What are the corollaries or consequences of the First Law of Thermodynamics?
The first law of thermodynamics is a simple statement for the law of conservation of energy.
This is the assertion that the perpetual machines of the first kind do not exist.
Any machine which works without expenditure of energy is called the perpetual machine of the first kind. The first law of thermodynamics states that perpetual machines of the first kind don't exist.
Although, apparently first law of thermodynamics is so obvious from what we learn in middle school, it's mathematical formulation a powerful tool for working with problems in thermodynamics and deriving useful results in thermodynamics.