Changes

<math>

<math>

\,dU = \delta Q + \delta W

\,dU = \delta Q - \delta W

</math>

</math>

where <math>\,dU</math> is the infinitesimal change in internal energy, <math>\delta Q</math> is the heat that flows into the system and <math>\delta W</math> is the work done on the system.

where <math>\,dU</math> is the infinitesimal change in internal energy, <math>\delta Q</math> is the heat that flows into the system and <math>\delta W</math> is the work the system performs.

This principle notably included heat as a form of energy, which was the first law of thermodynamics.  Hot objects contain potential energy in the form of their heat, and all the usual rules of transformation between potential and kinetic energy apply.  When an electric current is passed through a resistor, the electrical energy is converted to heat energy, and the resistor gets hotter.  Everything seemed to work out accurately.

This principle notably included heat as a form of energy, which was the first law of thermodynamics.  Hot objects contain potential energy in the form of their heat, and all the usual rules of transformation between potential and kinetic energy apply.  When an electric current is passed through a resistor, the electrical energy is converted to heat energy, and the resistor gets hotter.  Everything seemed to work out accurately.