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Heat is produced in anabolic or exothermic reactions. It is responsible for the mechanical work of a certain body or system. For example, heat produced in the combustion of fuel is used to keep an automobile running.
Heat is a form of energy which flows from a hotter to a colder body (Brown, et al., 2004). There is a number of mechanisms to transfer heat, but, generally, it is transferred due to the temperature difference between two bodies.
Heat transfer plays an important role in predicting weather conditions. Heat transfer mechanism can be observed in the surrounding environment and people’s daily lives, for example, the rise of a hot air balloon (Young, et al., 2004). There are many spheres that apply to the heat transfer, but its very important function can be observed in its role to sustain and support life.
The Mechanisms of Heat Transfer
In conduction, heat is transferred from one body to another when the bodies have thermal interactions. Heat transfers due to the difference in temperature between the bodies in contact (Young, et al., 2004).
For example, in an isolated system an aluminum spoon gets hotter when a person immerses it in the hot water. An isolated system means that all the heat lost by the hot water is gained by the aluminum spoon. Considering the molecular level, heat is the energy released by the molecules during collision. Kinetic energy of molecules increases as the temperature increases.
As a result of the molecular increase in kinetic energy, molecules tend to move randomly and thus increasing the probability of the molecules to collide with each other. The Law of Conservation of Energy states that energy can neither be created nor destroyed, we can say that upon collision of the molecules the energy, in the form of heat, is transferred to another molecule making the molecules lose and gain energy (Atkins, et al., 2002). This phenomenon ceases and eventually stops at a point when the temperature between the two bodies is equal, the resultant temperature. It stops because the heat transfer is possible only if there is a temperature difference between the two bodies in contact.
In the case of the aluminum spoon, heat travels from the immersed part of the spoon up to the top since the spoon and the hot water, which comprise the system, are initially at different temperatures. The cooling of the spoon and hot water is also an example of conduction. In this case, the first body will be comprised of the spoon and the hot water initially will be at the resultant temperature. The second body will be comprised by the surroundings in which its temperature is equal to the room temperature.
The mechanism, which is influenced by the mass motion of a fluid from one region of space to another, is called convection (Young, et al., 2004). It is the most dominant mechanism of heat transfer between fluids. Take note that in physics, fluids include not only liquids but also gases.
There are two types of convection, the natural or free convection and the forced convection. In natural convection, the flow of fluids happens due to their difference in density due to the thermal expansion. An example of natural or free convection is the heat transfer in the oceans, which is an important mechanism for the aquatic organisms to survive during seasons of cold temperature.
During seasons of cold temperature, for example, the upper surface of the ocean is at a lower temperature compared to the temperature at the bottom. In terms of density, cold water is denser than hot water. As a result of this difference in density, the hot water rises and the cold water settles at the bottom of the ocean. Then the water at the bottom becomes warmer, and the water on the surface becomes colder. The rising of the hot water and settling of the cold water at the bottom will continue like a cycle. During the transfer of the fluid with different densities, a certain current is produced which is called the convection current (Young, et al., 2004).
The second type of convection, which is the forced convection, happens when there is a use of some external force in order for the convection to happen. The fluid transfers from one region of space to another not because of the influence of temperature, but because there is some force that triggers the convection to happen.
An example is the forced convection of the blood in the body to maintain the internal body temperature wherein the heart serves as the pump or the external force to drive this heat transfer (Young, et al., 2004).
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In atomic level, electrons tend to occupy the lowest possible energy level. When electrons from higher energy level jump back to a lower energy level, they emit energy to balance the electron’s energy with the required energy in the lower level. This energy is emitted through radiation.
Radiation is another type of heat transfer mechanism in which heat can be transferred even if there are no matter those bridge two bodies to be in contact (Heat Transfer, 2002). This means that two bodies do not need to be in thermal contact or have density gradient to transfer heat.
This type of heat transfer mechanism uses electromagnetic waves, which include the visible light, infrared, and ultraviolet radiation, to transfer heat from the source to another body (Young, et al., 2004). An obvious example of this type of heat transfer is observed on how living organisms on the earth feel the warmth of the sun.
Heat is transferred from the sun to the organisms neither by conduction nor convection with the surrounding air. It is transferred directly from the sun to the living organisms by the ultraviolet radiation. Another manifestation of this heat transfer mechanism is the intense heat we feel near a fireplace with glowing charcoal. Materials and surfaces have the ability to absorb radiation. A blackbody is a material or a surface which absorbs all the emitted radiation by a source while a reflector is a material or a surface that absorbs no radiation at all (Young, et al., 2004).
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Heat transfer, or energy transfer, is a very important mechanism indeed. However, heat is sometimes not needed or transfer of heat should be prevented in some environments. These materials that do not permit the transfer or flow of heat are called insulators while those materials that permit the heat transfer mechanisms are called conductors, an iron rod and an aluminum cup are some of the examples.
Heat transfer may be of less importance at first glance. However, if we analyze and understand deeper the things that surround us, we will realize how important heat transfer is. Starting from the molecules that compose us up to the things that we see, heat transfer plays a vital role to living organisms. It does not only restore the balance in the environment, but it is also a vital factor for the living organisms to survive.