First Law of Thermodynamics

The first law of thermodynamics is fundamentally linked to the conservation of energy. It deals with the conversion of heat energy into work and vice versa. In the context of closed systems, energy can be transferred through both work and heat transfer.

In thermodynamics, “work” represents a method for transferring energy. When work is done by a system, it is considered positive (W > 0), whereas work done on a system is considered negative (W < 0). Similarly, when heat is added to a system, it is considered positive (Q > 0), and when heat is released by a system, it is considered negative (Q < 0).

When a small amount of work (dw) is supplied to a closed system undergoing a cycle, the work supplied is equal to the heat transfer or heat produced (dQ) within the system. This relationship is expressed as:

dw = ∫J ∫ dQ

Here, J is a constant in joules, with 1 calorie equal to 4.18 joules. If Q amount of heat is added to a system undergoing a change of state, and W represents the work done by the system during the process, the net energy (Q – W) is stored within the system as internal energy or simply energy (∆U):

Q – W = ∆U

Sign Convention:

  • dQ > 0: Heat is added to the system, or the system absorbs heat.
  • dQ < 0: Heat is removed from the system, or the system rejects heat.
  • dW > 0: Work is done by the system.
  • dW < 0: Work is done on the system.
  • ∆U > 0: Internal energy of the system increases.
  • ∆U < 0: Internal energy of the system decreases.

The paragraph also mentions the concept of the first kind of perpetual motion machine (PMM1), which is a hypothetical machine that continuously supplies mechanical work without any energy loss, and it concludes that PMM1 is impossible, representing a fictitious concept.

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