Energy cannot be created or destroyed although, in many processes, energy is transferred to the environment as heat.
Heat and Thermodynamics
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Heat flow and work are two forms of energy transfer between systems.
The work done by a heat engine that is working in a cycle is the difference between the heat flow into the engine at high temperature and the heat flow out at a lower temperature.
The internal energy of an object includes the energy of random motion of the object's atoms and molecules. The greater the temperature of the object, the greater the energy of motion of the atoms and molecules that make up the object.
Most processes tend to decrease the order of a system over time, so that energy levels eventually are distributed more uniformly.