Conduction type, convectonsand its types, radiations and its types .
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Conduction transfers heat through direct contact between molecules. Heat flows from hotter to colder areas as faster molecules collide with and transfer energy to slower ones. The rate of conduction depends on temperature difference, object size/shape, material properties like thermal conductivity. Metals conduct heat better than fabrics due to higher conductivity. Convection transfers heat via fluid movement. Heated fluid expands, becomes less dense and rises, while cooler fluid sinks below. This drives convection currents that circulate heat through a fluid. Natural convection occurs without external influence, like a heated pan cooling in air. Forced convection uses devices like fans to enhance fluid and heat flow. Radiation emits electromagnetic waves to transfer heat
![Conduction:
Conduction transfers heat via direct molecular collision. An area of greater kinetic energy will
transfer thermal energy to an area with lower kinetic energy. Higher-speed particles will collide
with slower speed particles. The slower-speed particles will increase in kinetic energy as a result.
Conduction is the most common form of heat transfer and occurs via physical contact. Examples
would be to place your hand against a window or place metal into an open flame. The process of
heat conduction depends on the following factors: temperature gradient, cross-section of the
material, length of the travel path, and physical material properties. The temperature gradient is
the physical quantity that describes the direction and rate of heat travel. Temperature flow will
always occur from hottest to coldest or, as stated before, higher to lower kinetic energy. Once
there’s thermal equilibrium between the two temperature differences, the thermal transfer stops.
Cross-section and path of travel both play an important part in conduction. The greater the size
and length of an object, the more energy that’s required to heat it. And the greater the surface
area that’s exposed, the more heat is lost. Smaller objects with small cross-sections have minimal
heat loss.
Physical properties determine which materials transfer heat better than others. Specifically, the
thermal conductivity coefficient dictates that a metal material will conduct heat better than cloth
when it comes to conduction. The following equation calculates the rate of conduction:
Q = [k ∙ A ∙ (Thot – Tcold)]/d
where Q = heat transferred per unit time; k = thermal conductivity of the barrier; A = heat-
transfer area; Thot = temperature of the hot region; Tcold = temperature of the cold region; and d =
thickness of the barrier.](https://image.slidesharecdn.com/conductiontheory1-190521115233/85/Conduction-type-convectonsand-its-types-radiations-and-its-types-1-320.jpg)
Conduction type, convectonsand its types, radiations and its types .
- 1. Conduction: Conduction transfers heat via direct molecular collision. An area of greater kinetic energy will transfer thermal energy to an area with lower kinetic energy. Higher-speed particles will collide with slower speed particles. The slower-speed particles will increase in kinetic energy as a result. Conduction is the most common form of heat transfer and occurs via physical contact. Examples would be to place your hand against a window or place metal into an open flame. The process of heat conduction depends on the following factors: temperature gradient, cross-section of the material, length of the travel path, and physical material properties. The temperature gradient is the physical quantity that describes the direction and rate of heat travel. Temperature flow will always occur from hottest to coldest or, as stated before, higher to lower kinetic energy. Once there’s thermal equilibrium between the two temperature differences, the thermal transfer stops. Cross-section and path of travel both play an important part in conduction. The greater the size and length of an object, the more energy that’s required to heat it. And the greater the surface area that’s exposed, the more heat is lost. Smaller objects with small cross-sections have minimal heat loss. Physical properties determine which materials transfer heat better than others. Specifically, the thermal conductivity coefficient dictates that a metal material will conduct heat better than cloth when it comes to conduction. The following equation calculates the rate of conduction: Q = [k ∙ A ∙ (Thot – Tcold)]/d where Q = heat transferred per unit time; k = thermal conductivity of the barrier; A = heat- transfer area; Thot = temperature of the hot region; Tcold = temperature of the cold region; and d = thickness of the barrier.
- 2. A modern of use of conduction is being developed by Dr. Gyung-Min Choi at the University of Illinois. Dr. Choi uses spin current to generate spin transfer torque. Spin transfer torque is the transfer of the spin angular momentum generated by the conduction electrons to the magnetization of a ferromagnet. Instead of using magnetic fields, this allows the manipulation of nanomagnets with spin currents. (Courtesy of Alex Jerez, Imaging Technology Group, The Beckman Institute). Types of Conduction: 1. Electric Conduction: Electric conduction refers to the ability of a material to transfer an electric current. Conductivity is determined by how dense an object is compared to the strength of the electric field it can maintain. Metals are substances with a high level of conductivity (also known as a conductor) since they display minimal resistance to an electrical charge. Insulators, such as glass, are materials that are resistant to electrical charges. Televisions, radios and computers are examples of inventions that rely on the current provided by electric conduction.
