Conduction, Convection and Radiation
Heat is a form of thermal energy that can be transferred from one point to another. Heat transfer takes place through conduction, convection and radiation. Heat generally flows from a hot object to a colder one. This process continues until the two regions with temperature differences achieve a thermal equilibrium, and as long as the external source of heat is removed (Sundén, 2012). All metals are good conductors of heat. On the other hand, gases and non-metals fall into the category of poor heat conductors. These are also referred to as insulators. Heat conduction involves heat transfer within the object itself, while radiation may involve flow of heat between objects that are separated spatially. Heat transfer through convection occurs through a material carrier that facilitates flow of heat energy. This paper will analyze in detail the above mentioned heat transfer mechanisms, giving relevant examples.
According to Baukal (2012), heat conduction is the flow of thermal energy between objects that are in direct contact, or heat transfer within the object itself. All matter is made up of particles or atoms. When particles get heated, they move randomly. In objects such as metal which are good conductors of heat, the heated particles move randomly causing the neighboring particles to move faster. As particles move within the object, they collide with each other in a haphazard manner. As the particles move in random manner, they transfer both potential and kinetic energy throughout the metallic object (Baukal, 2012). This energy is referred to as the internal energy.
The internal energy in objects is transferred by the action of randomly moving atoms as they interact with neighboring particles. The heated atoms move and collide with the adjacent ones, and in the process transfer potential and kinetic energy to the adjacent atoms. As long as there is a source of heat, heat transfer from the hotter to the colder parts of the object continues. Electrons also flow within the object but in a back and forth manner. This prevents formation of electric current within the object. Conduction occurs in all states; in solids, liquids and gaseous substances. However, it is more pronounced in solids than in other forms due to the nature of the arrangement of particles in various states. In solids such as metals, atoms are packed closely together. This enables the vibrating atoms to transfer some of their energy to the neighboring atoms easily (S. Blundell and K. Blundell, 2006). In liquids and gases, the atoms are further apart, which means that there are fewer collisions between the heated and adjacent atoms.
Conduction is best explained through heating a metallic pan. When the pan is placed on fire, a person can comfortably touch the pan’s surface. As time goes by, the surface of the pan becomes hotter followed by the sides. Lastly, the handle also becomes hot. When the pan is heated, particles making up the base of the pan starts to move rapidly. These particles move and collide with neighboring particles causing them to move randomly. As the particles move, they transfer potential and kinetic energy obtained from the fire to the adjacent particles. The successive collision of particles within the pan causes temperatures to increase within the whole pan. This explains how the handle which is not in direct contact with the fire also becomes hot – conduction transfers heat throughout the pan. When one touches a cold object such as a metal rod it usually feels cold to touch. Heat is transferred from the body to the metal rod through conduction. This can explain why after holding the metal rod for sometimes it seizes to feel cold to touch.
According to Lienhard (IV) and Lienhard (V) (2008), convection is the transfer of heat through movement of liquids or gases from one point to another. Convection takes place in fluids and gaseous substances. In this method, heat transfer occurs through both fluid flow and diffusion. Convection mainly occurs through movement of fluid. Nonetheless, diffusion also causes molecular motion leading to heat transfer within fluids. When fluid or gas is heated, it becomes less dense. Thus, the warmer areas of the liquid which are in contact with the hot object rise to the cooler parts of the fluid. On the other hand, the cooler and dense part of the fluid moves to occupy the lower parts. This continuous cycle eventually results to the whole fluid obtaining even temperatures.
When fluids are heated, they expand just like solids. This is mainly due to the fact that heated particles move faster and randomly compared to cold particles within the fluid (Lienhard (IV) and Lienhard (V), 2008). As the particles move faster, the gaps between them increases and they take up more volume. However, the particles themselves do not change is size or shape. This leads to the hot parts having a lower density compared to colder parts of the fluid. This leads to the less dense part of the liquid rising and the colder parts of the fluid fall to warm regions. This process continues in what is known as the convectional currents. The process is maintained as long as there is a temperature gradient. When heating is removed, the convectional currents continue until a uniform density and temperature is obtained (Lienhard (IV) and Lienhard (V), 2008).
