<P> Thus, conservation of energy (total, including material or rest energy), and conservation of mass (total, not just rest), each still holds as an (equivalent) law . In the 18th century these had appeared as two seemingly - distinct laws . </P> <P> The discovery in 1911 that electrons emitted in beta decay have a continuous rather than a discrete spectrum appeared to contradict conservation of energy, under the then - current assumption that beta decay is the simple emission of an electron from a nucleus . This problem was eventually resolved in 1933 by Enrico Fermi who proposed the correct description of beta - decay as the emission of both an electron and an antineutrino, which carries away the apparently missing energy . </P> <P> For a closed thermodynamic system, the first law of thermodynamics may be stated as: </P> <Dl> <Dd> δ Q = d U + δ W (\ displaystyle \ delta Q = \ mathrm (d) U+ \ delta W), or equivalently, d U = δ Q − δ W, (\ displaystyle \ mathrm (d) U = \ delta Q - \ delta W,) </Dd> </Dl>

Who established the relationship between work done and the change in energy that results