The transfer of heat and energy has been the focus of scientific discovery and engineering for a large portion of human history as the understanding of heat and energy and the mechanisms by which it is transferred allows for meaningful work to be performed. In this post, I will be discussing what heat is and some mechanisms by which it is transferred between systems.
What is Heat
Heat is energy that can be transferred between bodies and objects. This transfer of heat happens spontaneously between hot and cold systems, with heat transferring from the hot system to the cold system. “Hot” and “cold” refer to the thermodynamic temperature (absolute temperature) which is a measure of the energy arising from the relative motion of atoms and molecules within a substance. Thermodynamic temperature is an absolute measure of temperature in that at 0 degrees on an absolute temperature scale (Kelvin or Rankine) refers to a state where there is no molecular motion present. Absolute zero is impossible to achieve, however, temperatures only 1 billionth of a degree kelvin above absolute zero have been. The mechanisms by which heat is transferred between systems are conduction, convection, and radiation. The three mechanisms by which heat transfer listed all occur spontaneously between hot and cold systems. However, the exact way and rate at which heat is transferred is the main differentiator between the mechanisms.
Mechanisms of Heat Transfer
In the case of conductive heat transfer, heat is transferred between two materials of differing temperature in direct contact with each other. This transfer of energy arises from the direct interaction of higher energy, faster moving particles in the hot system colliding with the more slowly moving particles within the cold system, transferring some of their kinetic energy. Convective heat transfer is like conduction except with the addition of fluid flow to transfer mass along with heat. An example of convection would be the case of a convection oven. As the air is heated via conduction by the oven walls, motion is induced by density differences in the air creating drafts and currents. These drafts and currents move the hot air that then can conduct its heat to the food in the oven. Gases and liquids are typically very poor conductors of heat, however, due to the additional mass transfer by convective currents, convection is a very effective mechanism of heat transfer. Thermal radiation is different from the previous two discussed mechanisms in that it occurs between systems that can be vast distances apart and don’t require a medium in-between to be heated. An example of radiative heat transfer would be the sun which heats the Earth through millions of miles of vacuous space. This mechanism occurs by way of electromagnetic energy released by objects depending on the heat of that object. Hotter objects release more energy than cooler objects and that energy can interact with the atoms and molecules of the cooler object to increase their heat.
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