The amount of this may be better appreciated by comparison. Thus, the pyramid of Cheops contains less than ,, cubic feet and weighs less than 7,, tons, and this water, then, in the form of ice. If we had to cart it away, it would require 2,, cars, carrying 12 tons each to remove it, and these, at an average inch of rain spread over the whole area of the United States is not an extraordinary day's rainfall throughout its territory, but it will be found by any one who wishes to make the computation that such a day's rain represents a good deal over the round sum of ten thousand of millions of tons, and that all the pumping engines which supply Philadelphia, Chicago, and our other large cities, dependent more or less on steam for their water supply, working day and night for a century, would not put it back to the height to which it was raised by the sun before it fell.
Every ton was lifted by the silent working solar engine, at the expense of a fixed amount of heat, as clearly as in the case of any steam pump, and this is the result of an almost infinitesimal fraction of the heat daily poured out from the sun! When we hold the right hand in warm water, the other in cold, for a few moments, and then plunge both in the same basin of tepid water, the two hands will give different reports; to the right the fluid is cold, to the left it will feel warm, though it is the same really to both, and we might vary the experiment by trying it with shade and sunshine.
In either case the experiment would convince us that our sensations were very untrustworthy, and that if we were going to measure the sun's heat we must depend on some sort of instrument and not on anything that can feel. The first things we have to do about the sun's heat is to measure it, not to guess at it—to measure it as accurately as we would anything which we could try with a foot rule or put in a pair of scales.
When we have done this we have a solid foundation to work on, and the doing this has been thought a worthy occupation of a considerable part of their lives by many able men. One of the first of these was Pouillet; others, such as Saussure and Herschel, had been at the problem before him, but his results were the most accurate until very recently, and even recent work has not materiallyaffected his conclusions.
His instrument is easily understood with a little attention. We have it represented in Fig. Let lis first remark, that what we want to get is the sun's direct or radiant heat, quite irrespective of that of the atmosphere around us, and that to get definite results.
We may reckon it by any one of the numerous effects heat produces; practically it is convenient to let it warm water, and to see how much it heats, through how many degrees, and in how many minutes.
Pouillet's pyrheliometer is substantially nothing but a very shallow cylindrical box, A A', filled with a measured quantity of water. It is mounted on the end of a hollow rod, having at its other extremity a metal disk of the same size as the water box.
When the shadow of the box exactly covers M. Now this is for a very small part of a single year's work of the sun in raising water to produce rain on the little spot of Manhattan Island alone—a spot, geographically speaking, hardly visible on the map of the country. Again, lr of an the disk the ind nanent is pointed true on the sun. Held in the hollow rod is an inverted thermometer, whose bulb is within the water box. This enables us to read the temperature of the water from moment to moment.
It is not enough to expose it for a time to the sun and read the thermometer—this would give too small a result, because the instrument as soon as it is warmed commences to radiate the heat away again, like any other hot body; and we would like, if we could, to keep all this heat in it to measure. As we cannot, we reach the same result by finding how much is? Thus, the observer first leaves the apparatus in the shade for instance five minutes, and notices whether it loses or gains from its own radiation to surrounding objects, Then he leaves it directed to the sun, which shines full on it for five minutes more, the thermometer be- j ing read at the end of this exposure; and finally, at the end j of another five minutes, during which the instrument has.
Ericsson, the celebrated engineer, who has improved ' on Pouillet's apparatus, has in fact shown that we do in accurate experimenting always get more heat other things being equal on a day in winter than in summer, as we should, if it is the direct solar radiation alone we are after; for that will be the greatest when the sun is nearest, as it is in our northern winter.
Again, measuring when the sun is high, and at all altitud es down to the horizon, we find less and less heat, as the rays go through more of our atmosphere, and hence we can make a table showing how much this absorbs for every al ti tude, ami consequently how much we should gain if it were taken away altogether, When this is done we find, according to Mr.
