# Thermodynamics: Energy Transferred by Heat

An example of energy transferred by heat is when you leave a hot cup of coffee out in room temperature for too long it will cool down to room temperature. From this example, a simple definition for energy transferred by heat can be derived. Simply if energy is being transferred by heat then there has to be a temperature difference between two systems, or a system and its environment. Due to this temperature difference, energy will be exchanged until the two systems are in thermal equilibrium. Once the two systems are in thermal equilibrium (at the same temperature) then energy can no longer be transferred by heat, and if energy is still crossing the boundary of the system then the energy transfer must be due to work. During a thermodynamic process if no heat crosses the boundary of the system then it can be considered an adiabatic process. Heat transfer can be represented in per unit mass form by using the following equation.

(1)

There are three distinct forms of heat transfer. They are conduction, convection, and radiation. Conduction occurs when two particles come in contact with each other and the more energetic particle transfers energy to the least energetic particle. Convection is very similar to conduction, except the heat exchange occurs when a fluid flows over a solid. Finally, heat transfer due to radiation occurs due to the emission of electromagnetic waves from the object. A prime example of heat transfer due to radiation is the heat that comes from the sun.