Basic Concepts and Definitions

Thermodynamics studies energy, heat, work, and the properties of matter and their changes. A system is a specified portion of the universe; everything else is the surroundings. Systems are classified as closed (no mass exchange, but energy exchange allowed), open (mass and energy exchange allowed), or isolated (no mass or energy exchange). Properties describe the state of a system and are either intensive (independent of system size, e.g., temperature, pressure, density) or extensive (depend on size, e.g., mass, total volume, internal energy). State functions depend only on the state (e.g., internal energy U, enthalpy H, entropy S); path functions depend on the process path (e.g., heat Q, work W). Processes are characterized as isothermal (constant T), isobaric (constant p), isochoric (constant V), adiabatic (no heat transfer), reversible (idealized, no entropy generation and quasi-static), or irreversible (real processes with entropy generation). The first law is conservation of energy; the second law introduces entropy and irreversibility. For ideal gases, simple relations link pressure, volume and temperature and allow specific internal energy and enthalpy to depend only on temperature.

Governing FormulaKey definitions and relations (SI units): - Ideal gas (per unit mass): p v = R T (p in Pa, v in m^3/kg, T in K, R in J/kg·K) - Universal gas constant: R_universal = 8.314462618 kJ/kmol·K (use with molar mass) - Specific/internal/enthalpy relations for ideal gas: du = c_v dT, dh = c_p dT, c_p - c_v = R - First law (closed system): dU = δQ - δW; for boundary (PV) work δW = p dV - Enthalpy: H = U + pV (specific: h = u + p v) - Steady-flow energy (per unit mass): q - w = (h2 - h1) + (ke2 - ke1) + (pe2 - pe1) - Isentropic ideal gas relations (reversible adiabatic, k = c_p/c_v): p v^k = const, T v^{k-1} = const, T2/T1 = (p2/p1)^{(k-1)/k} - Entropy change for ideal gas (per unit mass): s2 - s1 = c_p ln(T2/T1) - R ln(p2/p1) (assuming c_p const.) - Sign conventions used here: δQ positive when heat is added to the system; δW positive when work is done by the system.

Knowledge Check

10 Questions

1.Which statement correctly defines an isolated system in classical thermodynamics?

2.Which of the following is an intensive property?

3.Which one of the following is a path function (i.e., depends on process path rather than only end states)?

4.A closed container holds 2.00 kg of an ideal gas with specific gas constant R = 287 J·kg^{-1}·K^{-1} at pressure p = 100 kPa and temperature T = 300 K. What is the total volume of the gas? (Use v = R T / p.)

5.A closed system initially has internal energy U1 = 500 kJ. Heat of 200 kJ is added and the system does 150 kJ of work on the surroundings. What is the final internal energy U2?

6.For an ideal gas the specific heats at constant pressure and volume are c_p = 1005 J·kg^{-1}·K^{-1} and c_v = 718 J·kg^{-1}·K^{-1}. What is the specific gas constant R = c_p - c_v ?

7.An ideal diatomic gas (k = c_p/c_v = 1.4) undergoes a reversible adiabatic compression from p1 = 100 kPa and T1 = 300 K to p2 = 500 kPa. What is the final temperature T2 (use T2 = T1 (p2/p1)^{(k-1)/k})?

8.A closed rigid vessel (constant volume) contains 1.50 kg of an ideal gas with c_p = 1005 J·kg^{-1}·K^{-1}. The temperature increases from 290 K to 330 K. What is the change in enthalpy ΔH of the gas? (Use Δh = c_p ΔT and ΔH = m Δh.)

9.For an ideal gas with constant c_p = 1005 J·kg^{-1}·K^{-1} and R = 287 J·kg^{-1}·K^{-1}, the state changes from T1 = 300 K, p1 = 100 kPa to T2 = 360 K, p2 = 80 kPa. What is the specific entropy change Δs = s2 - s1 (use Δs = c_p ln(T2/T1) - R ln(p2/p1))?

10.Which of the following descriptions best characterizes a reversible process in classical thermodynamics?