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UW ENVIR 215 - Energy Experiments

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ENVIR215 Spring 2005 1 Energy Experiments: Background A document prepared by Peter Rhines (the previous instructor of this class) Because our textbook relates to energy use, technology and impacts only, we need some more explanation of the science behind the lab experiments. This can help you in doing your final lab-book write ups of the experiments. We come from many backgrounds, so some of you will know most of what follows, others will not. The idea is to move from where you are now (in scientific training) a step or two higher. Read the sections that are relevant to your experiments with the most attention, and then begin to spend some time reading the others. As you read this everyone should think about the basics of: • energy conservation (that is using ‘conservation’ in its scientific sense: energy is neither created nor destroyed), • energy transformation • energy transmission • energy efficiency. • energy ‘evaluation’ (how do we measure it) • the quantitative idea that ‘work’ transfers energy from one object to another, and ‘work’ (again, by its scientific definition) is equal to force times distance, the force exerted on a body times the distance the body moves. The example below is the heat engine in E4 Notice that some of the energy converting devices in the experiments can be reversed: and electric motor becomes a generator (perhaps generating hydropower); water is split into hydrogen and oxygen gas by passing an electric current through the water, and the reverse reaction is the fuel cell, with hydrogen gas used to make electricity without burning it. It is less easy to see this ‘reversibility’ in the long chain of energy transformation from sunlight to fossil fuel to gasoline engines: in fact we overwhelmingly feel that fossil fuel burning is using up sunlight stored over millions of years. Still some of the chemistry involved can be reversed. Amory Lovins describes chemistry labs as places where fairly natural basic chemical substances are combined to make lots of toxic waste. There is forward-looking university course in a Swiss university where the students reverse this, taking toxic chemicals and turning them back into harmless elemental substances. As you read through this follow some of the web links, and find more on your own. For example, a simple Google search for ‘diffraction grating’ brings up a wonderful set of descriptions. E1: SUNS AND RAINBOWS: Examining the solar spectrum. We think of sunlight as ‘pure’ light or ‘white’ light. Actually it is light radiated from a hot object, and its intensity is a maximum at a particular wavelength…or color…and drops off at other wavelengths. It is the prime source of energy for nearly everything on Earth. There is also some heat coming up from deep in the Earth, some nuclear energy, natural and manmade, and some gravity forces that raise the ocean tides. Even fossil fuels like oil (‘liquid sunshine’) are stored solar energy. The intensity of sunlight is about 1390 watts per square meter, outside the Earth’s atmosphere. The intensity varies a bit over the course of the year because the Earth’sENVIR215 Spring 2005 2 orbit about the sun is an ellipse, not a circle. This variation ranges from 1345 to 1438 watts per square meter; this variation is one of the driving forces of the huge climate variations that result in the ice ages. Because of the various angles and shadows involved, the average sunlight hitting the Earth over the course of the year is ¼ of the above numbers, averaging 344 watts per square meter of Earth’s surface. That is, if the atmosphere were not in the way. The peak sunshine at noon in Seattle, including the atmosphere’s effects, is roughly 1000 watts per square meter (written watt m-2), but this is just for a short time each day. Have a look at the E2 experiment on prisms and lenses as well as this one. When the sun shines through a prism its path is refracted or bent. The speed of light in glass or plastic varies slightly from the values below, for a variety of wavelengths. This splits sunlight into a rainbow of colors, with correspondingly different wavelengths. We use a low-power red laser to explore the refraction of sunlight one ray, that is one color, at a time. The figure above shows the actual intensity of light above the atmosphere and a the Earth’s surface, as it varies with wavelength of light*1 Also shown is the curve calculated using theory 1 * Planck’s formula describing the ideal solar radiation above the atmosphere is 252(exp( / ) 1)c hMhc kT!" "=# in units of watts per square meter, per meter of wavelength. c is the speed of light, h is Planck’s constant and k is Boltzmann’s constant, T is the absolute (Kelvin) temperature. here exp(x) is a way of writing the constant e (= 2.718) raised to the power x.ENVIR215 Spring 2005 3 by Max Planck for the radiation of a heated body at 59000 C, which is in very good agreement; apparently the sun acts like a simple heated body at that temperature, which is close to what exists at the outer visible layers of the sun. This ideal curve of radiation intensity vs. wavelength is known as black-body radiation. On the second figures below, the light from various stars differs in the wavelength (and hence color) of its maximum radiation: the cooler stars radiate at longer wavelength, as we have suggested. Figures from RCA Electro-Optics Handbook, 1978. Sunlight is not quite ‘pure’; heavy elements in the sun’s atmosphere like iron block certain wavelengths, leaving thin black bands cut out of the spectrum. Also in passing through the atmosphere there are bites taken out of the curve, absorption lines, by interfering molecules everywhere in the air. Water vapor, carbon dioxide (CO2), methane (CH4), ozone (O3) and other gases are excited by light and absorb it; the energy becomes heat or acts to stimulate other chemical reactions, it cannot just vanish! In fact sunlight produces ozone and, in the upper atmospheric region known as the stratosphere this ozone protects us from the ultraviolet parts of sunlight. Ultraviolet rays have wavelengths shorter than violet, are not visible, and are veryENVIR215 Spring 2005 4 damaging to living tissue. Smog (‘photochemical smog’) is something we will meet later in this course, and it is a strong reaction involving oxides of nitrogen, ozone


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