They are then said to be in thermal equilibrium. When the flow of heat stops, they are said to be at the same temperature. Heat transfer occurs by conduction or by thermal radiation. When a temperature difference does exist, heat flows spontaneously from the warmer system to the colder system. In general, when two objects are brought into thermal contact, heat will flow between them until they come into equilibrium with each other. In this case, there is energy transfer between these two blocks, and heat flows from the hotter to the cooler block by these random vibrations. If a hotter block of metal is put in contact with a cooler block, the intensely oscillating atoms at the edge of the hotter block give off their kinetic energy to the less oscillating atoms at the edge of the cool block.
At low temperatures, the atoms continue to oscillate but with less intensity. However, the transfer of energy as heat occurs at the molecular level due to a temperature difference.Ĭonsider a metal block at high temperature that consists of atoms oscillating intensely around their average positions. Heat is a form of energy, but it is energy in transit. While internal energy refers to the total energy of all the molecules within the object, heat is the amount of energy flowing spontaneously from one body to another due to their temperature difference. Both work by moving heat from a cold space to a warm space.
There is no difference between the thermodynamics of refrigerators and heat pumps. These devices are driven by electric motors, requiring work from their surroundings to operate. In a refrigerator, heat pump, or air conditioner, heat flows from cold to hot, but only when forced by external work. This is exactly what refrigerators and heat pumps accomplish. Heat cannot spontaneously flow from cold system to hot system without external work being performed on the system. “It is impossible to construct a device which operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body.” But it doesn’t happen in nature.ĭirections of thermodynamic processes are subject of the second law of thermodynamics, especially of the Clausius Statement of the Second Law, which states: In fact, such heat flow (from a colder body to a warmer system) would not violate the first law of thermodynamics, i.e., energy would be conserved. For example, when a temperature difference does exist, heat flows spontaneously from the warmer system to the colder system, never the reverse. According to the second law of thermodynamics, many thermodynamic processes proceed naturally in one direction but not the opposite.
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