A good example of heat transfer via convection is heating of water using a stove. When boiling something such as elbow pasta, it is possible to observe the pasta rise from the bottom of the pan, move to the top and then sideways. Finally, the pasta sinks to the bottom of the pan and the cycle continues. When the water particles at the bottom of the pan become heated, they move faster and expand as well. This makes them less dense compared to the particles in the colder parts. This causes the less dense particles to rise, while the denser particles at the top fall to replace the rising water particles. Thus, movement of warmed water results to the whole water being heated up.
Convection is also observed in the heating of the atmosphere. The sun’s rays strike the earth and warm the earth’s surface. The air adjacent to the earth’s surface becomes warm and rises up, while the cold dense air falls to replace the rising warm air. Lastly, an electric heater placed in a room heats up the entire room through convection. The air close to the heater becomes warm and rises up, while the air at the top moves towards the heater. This results in the entire room becoming hot.
Radiation is transfer of heat through electromagnetic waves or photon emission. As such, heat radiation can occur without the need for intermediate matter unlike heat transfer through conduction and convection. This simply means that heat radiation can take place even in a vacuum. According to Sundén (2012), heat radiation occurs as part of electromagnetic spectrum found in energy emissions. Thus, heat radiation occurs as waves similar to sound waves or light waves. However, the range of waves differs in their frequency and wavelength. This is what distinguishes the various forms of waves. Heat radiation comprise of the infrared radiation in the electromagnetic spectrum. All objects with temperatures above absolute zero are capable of emitting electromagnetic waves. However, the amount of thermal radiation emitted depends with the temperatures of the object emitting thermal energy. Generally, hotter objects emit more of the thermal energy.
According to Sundén (2012), thermal radiation is generated when charged particles move about in matter. This occurs only when the temperature of the matter is above absolute zero, resulting to atomic collisions in particles within the matter. The atomic collisions lead to changes in the kinetic energy of the colliding particles. The resulting dipole oscillation produces electromagnetic radiation. In addition, a whole range of electromagnetic spectra is produced. It is important to note that transfer of heat through radiation does not depend on interaction of the matter.
The wavelength and frequency of the electromagnetic waves is determined by the temperature of the radiating object (S. Blundell and K. Blundell, 2006). The most common form of electromagnetic radiation comes from the sun in form of heat. The heat from the sun travels to Earth in form of electromagnetic waves. The waves travel through empty space, and millions of kilometers to reach Earth. Objects at room temperature also produce electromagnetic radiation, mainly as infrared rays. This form of radiation is invisible to the naked human eye. Nonetheless, it can be detected by using an infrared camera. Thus, even the human body produces thermal radiation which is invisible to the naked eye.
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The coil of an electric cooker produces thermal radiation that is visible to the human eye. This occur when the coil is considerably hot and way above room temperatures. The glow on the coil which is thermal energy acts as a warning to users that the coil is hot. The incandescent light bulbs also produce thermal radiation that is visible to the human eye. This thermal energy also warms the bulb which is usually hot when one touches it.
In conclusion, heat is transferred through three methods; conduction, convection and through radiation. In conduction and convection, transfer of heat occurs through matter. Matter is made up of particles which vibrate and move randomly. In conduction, the particles move and vibrate vigorously, causing collisions with the neighboring particles and hence transfer of heat. Heat transfer occurs due to the disequilibrium that exists between temperatures of the object radiating heat and the surrounding objects. Heat transfer through convection occurs when warm fluid rises and is replaced by dense colder fluid. In radiation, heat transfer can occur through a vacuum. Heat transfer in radiation occurs in form of electromagnetic radiation.
Baukal, E. C. (2012). The John Zink Hamworthy Combustion Handbook, Second Edition. New York, NY: CRC Press.
Blundell, S. & Blundell, K. (2006). Concepts in Modern Physics. Oxford: Oxford University Press.
Lienhard, J. H. (IV)., & Lienhard, J. H. (V). (2008). A Heat Transfer Textbook. Cambridge, Phlogiston Press.
Sundén, B. (2012). Introduction to heat transfer. Southampton: WIT Press.