Eri csson's late determinations which we substitute for Pouillet's , that the direct heat of the sun on 1 square foot in March is competent to raise 7'11 pounds of water 1 ' Fah, in one minute, This is what it would do if we got outside of our atmosphere; but owing to the absorbing action of this, the radiation which actually reaches us, under a vertical sun, will so heat only about 5 6 pounds, According to the mechanical theory of heat this effect is that which would be required to drive an engine of 5'6 X ' —tjtt.
In other words, the heat of j a vertical sun after absorption by our atmosphere represents rather over one horse power to each square yard. It must be remembered that, according to what has been stated, the sun offers us a source of power which is prac-tjcally infinite both in amount and duration. According to what we believe we know with assurance, we can say that the sun is not a fire, fed by any fuel, but a glowing gas ball, maintained at an enormous temperature, and radiating enormous heat from a fund of energy maintained by the contraction of its volume, and by the impact of meteoric bodies, We can reckon with confidence that there will be no material diminution of its supply from these sources for a duration only to he reckoned by hundreds of thousands of years.
As to the amount of heat supplied, it is i nconceivable, The writer has made a computation of the time all the coal of the world would suffice to main-tain the sun's radiation, were the actual source of it to fail, and were our whole supply of coal transported to its surface and burned there in its place. The result, otherwise stated, is that in any one second the sun radiates into space an amount greater than could be made good by totally consuming all the known coal beds of the world!
Most solar water heaters that operate in winter use a heat transfer fluid, similar to antifreeze, that will not freeze when the weather turns cold. Today over 1. Solar Electricity. Besides heating homes and water, solar energy also can be used to produce electricity. Two ways to generate electricity from solar energy are photovoltaics and solar thermal systems.
Photovoltaic comes from the words photo meaning "light" and volt , a measurement of electricity. Sometimes photovoltaic cells are called PV cells or solar cells for short. You are probably already familiar with solar cells.
Solar-powered calculators, toys, and telephone call boxes all, use solar cells to convert light into electricity. A photovoltaic cell is made of two thin slices of silicon sandwiched together and attached to metal wires. The top slice of silicon, called the N-layer, is very thin and has a chemical added to it that provides the layer with an excess of free electrons.
The bottom slice, or P-layer, is much thicker and has a chemical added to it so that it has very few free electrons. When the two layers are placed together, an interesting thing happens-an electric field is produced that prevents the electrons from traveling from the top layer to the bottom layer. This one-way junction with its electric field becomes the central part of the PV cell.
When the PV cell is exposed to sunlight, bundles of light energy known as photons can knock some of the electrons from the bottom P-layer out of their orbits through the electric field set up at the P-N junction and into the N-layer. The N-layer, with its abundance of electrons, develops an excess of negatively charged electrons. This excess of electrons produces an electric force to push the additional electrons away. These excess electrons are pushed into the metal wire back to the bottom P-layer, which has lost some of its electrons.
This electrical current will continue flowing as long as radiant energy in the form of light strikes the cell and the pathway, or circuit, remains closed. Current PV cell technology is not very efficient. Today's PV cells convert only about 10 to 14 percent of the radiant energy into electrical energy. Fossil fuel plants, on the other hand, convert from percent of their fuel's chemical energy into electrical energy.
The cost per kilowatt-hour to produce electricity from PV cells is presently three to four times as expensive as from conventional sources.
However, PV cells make sense for many uses today, such as providing power in remote areas or other areas where electricity is difficult to provide. Scientists are researching ways to improve PV cell technology to make it more competitive with conventional sources. Like solar cells, solar thermal systems use solar energy to make electricity.
But as the name suggests, solar thermal systems use the sun's heat to do it. Most solar thermal systems use solar collectors with mirrored surfaces to concentrate sunlight onto a receiver that heats a liquid. The super-heated liquid is used to make steam that drives a turbine to produce electricity in the same way that coal, oil, or nuclear power plants do.
