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The most obvious and trivial way in which energy is transported is when an object that possesses energy simply moves from one place to another. For example, a baseball flying through the air is a simple form of energy transport.
Kinetic energy can also be transferred from one object to another when objects collide. This is also pretty trivial, except that we also know that the total energy, including any heat or other forms of energy generated during the collision, is conserved in this process, regardless of the relative sizes, shapes, and materials of the objects.
In general, the important modes of transfer for renewable energy technology are:
*Note the two "triads" above: (transmission-reflection-absorbtion & conduction-convection-radiation). You should memorize these and know what they mean!
Light (essentially pure energy that can be thought of as either "photons" or electro-magnetic waves) propagates by itself in a vacuum at very high speed (the speed of light that is! Always the same value in a vacuum).
Light interacts with materials in various ways that impact its transfer. In general, light is either:
Transmitted: It passes through an object - an object is either transparent (the light passes straight through), or translucent (the light passes through, but its direction "scattered" by the material).
Reflected: The light bounces off. Reflection can either be coherent (the angle of incidence equals the angle of reflection) or diffuse (the reflected direction is randomly scattered):
Absorbed: The light enters the material but does not pass through - Instead, its energy is converted into the form we call "heat", that is, microscopic vibrations of the material, or is absorbed by chemical reactions triggered by the light (photochemical effect).
There are three important ways that heat energy can be transported or transferred, called conduction, convection, and radiation. The first two refer to transfer of the thermal energy, whereas the last is really a conversion of energy to a different form, (photons of light) and the subsequent travel (transport) of those photons.
The "diffusion" of
thermal energy (heat) through a substance, which occurs because hotter
molecules (those that are vibrating, rotating, or traveling faster), interact
with colder molecules, and in the process
transfer some of their energy. For example, conduction of thermal energy is
what makes the handle of a metal frying pan on the stove get hot, even though
the handle is not exposed to the flame. Metals
are excellent conductors of heat energy, whereas things like
The rate H, at which heat conducts through a slab of material across a fixed temperature difference DT, for example, from the inside of a warm house to the outside through a wall, is given by the area A of the surface, times the temperature difference DT, divided by the thermal resistance R,
H = (A DT) / R.
R is also called the "R-factor" of the material. When considering the insulating power of the walls of a house, R is likely to have units of square feet divided by BTUs per hour (ft2/(BTU-hr)). BTU stands for British Thermal Unit, and is the amount of energy needed to raise the temperature of one pound of water one degree Fahrenheit. Technical discussions involving conduction may also refer to the thermal conductivity K of a material, which is related to the R-factor by
R = L / K,
where L is the thickness of the material. Thus, if you look up the thermal conductivity of some material, then you can calculate the R value for a slab of that material with thickness L.
Convection: The transfer of heat energy by the movement of a substance, such as a heated gas or liquid from one place to another. For example, hot air rising to the ceiling is an example of convection (in this case called a convection current).
Radiation: In general, you are probably familiar with the fact that the word "radiation" applies to both the light waves (photons), and also rays consisting of other subatomic particles, such as electrons (beta rays) and helium nuclei (alpha rays), that are emitted by radioactive materials.
In the context of heat transfer, however, the term "radiation" refers just to light (electro-magnetic waves), and in particular, to the surprising fact that all objects, even those that are in equilibrium (at equal temperature) with their surroundings, continuously emit, or radiate electromagnetic waves (that is, light waves) into their surroundings. The source of this radiation is the thermal energy of the materials, that is, the movement of the object's molecules.
The amount of light wave radiation radiated by ordinary objects is surprisingly large, even though we usually don't notice it. For example, an object at 70 degrees Fahrenheit (room temperature), radiates about 460 watts per square meter of its surface! If this is true, you might wonder, then why doesn't everything grow cold right away, and why don't we feel this radiation? In fact, if an object is suddenly placed in outer space, far away from any strong energy input, then the object would indeed grow cold quite rapidly by the radiation process. Normally, however, an object is completely surrounded by other objects of the same temperature (such as by the air itself), and these objects also radiate energy at the same rate. Thus the energy loss from radiation leaving the objects is balanced by the incoming radiation coming from the others. We don't feel the effects of the radiation because of this balance, unless we happen to stand between objects that have different temperature, for example, if we stand next to a wall that was being warmed by the Sun right after the Sun goes down.
