1ENVIR 202: EARTH, AIR, WATER 22 Jan 2003 BACKGROUND DISCUSSION FOR THE SCIENCE CORE: ENERGY P.B. RHINES 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 most relevant to your experiments with the most attention, and then begin to spend some time reading the others. Everyone should look at 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, 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: sunlight’s colors, its power and energy, and what happens when it passes through the atmosphere E2 Lenses and Mirrors: concentrating energy and taking apart sunlight, ray by ray E3 A Model River: generating a flow in a water channel with electric propulsion (energy conversion, electrical to mechanical, the reverse of hydropower): E4 A Heat Engine: an engine using heated air to make mechanical energy E5 My Candle Burns at Both Ends: measuring the useful energy content of fuels (energy conversion, hydrocarbon => heat) E6 Blowing on Your Soup: conduction and convection (thermal energy flow in solids and fluids) E7 A Solar Pond (energy conversion and storage, solar to thermal): E8 Your Next Car? The hydrogen fuel cell (energy conversion, chemical to electrical and reverse). E9 Bicycle Power: generating electricity from mechanical energy (which comes from chemical energy) E10 The World’s Simplest Electric Motor: a solar powered motor based on simplicity.2 E1 SOLAR SPECTRUM: SUNLIGHT’S COLORS AND ITS ABSORPTION 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’s orbit about the sun is an ellipse, not a circle. This variation ranges from 1345 to 1438 watts per square meter, and we will see later this term that this variation is one of the driving forces of the huge climate variations that are 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 .. bent. The speed of light in glass o 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 figures below show the actual intensity of light above the atmosphere and a the Earth’s surface, as it varies with wavelength of light.** Also shown is the curve calculated using theory 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. Notice on the second of the two figures how 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.34 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