Basic TD Concepts

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. Qkdt/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.