First Law of Thermodynamics in Closed System by PK NAG (Chapter 04)

A: In the First Law of Thermodynamics, work (W) and heat (Q) are two different modes of energy transfer. Work is energy transfer due to any force acting through a distance, such as boundary work during expansion or compression of a gas. Heat, on the other hand, is energy transfer due to temperature difference between the system and its surroundings. While both affect the internal energy of the system, they are fundamentally different processes. PK NAG’s book provides examples and detailed explanations that help clarify this distinction.

A: During a quasi-static process in a closed system, the process occurs slowly enough for the system to remain nearly in equilibrium at all times. The First Law applies as usual: the change in internal energy equals heat added to the system minus work done by the system. Because the process is quasi-static, the work done can often be easily calculated using \( W = \int P dV \), where P is the pressure and V is the volume. The quasi-static nature helps simplify the analysis by making the pressure and temperature well-defined at each intermediate state. More rigorous treatment appears in PK NAG's Chapter 04.

A: Internal energy (U) is the total energy stored within the system due to molecular motion and interactions. Enthalpy (H) is defined as \( H = U + PV \) and often used in processes involving pressure-volume work, especially at constant pressure. Specific heats at constant volume (\( C_v \)) and constant pressure (\( C_p \)) relate changes in internal energy and enthalpy to temperature changes: \( dU = C_v dT \) and \( dH = C_p dT \) respectively, assuming ideal gas behavior. These relationships help apply the First Law in closed systems when temperature changes occur, as discussed in PK NAG's textbook.

A: In an adiabatic process, the system exchanges no heat with its surroundings (Q = 0). According to the First Law \( \Delta U = Q - W \), this reduces to \( \Delta U = -W \). This means that any work done by the system leads to a decrease in internal energy, and work done on the system increases internal energy. Adiabatic processes are common in thermodynamics, and PK NAG discusses them in detail, including how they affect temperature, pressure, and volume changes in closed systems.

A: In a cyclic process, the system returns to its initial state at the end of the cycle, so the net change in internal energy over one complete cycle is zero (\( \Delta U = 0 \)). According to the First Law, \( 0 = Q - W \) or \( Q = W \). This means the net heat added to the system equals the net work done by the system during the cycle. This principle is fundamental in thermodynamic cycles such as engines and refrigerators. PK NAG’s chapter on the First Law covers cyclic processes and their importance in practical thermodynamics.

A: The control volume concept involves an open system where mass crosses the system boundaries, whereas a closed system does not allow mass transfer. The First Law for a closed system considers only heat and work interactions changing the internal energy. In contrast, the First Law for an open system (control volume) also accounts for energy associated with mass flow (enthalpy, kinetic and potential energy). PK NAG distinguishes these concepts and provides detailed explanations in separate chapters, emphasizing their application in different engineering problems.