To give you a feel for how much the imbalance of radiation between objects is in such cases, a temperature difference of about 20 degrees Fahrenheit leads to a net radiation transfer from the hotter object to the cooler of about 11 watts per square meter, which is enough to notice, yet still not much compared to the total radiation coming from each object. As another example, if the sky is cloudy then heat radiating from the ground will largely be absorbed and reradiated back to Earth by the clouds, keeping the air near the surface warm. On a clear night, however, the ground and nearby air can cool very dramatically by radiating out into space, and you will sometimes hear people call this effect "radiation cooling".
The Black-Body spectrum of radiation from objects
For those that are curious about how to calculate the amount of energy that is radiated, it is interesting to know that the spectrum of light radiated by objects, that is, how much energy is radiated at each frequency, has approximately the same mathematical form for all objects, and thus depends only on the temperature of the object and not the specific kind of material. The spectrum is called the "black-body spectrum", because it is most perfectly exhibited by objects which absorb all the light falling upon them (which means they are perfectly black in color). For relatively low temperatures, such as room temperature, most of the black-body spectrum is at long wavelengths of light, that is, in the infrared or longer wavelengths, which are invisible to the human eye, while at high temperatures the spectrum lies at shorter wavelengths, and can become visible if the temperature is very high.
For example, an ordinary object sitting on a desk appears not to radiate anything (although it does), because most of its radiation is at wavelengths longer that light waves in the visible range. On the other hand, when an electric stove burner starts to glow red, it does so because its reaching a temperature at which the black body spectrum is starting to strongly overlap the region of visible light.
It is very interesting to note that the Sun itself is, to a very good approximation, also a black-body radiator, and that the black-body spectrum of the Sun largely lies in the visible because the Sun is so hot. For any temperature, the wavelength at which the blackbody spectrum has its peak is given by "Wien's Displacement Law",
Peak wavelength in micrometers = 2900 / T,
where T is the temperature in Kelvin degrees. For the Sun, which has a surface temperature of about 6000 Kelvin, we find that the Sun's peak wavelength is about .5 micrometers, which corresponds roughly to the color yellow, approximately in the middle of our visible range. Thus, not surprisingly, we find that our eyes are well adapted to the peak wavelengths given off by the Sun!
The total amount of energy radiated per second, that is, the total power of the radiation, also has a simple formula, which gives the power as a function of the temperature of the body raised to the fourth power,
P = s e A T4.
This is called the Stefan - Boltzmann Law. In this equation, the parameter s is called the Stefan-Boltzmann constant, equal to 5.67 x 10-8 watts/(meter2 - degree Kelvin), A is the surface area of the object, and e is the "emissivity" of the object, which ranges for 0 to 1. Because the temperature is raised to the fourth power, then an object with twice the (absolute) temperature of another object will radiate 16 times more strongly!.
Finally, it is interesting to know that this law, along with the form of the black-body spectrum and Wien's Displacement Law above, can be explained only by using the theory of physics called "quantum mechanics", and therefore is actually a very deep result of science. In fact, quantum mechanics arose during attempts to explain the experimentally measured form of the blackbody spectrum, which contradicted the predictions of Newtonian (classical physics). Newtonian physics actually predicted, absurdly, that objects would radiate at infinite power, so people realized that there must be something wrong with Newtonian physics at microscopic scales.
Convection, Conduction, and Radiation, all at once
As a good example of conduction, convection, and radiation, all happening at once, consider sitting by the side of a hot fire, holding a metal rod in the fire with a marsh mellow on it. The rod gets to hot to hold after awhile, because heat conducts down the rod to the handle. Likewise, you can see that there is hot air rising above the fire, carrying smoke. The energy carried by the hot air is an example of convection. Finally, you can feel your face and body get hot from being near the fire. This isn't likely to be because hot air is coming to you from the fire, because the fire is actually drawing air into the fire. The heat you feel is actually coming from radiation - that is, the light given off by the fire.
Transmission-Reflection-Absorbtion & Conduction-Convection-Radiation all at Once
Consider a passive solar home in the winter (see figure below):
Finally, energy can be transferred by electrical transmission. Within a wire this is accomplished through electric fields associated with electrons in the metal wire. The electrons literally push on each other, and convey force through the wire, which thereby transfers energy. For example, the electro-chemical processes in a battery create positive and negative electric charges at the battery contacts which push on, and hence force, the free (moveable) electrons in the wires to move. Electrical energy is converted to heat when some of the electrons encounter resistance - that is, when the electrons are pushed through materials causing heat, that is, cause the atoms of the material to start vibrating. Alternatively, the movement of electrons may give rise to electric and magnetic fields (such as in coils of a motor), which do work, such as turning the motor shaft.
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