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From Wikipedia

Thermoelectric effect

This page is about the thermoelectric effect as a physical phenomenon. For applications of the thermoelectric effect, seethermoelectric materials, thermoelectric generator, and thermoelectric cooling.

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely when a voltage is applied to it, it creates a temperature difference (known as the Peltier effect). At atomic scale (specifically, charge carriers), an applied temperature gradient causes charged carriers in the material, whether they are electrons or electron holes, to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence, the thermally induced current.

This effect can be used to generate electricity, to measure temperature, to cool objects, or to heat them or cook them. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices make very convenient temperature controllers.

Traditionally, the term thermoelectric effect or thermoelectricity encompasses three separately identified effects, the Seebeck effect, the Peltier effect, and the Thomson effect. In many textbooks, thermoelectric effect may also be called the Peltier–Seebeck effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and Estonian-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a resistive material, is somewhat related, though it is not generally termed a thermoelectric effect (and it is usually regarded as being a loss mechanism due to non-ideality in thermoelectric devices). The Peltier–Seebeck and Thomson effects can in principle be thermodynamically reversible, whereas Joule heating is not.

Seebeck effect

The Seebeck effect is the conversion of temperature differences directly into electricity.

Seebeck discovered that a compass needle would be deflected when a closed loop was formed of two metals joined in two places with a temperature difference between the junctions. This is because the metals respond differently to the temperature difference, which creates a current loop, which produces a magnetic field. Seebeck, however, at this time did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect, thinking that the two metals became magnetically polarized by the temperature gradient. The Danish physicist Hans Christian Ørsted played a vital role in explaining and conceiving the term "thermoelectricity".

The effect is that a voltage, the thermoelectric EMF, is created in the presence of a temperature difference between two different metals or semiconductors. This causes a continuous current in the conductors if they form a complete loop. The voltage created is of the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature.

In the circuit:

(which can be in several different configurations and be governed by the same equations), the voltage developed can be derived from:

V = \int_{T_1}^{T_2} \left( S_\mathrm{B}(T) - S_\mathrm{A}(T) \right) \, dT.

SA and SB are the Seebeck coefficients (also called thermoelectric powerorthermopower) of the metals A and B as a function of temperature, and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear as a function of temperature, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:

V = (S_\mathrm{B} - S_\mathrm{A}) \cdot (T_2 - T_1).

The Seebeck effect is commonly used in a device called a thermocouple (because it is made from a coupling or junction of materials, usually metals) to measure a temperature difference directly or to measure an absolute temperature by setting one end to a known temperature. A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition, kept at a constant temperature, is held in contact with it. Industrial quality control instruments use this Seebeck effect to identify metal alloys. This is known as thermoelectric alloy sorting.

Several thermocouples connected in series are called a thermopile, which is sometimes constructed in order to increase the output voltage since the voltage induced over each individual couple is small.

This is also the principle at work behind thermoelectric generators (such as radioisotope thermoelectric generators or RTGs) which are used for creating power from heat differentials.

The Seebeck effect is due to two effects: charge carrier diffusion and phonon drag (described below).

Thermopower

The thermopowe


From Yahoo Answers

Question:"get a much greater air flow, allowing evaporative cooling to occur" Does this mean that without air flow evaporative cooling is not possible? if not/ if so please explain :D thnx "As water evaporate from a surface" is that sweat?

Answers:Not necessarily. Pre-existing airflow encourages this mode of cooling, although it can exist even if the air is initially quiescent. What does prevent it, prevents air flow, which is a layer of solid material which traps air in "pockets". Suppose you have a hot bowl of soup on a table that you'd like to cool to safe eating temperature. You see steam wafting off the top surface. This is evaporative cooling, since the steam used to be water and heat from the rest of the soup is used to cause it to vaporize, thus cooling the soup. Your soup is cooled by natural convection and natural evaporative cooling, both of which are driven by the gravitational field and the buoyant effects of hot and humid air. Waiting for your soup to cool, you become impatient and blow on it. By doing this, you impose a forced air flow over the soup surface. This forced air flow encourages convection and evaporation stronger than a natural condition of air. Now, you soup is cooled at a faster rate via forced convection and forced evaporative cooling than if you had been more patient and let buoyancy effects cool your soup. So in quiescent (think "quiet") air, evaporative cooling and natural convective cooling exist without a pre-existing airflow. If a layer of downfibers covers the soup, evaporative cooling and natural convective cooling aren't possible. This is because, although they exist without pre-existing airflow, buoyancy drives a newly existing airflow in order for these modes of cooling to work. With a downfiber layer covering the soup, it can only be cooled by conduction and radiation (which work without flow of any fluid).

