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Thermodynamics

Thermodynamic Terms : System and Surroundings, Types of System

The System and the Surroundings

• System − Part of the universe in which observations are made

• Surroundings − Part of universe excluding system

• Universe = System + Surroundings Types of the System

• Open system − There is exchange of energy and matter between system and surroundings. (shown in figure) Example − Presence of reactants in an open beaker

• Closed system − There is no exchange of matter, but exchange of energy is possible between the system and the surroundings. (shown in figure) Example − Presence of reactants in closed vessel made of conducting material

• Isolated system − There is no exchange of energy or matter between the system and the surroundings. (shown in figure) Example − Presence of reactants in a thermos flask or any other closed insulated vessel

The State of System

• State of a thermodynamic system can be described by properties such as its pressure (p), temperature (T), volume (V), composition of the system, etc.

• Variables such as p, V, T are called state variables or state functions.

• The values of state functions or state variables depend only on the state of the system and not on how it is reached.

• To define the state of a system, it is not necessary to define all the properties of the system.

The System and the Surroundings

• System − Part of the universe in which observations are made

• Surroundings − Part of universe excluding system

• Universe = System + Surroundings Types of the System

• Open system − There is exchange of energy and matter between system and surroundings. (shown in figure) Example − Presence of reactants in an open beaker

• Closed system − There is no exchange of matter, but exchange of energy is possible between the system and the surroundings. (shown in figure) Example − Presence of reactants in closed vessel made of conducting material

• Isolated system − There is no exchange of energy or matter between the system and the surroundings. (shown in figure) Example − Presence of reactants in a thermos flask or any other closed insulated vessel

The State of System

• State of a thermodynamic system can be described by properties such as its pressure (p), temperature (T), volume (V), composition of the system, etc.

• Variables such as p, V, T are called state variables or state functions.

• The values of state functions or state variables depend only on the state of the system and not on how it is reached.

• To define the state of a system, it is not necessary to define all the properties of the system.

• Internal energy (U) represents the total energy of a system (i.e., the sum of chemical, electrical, mechanical or any other type of energy).

• Internal energy of a system may change when:

• Heat passes into or out of the system
• Work is done on or by the system
• Matter enters or leaves the system

Work

• For an adiabatic system which does not permit the transfer of heat through its boundary (shown in the figure), a change in its internal energy can be brought by doing some work on it. • Initial state of the system, (1)

Temperature = T1

Internal energy = U1

• When some mechanical work is done, the new state (2) is obtained.

Temperature at state 2 = T2

Internal energy at state 2 = U2

• It is found that T2 >T1

Change in temperature, ΔT = T2T1

Change in internal energy, ΔU = U2U1

• The value of internal energy (U) is the characteristic of the state of a system.

• The adiabatic work (Wad) required to bring about a change of state is equal to the change in internal energy.

ΔU = U2U1 = Wad

• Thus, internal energy (U) of the system is a state function.

• When work is done on the system, Wad = + ve

• When work is done by the system, Wad = − ve

Heat

• Internal energy of the system can also be changed by transfer of heat from the surroundings to the system or vice versa, without doing any work.

• This exchange of energy, which is a result of temperature difference, is called heat (q).

• A system which allows heat transfer through its boundary is shown in the figure. • At constant volume, when no work is done, the change in internal energy is, ΔU = q

• When heat is transferred from the surroundings to the system, q is positive.

• When heat is transferred from the system to the surroundings, q is negative.

General Case

• When change in state is brought about bot…

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