<Ul> <Li> Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other . This law helps define the concept of temperature . </Li> <Li> First law of thermodynamics: When energy passes, as work, as heat, or with matter, into or out from a system, the system's internal energy changes in accord with the law of conservation of energy . Equivalently, perpetual motion machines of the first kind (machines that produce work with no energy input) are impossible . </Li> <Li> Second law of thermodynamics: In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases . Equivalently, perpetual motion machines of the second kind (machines that spontaneously convert thermal energy into mechanical work) are impossible . </Li> <Li> Third law of thermodynamics: The entropy of a system approaches a constant value as the temperature approaches absolute zero . With the exception of non-crystalline solids (glasses) the entropy of a system at absolute zero is typically close to zero, and is equal to the natural logarithm of the product of the quantum ground states . </Li> </Ul> <Li> Zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other . This law helps define the concept of temperature . </Li> <Li> First law of thermodynamics: When energy passes, as work, as heat, or with matter, into or out from a system, the system's internal energy changes in accord with the law of conservation of energy . Equivalently, perpetual motion machines of the first kind (machines that produce work with no energy input) are impossible . </Li> <Li> Second law of thermodynamics: In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases . Equivalently, perpetual motion machines of the second kind (machines that spontaneously convert thermal energy into mechanical work) are impossible . </Li>

1st law of thermodynamics is best described as
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