Question:

Answers:The surface tension is the key. Lower surface tension lets surface activity proceed more rapidly As the medium evaporates it also carries off heat,a lower surface allows this activity to take place. Dab alcohol on one spot on your arm and water on another the effect will be obvious.

Question:I thought the opposite would occur. Please explain the reason/s in a basic manner as I am new to this stuff

Answers:Air flow is only cooling because the atoms in the substance with the highest kinetic energy are removed. A liquid in equilibrium with its surroundings has a certain partial pressure of its gaseous form coexisting with it. Air flowing over the surface removes the gaseous form and, normally, replaces it with air with a lower amount of those molecules. So some of the molecules in the liquid evaporate to replace them. Those molecules with the highest energy (moving fastest) are the most likely to evaporate and so the average energy of the liquid molecules falls -- that is the definition of cooling. Now, the liquid absorbs heat from the surroundings and reestablishes the same average and range of energy in the liquid so more molecules can evaporate. The faster the evaporated molecules are removed, the faster the liquid evaporates. The most obvious case of this is sweating. Sweat on your brow (or wherever) evaporates and cools you. If you stand in front of a fan, the water vapor formed from your sweat is removed, the humidity in your local area goes down, and more sweat evaporates, cooling you more. If the fan is turned up faster then you may become quite cold. But the air blowing across you is not at any lower temperature than that elsewhere -- rather it causes your body to cool more rapidly. If the humidity of the air is really high, the movement of air does not result in much lower water vapor pressure and does not increase the evaporation rate. In that case the air flow is uncomfortable.

Question:My cousin and I have a long term project to do that our Earth Science teacher wants us to present in an Academic Fun Fair that our school holds every year. She said it would help our displays out if we could think of a cool activity that people, particularly young kids could do that would help educate them about the pollution problem. Our main focus is rivers, so we would want to incorporate the use of the rivers into it. Any ideas?

Answers:This is really fun: Get or make a model of some land. Have some rivers, some trees, a farm, some house, a street..have elevation, like a mountain that goes down to a lake. Get small materials that can represent fertilizer, lawn chemicals, pollution, trash..scatter them about. Get a spray can of "rain" or even "Acid rain." spray everything and watch it run everywhere pollutiing things... i dont know how to explain it, but make a model!

From Youtube

Honeywell Evaporative Air Coolers Audio Visual Presentation :Evaporative air cooling is the natural way of cooling, similar to a breeze flowing across a lake. This breeze drops the temperature and has a relaxing cooling effect on people.The evaporative air cooler cools the air by means of the evaporation of water. When water evaporates into the air, the result is a mixture of air and water molecules. This chemical change requires heat, thus energy or latent heat is taken from the air molecules - dropping the actual temperature of the air.The recommended humidity maximum level is 60% or less, which allows a noticeable temperature decrease* and causes people to feel more comfortable.The temperature decrease* will be larger in more dry climates because the lower the humidity, the more evaporation can occur.When used for cooling the evaporative air cooler should not be used in enclosed spaces. It must be kept level and there must be water in the water tank. The room should have doors and windows opened to allow free air flow. The evaporative air cooler works best when placed near an open window, so that fresh air is drawn into the evaporative air cooler and the air circulates in the room, and then leaves via the door. The maximum cooling effect is felt when a person is in the flow of air coming out of the evaporative air cooler.The evaporative air cooler can also be used to humidify dry air during the cool weather months. To be used for humidification, the windows and doors should be closed. This will allow the humidified air to ...

Filmmaking: Color temperature & Kelvins Explained :Learn how to achieve the best color in your independent films. This video discusses different color temperatures and the reasons why you shouldn't mix them. We look at both outdoor 5600K and indoor 3200K (tungsten) and we show examples of them both. We also discuss CTO gels and CTB gels and discuss the different effects of warm and cool lighting on your films.