested by wind, plants, or the water cycle. In any case, we're harnessing today's flow of energy for our current needs, rather than relying on energy stored up in eons past. These sources will be there so long as the Sun shines, another five to six billion years. I'm happy to say, with great propriety, that's long enough for me.
So while the details matter, the big picture is clear. We've had an impressive run, wildly releasing the energy locked up long ago in fossil fuels and heavy elements. That approach can't last forever. Fortunately the Sun provides a wonderful, practically permanent energy source. It rains down energy on the Earth constantly. We have but to harness that flow more directly, live within its limits, and our energy future is assured.
grew up in Michigan, in love with the natural world. He studied physics at Temple University and the University of Chicago, and now teaches it to anyone who will listen.
McKay joined the University of Michigan faculty in 1995, where he is currently Arthur Thurnau Professor of Physics and Astronomy, and Director of the LSA Honors Program.