Solar thermal systems may be one of three types: central receiver, dish, or trough. A central receiver system uses large mirrors on top of a high tower to reflect sunlight onto a receiver.
This system has been dubbed a "solar power tower. This system resembles a television satellite dish. A third system uses mirrored troughs to collect sunlight.
Until recently, trough systems seemed the most promising. The rest of the sun is heated by the energy that is transferred from the core through the successive layers, eventually reaching the solar photosphere and escaping into space as sunlight or the kinetic energy of particles. The sun releases energy at a mass—energy conversion rate of 4. To put that in perspective, this is the equivalent of about 9.
This is the zone immediately next to the core, which extends out to about 0. There is no thermal convection in this layer, but solar material in this layer is hot and dense enough that thermal radiation is all that is needed to transfer the intense heat generated in the core outward. Basically, this involves ions of hydrogen and helium emitting photons that travel a short distance before being reabsorbed by other ions. Temperatures drop in this layer, going from approximately 7 million kelvin closer to the core to 2 million at the boundary with the convective zone.
Density also drops in this layer a hundredfold from 0. Here, the temperature is lower than in the radiative zone and heavier atoms are not fully ionized. As a result, radiative heat transport is less effective, and the density of the plasma is low enough to allow convective currents to develop. Because of this, rising thermal cells carry the majority of the heat outward to the sun's photosphere. Once these cells rise to just below the photospheric surface, their material cools, causing their density increases.
This forces them to sink to the base of the convection zone again — where they pick up more heat and the convective cycle continues. At the surface of the sun, the temperature drops to about 5, K.
The turbulent convection of this layer of the sun is also what causes an effect that produces magnetic north and south poles all over the surface of the sun. It is also on this layer that sunspots occur, which appear as dark patches compared to the surrounding region. These spots correspond to concentrations in the magnetic flux field that inhibit convection and cause regions on the surface to drop in temperature to compared to the surrounding material.
Lastly, there is the photosphere, the visible surface of the sun. It is here that the sunlight and heat that are radiated and convected to the surface propagate out into space. Because the upper part of the photosphere is cooler than the lower part, an image of the sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening.
The photosphere is tens to hundreds of kilometers thick, and is also the region of the sun where it becomes opaque to visible light. The reasons for this is because of the decreasing amount of negatively charged Hydrogen ions H— , which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H— ions.
The energy emitted from the photosphere then propagates through space and reaches Earth's atmosphere and the other planets of the solar system. Here on Earth, the upper layer of the atmosphere the ozone layer filters much of the sun's ultra-violet UV radiation, but passes some onto the surface. The energy that received is then absorbed by the Earth's air and crust, heating our planet and providing organisms with a source of energy.
The sun is at the center of biological and chemical processes here on Earth. It is here, in the core, where energy is produced by hydrogen atoms H being converted into molecules of helium He. This is possible thanks to the extreme pressure and temperature that exists within the core, which are estimated to be the equivalent of 2 50 billion atmospheres The net result is the fusion of four protons hydrogen molecules into one alpha particle — two protons and two neutrons bound together into a particle that is identical to a helium nucleus.
Two positrons are released from this process, as well as two neutrinos which changes two of the protons into neutrons , and energy. The core is the only part of the Sun that produces an appreciable amount of heat through fusion.
The rest of the Sun is heated by the energy that is transferred from the core through the successive layers, eventually reaching the solar photosphere and escaping into space as sunlight or the kinetic energy of particles. The Sun releases energy at a mass—energy conversion rate of 4. To put that in perspective, this is the equivalent of about 9.
Radiative Zone: This is the zone immediately next to the core, which extends out to about 0. There is no thermal convection in this layer, but solar material in this layer is hot and dense enough that thermal radiation is all that is needed to transfer the intense heat generated in the core outward.
Basically, this involves ions of hydrogen and helium emitting photons that travel a short distance before being reabsorbed by other ions.
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