Thermodynamics. The Carnot principle states that work cannot be created in a closed cycle with only a single heat source. A minimum of two sources at temperatures T1 and T2 are needed to produce driving power. The maximum theoretical efficiency then only depends on the temperatures at which the heat is exchanged; the Carnot principle then implies Q1 / T1 + Q2 / T2 <= 0 and equal for reversible transformations, , Q1 (resp. Q2) denoting the quantity of heat (counted algebraically) provided by source 1 (resp.to source 2).
This is the first statement in the second law of thermodynamics.