First Law of Thermodynamics

The First Law of Thermodynamics is that the increase in internal energy of a closed system equals the amount of heat energy added minus the work performed by the system.

Mathematically, this is described as follows:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathrm{d}U=\delta Q-\delta W\,}

where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathrm{d}U} is the infinitesimal increase in the internal energy, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta Q} is the infinitesimal amount of heat added, and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta W} is the infinitesimal amount of work performed.

Note that if no energy is added, then the maximum amount of work that can be performed by the system is equal to its initial energy. This prevents the existence of a type of perpetual motion machine.

This is however being questioned by many scientists around the world. It is also only a theory.