Fuel and the first law of thermodynamics

Energy is neither created nor destroyed, it only changes from one kind to another; the first law of thermodynamics is this law applied to heat phenomena (R. Mayer, H. Helmholtz, J. Joule, mid-19th century). The heat given to a system is spent on changing its internal energy and on doing work against external forces: Q = ΔU + A. In an isothermal process T = const, ΔU = 0, so Q = A. In an isobaric process Q = ΔU + pΔV. In an isochoric process A = 0, so Q = ΔU. In an adiabatic process, with no heat exchange with the surroundings, Q = 0 and A = –ΔU: when the gas expands, the work is done at the expense of internal energy and the gas cools; when compressed it warms. Fast processes (compressing air in a pump, expansion of rising air and cloud formation) are close to adiabatic. A thermos flask is close to adiabatic insulation.