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Basic TD Concepts

Basic TD Concepts. A system is the region enclosed by an imaginary boundary which can be rigid, flexible, stationary, moving, solid or with through-flow. Everything outside this boundary is called the surroundings .

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Basic TD Concepts

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  1. Basic TD Concepts • A system is the region enclosed by an imaginary boundary which can be rigid, flexible, stationary, moving, solid or with through-flow. • Everything outside this boundary is called the surroundings. • A reservoir is a body large enough not to be affected by small transfers of energy

  2. Closed vs. Open System • In a closed system no mass can cross the system boundary. • In an open system mass can cross the system boundary. • Energy without mass can cross the boundaries of an open system. • If neither mass nor energy cross the boundary the system is isolated.

  3. Control Features • A control surface surrounds a system or part of a system. • The volume contained inside a control surface is the control volume. • The mass contained inside a control surface is the control mass.

  4. Energy • A macroscopic body can have: • Kinetic energy (=1/2 m v2) • Potential energy • Gravity (=mgh) • Spring (=1/2kx2) • Electric/magnetic field • Internal energy

  5. Internal energy (U) • The internal energy (hotness) of a body is due to the following microscopic (molecular energies): • Translational (KE) • Rotational (KE) • Vibrational (KE & PE) • Electronic • Molecular/chemical • Nuclear

  6. Total Energy & its Transfer • Internal energies depend on temperature U = fn(T) • Total energy = KE + PE + U • Energy can be transferred as: • Work - random molecular motion becomes organized motion. W = PdAdx = PdV • Heat across a temperature gradient. Q=kdt/dx

  7. Thermodynamic Properties • Thermodynamic properties are independent of location, velocity or acceleration. • Intensive variables do not depend on system size (p, T, , specific values). • Extensive variables depend on system size (m, V, U). • Ratios of extensive variables are intensive.

  8. Equilibrum • A system reaches equilibrium when it will no longer change even if we wait forever. • Thermal equilibrium: T uniform (no heat transfer). • Mechanical equilibrium: p uniform (no work). • Chemical equilibrium: composition fixed. • Thermodynamic equilibrium: all of the above. • State - a unique combination of all properties of the system.

  9. State variables • State variables are variables that are fixed for a given state. • For a given change in state they change by a fixed amount. • Examples of state variables: p, T, , V, u, s, h, etc. • Not state variables (path dependent) are w, q. • On a p-V or s-T diagram a state is a point. • Path between two points is a process.

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