<P> Solids include complex crystalline materials with sometimes complicated behavior . By contrast , the behavior of compressible fluids , and especially gases , demonstrates the essence of elastic energy with negligible complication . The simple thermodynamic formula : d U = − P d V , ( \ displaystyle dU = - P \ , dV \ , ) where dU is an infinitesimal change in recoverable internal energy U , P is the uniform pressure ( a force per unit area ) applied to the material sample of interest , and dV is the infinitesimal change in volume that corresponds to the change in internal energy . The minus sign appears because dV is negative under compression by a positive applied pressure which also increases the internal energy . Upon reversal , the work that is done by a system is the negative of the change in its internal energy corresponding to the positive dV of an increasing volume . In other words , the system loses stored internal energy when doing work on its surroundings . Pressure is stress and volumetric change corresponds to changing the relative spacing of points within the material . The stress - strain - internal energy relationship of the foregoing formula is repeated in formulations for elastic energy of solid materials with complicated crystalline structure . </P>
<P> Solids include complex crystalline materials with sometimes complicated behavior . By contrast , the behavior of compressible fluids , and especially gases , demonstrates the essence of elastic energy with negligible complication . The simple thermodynamic formula : d U = − P d V , ( \ displaystyle dU = - P \ , dV \ , ) where dU is an infinitesimal change in recoverable internal energy U , P is the uniform pressure ( a force per unit area ) applied to the material sample of interest , and dV is the infinitesimal change in volume that corresponds to the change in internal energy . The minus sign appears because dV is negative under compression by a positive applied pressure which also increases the internal energy . Upon reversal , the work that is done by a system is the negative of the change in its internal energy corresponding to the positive dV of an increasing volume . In other words , the system loses stored internal energy when doing work on its surroundings . Pressure is stress and volumetric change corresponds to changing the relative spacing of points within the material . The stress - strain - internal energy relationship of the foregoing formula is repeated in formulations for elastic energy of solid materials with complicated crystalline structure . </P>
​ stress that is converted to positive energy is called
​ stress that is converted to positive energy is called
elastic energy
elastic energy