That radiation comes in the form of very high energy electromagnetic waves, as well as nuclear particles. Note that the radiation we're talking about here, in regard to heat transfer, is not the same thing as the dangerous radiation associated with nuclear bombs, etc. The only difference between the different kinds is the frequency and wavelength of the wave. Don't worry if that definition goes over your head, because you're already familiar with many kinds of electromagnetic waves, such as radio waves, microwaves, the light we see, X-rays, and ultraviolet rays. We'll talk about electromagnetic waves in a lot more detail in PY106 an electromagnetic wave is basically an oscillating electric and magnetic field traveling through space at the speed of light. The third way to transfer heat, in addition to convection and conduction, is by radiation, in which energy is transferred in the form of electromagnetic waves. With a layer of ice covering the walls, the rate of heat transfer is reduced by a factor of more than 300! It's no wonder the freezer has to work much harder to keep the food cold. Now, instead of heat being transferred through the aluminum with a temperature difference of 15°, the difference is only 0.041°. The thermal conductivity of ice is 2.2 J / (s m ☌). Setting the heat-transfer rates equal gives: The rate of heat transfer must be the same through the ice and the aluminum this allows the temperature at the ice-aluminum interface to be calculated. What happens if 5 mm of ice builds up inside the freezer, however? Now the heat must be transferred from the freezer, at -10 ☌, through 5 mm of ice, then through 1.5 mm of aluminum, to the outside of the aluminum at -25 ☌. This is quite a large heat-transfer rate. With a temperature difference of 15°, the amount of heat conducted through the aluminum per second per square meter can be calculated from the conductivity equation: The aluminum is 1.5 mm thick, and the thermal conductivity of aluminum is 240 J / (s m ☌). The inside of the freezer is kept at -10 ☌ this temperature is maintained by having the other side of the aluminum at a temperature of -25 ☌. Under normal operation, a freezer keeps food frozen by transferring heat through the aluminum walls of the freezer. When this happens, the freezer is much less efficient at keeping food frozen. Styrofoam, on the other hand, a good insulator, has a thermal conductivity of 0.01 J / (s m ☌).Ĭonsider what happens when a layer of ice builds up in a freezer. K is the thermal conductivity, a constant depending only on the material, and having units of J / (s m ☌).Ĭopper, a good thermal conductor, which is why some pots and pans have copper bases, has a thermal conductivity of 390 J / (s m ☌). The equation governing heat conduction along something of length (or thickness) L and cross-sectional area A, in a time t is:
SECOND LAW OF THERMODYNAMICS FORMULA FREE
Metals have many free electrons, which move around randomly these can transfer heat from one part of the metal to another. Electrons can also carry heat, which is the reason metals are generally very good conductors of heat. When heat is transferred via conduction, the substance itself does not flow rather, heat is transferred internally, by vibrations of atoms and molecules. Birds and gliders make use of upward convection currents to rise, and we also rely on convection to remove ground-level pollution.įorced convection, where the fluid does not flow of its own accord but is pushed, is often used for heating (e.g., forced-air furnaces) or cooling (e.g., fans, automobile cooling systems). Convection currents are set up in the fluid because the hotter part of the fluid is not as dense as the cooler part, so there is an upward buoyant force on the hotter fluid, making it rise while the cooler, denser, fluid sinks. Heat transfer in fluids generally takes place via convection. The third way to transfer energy is by radiation, which involves absorbing or giving off electromagnetic waves. Another way to transfer heat is by conduction, which does not involve any motion of a substance, but rather is a transfer of energy within a substance (or between substances in contact). In fluids, heat is often transferred by convection, in which the motion of the fluid itself carries heat from one place to another. There are three basic ways in which heat is transferred. Heat transfer, and the first law of thermodynamics Heat transfer, and the first law of thermodynamics
0 kommentar(er)