- 3. 2. Heat Conduction: Where electric conduction refers to a transfer or electric current, heat conduction refers to a transfer of energy, specifically thermal energy. Heat conduction is sometimes called thermal conduction. The energy is transferred within a stationary object as a result of a change in temperature in parts of a material adjacent to one another. The energy will move quickly or slowly depending on what the object is made of, how large it is and, most importantly, the temperature gradient. Temperature gradient refers to the rate and direction in which the temperature changes from a specific point to another point. Diamonds and copper are materials with high thermal conductivity. 3. Photoconductivity: Photoconductivity occurs when a material absorbs electromagnetic radiation, resulting in a change in the substance's electrical conductivity. The electromagnetic radiation can be caused by something as simple as a light shining on a semiconductor or something as complex as a material being exposed to gamma radiation. When the electromagnetic event occurs, the number of free electrons increases, as does the number of electron holes, thus increasing the object's electrical conductivity. Common applications of photoconductivity include copy machines, solar panels and infrared detection equipment.
- 4. Convection: When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal energy along. This type of heat transfer is called convection. The fluid above a hot surface expands, becomes less dense, and rises.At the molecular level, the molecules expand upon introduction of thermal energy. As temperature of the given fluid mass increases, the volume of the fluid must increase by same factor. This effect on the fluid causes displacement. As the immediate hot air rises, it pushes denser, colder air down. This series of events represents how convection currents are formed. The equation for convection rates is calculated as follows: Q = hc ∙ A ∙ (Ts – Tf) where Q = heat transferred per unit time; hc = convective heat transfer coefficient; A = heat- transfer area of the surface; Ts = temperature of the surface; and Tf = temperature of the fluid.A space heater is a classic convection example. As the space heater heats the air surrounding it near the floor, the air will increase in temperature, expand, and rise to the top of the room. This forces down the cooler air so that it becomes heated, thus creating a convection current.
- 5. Types of Convection Natural Convection: Natural convection is a method of heat transfer in which natural means influence the motion of the fluid. There is no influence from external facts. This movement of molecules in the fluid is due to the differences between densities of different regions of the same fluid. The density of a fluid decreases when it heats and vice versa. That is because of the thermal expansion of the fluid (the speed of molecules increase with the temperature increase, which results in the increase of the volume of the fluid. Although the volume increases, the mass remains constant. Therefore the density decreases).When we heat a fluid in a container from its bottom, the density of the bottom layer of the fluid decreases. Then the lower density region tends to move to the top of the container. Then the cooler fluid at the top of the container replaces the bottom region. This continues, as a result, convection occurs. Examples of natural convection include cooling down a boiled egg when kept in the normal air, loss of cool of a cool drink can, etc. When considering the mechanism of natural convection, first, the temperature of the outside of a hot object (kept in cold air) drops down. At the same time, the temperature of the air adjacent to the object will rise due to heat transfer. Then the density of this adjacent layer of air decreases. As a result, the air rises upward. Cold air will replace this region. Then the convection continues. In the end, the object will cool down. Forced Convection: Forced convection is a method of heat transfer in which external means influence the motion of the fluid. There, external sources such as pumping, fans, suction devices, etc. are useful in generating the fluid motion. This method is very valuable because it can efficiently transfer heat from a heated object. Some common examples of this mechanism include air conditioning, steam turbines, etc.When considering the mechanism of forced convection, it is has a complicated mechanism than the natural way. That is because, in this method, we have to regulate two factors; fluid motion and heat conduction. These two factors have a strong connection since the
- 6. fluid motion can enhance the heat transfer. Ex: higher the rate of motion of the fluid, higher the heat transfers
- 7. Radiation Thermal radiation generates from the emission of electromagnetic waves. These waves carry the energy away from the emitting object. Radiation occurs through a vacuum or any transparent medium (either solid or fluid). Thermal radiation is the direct result of random movements of atoms and molecules in matter. Movement of the charged protons and electrons results in the emission of electromagnetic radiation. All materials radiate thermal energy based on their temperature. The hotter an object, the more it will radiate. The sun is a clear example of heat radiation that transfers heat across the solar system. At normal room temperatures, objects radiate as infrared waves. The temperature of the object affects the wavelength and frequency of the radiated waves. As temperature increases, the wavelengths within the spectra of the emitted radiation decrease and emit shorter wavelengths with higher-frequency radiation. Thermal radiation is calculated by using the Stefan-Boltzmann law: P = e ∙ σ ∙ A· (Tr 4 – Tc 4) where P = net radiated power; A = radiating area; Tr = temperature of the radiator; Tc = temperature of surroundings; e = emissivity; and σ = Stefan’s constant.Emissivity for an ideal radiator has a value of 1. Common materials have lower emissivity values. Anodized aluminum has an emissivity value of 0.9 while copper’s is 0.04.
- 8. Solar cell or photovoltaic cell, converts the energy of light into electricity via the photovoltaic effect. Light is absorbed and excites the electrcon to a higher energy state and the electric potential is produced by the separation of charges. Efficiency of solar panels has risen in recent years. In fact, those currently being produced by SolarCity, a company co-founded by Elon Musk, are at 22%. Black Body Radiation A black body or blackbody is an idealized physical body that absorbs all incidentelectromagnetic radiation, regardless of frequency or angle of incidence. (It does not only absorb radiation; It can also emit radiation. Grey Bodies Radiation Grey Bodies Heat Exchange. The easiest method to calculate radiative heat transfer between two bodiesis when they are assumed to be black bodies. ... While J represents the radiosity which is the total amount of radiation that is reflected off a surface per unit time and unit area.