Biophysics Terminology

  Biophysics Terminology

Open System Energy and matter Flows

Closed SystemEnergy flow but matter cannot

Sum of all energy –is called total energy E of a system

E = U (internal) + KE + PE

A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings.

Boundary: the real or imaginary surface that separates the system from its surroundings. The boundaries of a system can be fixed or movable. Mathematically, the boundary has zero thickness, no mass, and no volume

Closed system or control mass: consists of a fixed amount of mass, and no mass can cross its boundary. But, energy in the form of heat or work, can cross the boundary, and the volume of a closed system does not have to be fixed

Open system or control volume: is a properly selected region in space. It usually encloses a device that involves mass flow. Both mass and energy can cross the boundary of a control volume

Adiabatic system: A closed or open system that does not exchange energy with the surroundings by heat

  Total Energy

Total energy of a system has two groups:

  • macroscopic energy
  • microscopic energy

Macroscopic forms of energy:

forms of energy that a system posses as a whole with respect to some outside reference frame, such as kinetic and potential energy. The macroscopic energy of a system is related to motion and the influence of some external effects such as gravity, magnetism, electricity, and surface tension

Kinetic energy: energy that a system posses as a result of its relative motion relative to some reference frame, KE = ½ mv2

Potential energy: is the energy that a system posses as a result of its elevation in a gravitational field

PE = mgh


Microscopic forms of energy:

are those related to molecular structure of a system. They are independent of outside reference frames. The sum of microscopic energy is called the internal energy, U.

The total energy of a system consists of the kinetic, potential, and internal energies: where the contributions of magnetic, electric, nuclear energy are neglected

Internal energy is related to the molecular structure and the degree of molecular activity and it may be viewed as the sum of the kinetic and potential energies of molecules.

  • The sum of translational, vibrational, and rotational energies of molecules is the kinetic energy of molecules, and it is also called the sensible energy. At higher temperatures, system will have higher sensible energy
  • Internal energy associated with the phase of a system is called latent heat. The intermolecular forces are strongest in solids and weakest in gases
  • The internal energy associated with the atomic bonds in a molecule is called chemical or bond energy. The tremendous amount of energy associated with the bonds within the nucleolus of atom itself is called atomic energy

Energy interactions with a closed system can occur via heat transfer and work

  Thermodynamic Processes and Cycles

Process - any succession of events

Chemical process - a chemical or physical operation, or series of operations, which transforms raw materials into products

Thermodynamic process - the path of succession of states through which the system passes in moving from an initial state to a final state

Polytropic process - a thermodynamic process for which [PVn] is constant.  These processes are usually associated only to systems for which the ideal gas assumption holds (PV = nRT)

 Four special polytropic processes:

(1) Isobaric  - - - - - - - constant pressure   [n = 0]

(2) Isothermal  - - - - - - constant temperature [n = 1]

(3) Isentropic  - - - - - - constant entropy    [n = gamma,(Cp/Cv)]

(4) Isochoric (isometric) - constant volume     [n = infinity]