The First Law Of Thermodynamics The First Law Of Thermodynamics The first law of thermodynamics is a version of the law of conservation of energy, specialized for thermodynamical systems. It is usually formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work performed by the system on its surroundings. The law of conservation of energy can be stated: The energy of an isolated system is constant. Description :- The first law of thermodynamics was expressed in two ways by Clausius. One way referred to cyclic processes and the inputs and outputs of the system, but did not refer to increments in the internal state of the system. The other way referred to any incremental change in the internal state of the system, and did not expect the process to be cyclic. A cyclic process is one which can be repeated indefinitely often and still eventually leave the system in its original state. In each repetition of a cyclic process, the work done by the system is proportional to the heat consumed by the system. In a cyclic process in which the system does work on its surroundings, it is necessary that some heat be taken in by the system and some be put out, and the difference is the heat consumed by the system in the process.

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System : It is the space in the universe where the thermodynamic investigations or processes occur. Surroundings: This is the matter that lies all around the system. Universe: Total space that comprises of both system and surroundings. Systems are of three types :Open: Both matter and energy can be exchanged between the system and its surroundings. Closed: Heat can be exchanged between system and its surroundings but not matter. Isolated: Neither heat nor energy can move in and out of the system. Process: A process is the path taken by a system that alters or changes its state. Processes can be 1. Isothermal :- process occurs at constant temperature. 2. Isochoric :- process occurs at constant volume. 3. Isobaric :- process occurs at constant pressure. 4. Adiabatic :- process occurs with no exchange of heat. 5. Reversible :- If after a change in the state of a system, the system is able to go back to its initial state.

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6. Irreversible :- If after a change in the state of a system, the system is unable to go back to its initial state. Solved Problems Problem 1 :- A gas expands against a constant pressure of 1 atm from a volume of 10 L to 20 L. During this process, the system absorbs 600 J of heat from the surroundings. Calculate the internal energy of the system. Answer :ΔU = q + w = 600 J + (-PΔV) = -413 J = 600 J + {- 101.3 Pa (20-10) L} = 600 J + (-1013 J) = - 413 J Problem 2 :- q amount of heat is transferred to the system from the surroundings and w amount of work is done by the system. Write the expression for the internal energy. Answer :ΔU = q – w

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The First Law Of Thermodynamics