Page 22 - Advanced thermodynamics for engineers

1.5 INTERACTIONS BETWEEN SYSTEMS AND SURROUNDINGS 5

and for a process in which the pressure varies with volume the work is
Z
W ¼ pdV (1.2)

1.5.1.4 Unresisted expansion
Consider a system is at a pressure of p 1 and is separated from the surroundings, at a pressure of p 2 ,bya
diaphragm, where p 1 > p 2 . If the diaphragm is burst then the system expands against the surroundings:
but it can only do work against a reacting force. If the reacting force is not equal and opposite to the
internal force in the system then the work done by the system is reduced to that which can be absorbed
by the surroundings: in this case, !p 2 dV.
If the surroundings of the system were a vacuum then p 2 is zero and the work done by the
system in expanding against the vacuum would be zero. No work is done during an unresisted
expansion.

1.5.2 QUASI-STATIC PROCESSES
The rate of expansion is an important factor in assessing the work done. For the work to be !p c dV it is
necessary that
• the pressure in the cylinder is uniform, which means that the piston speed must be low relative
to the speed of sound;
• the speed of the piston is slow relative to the velocity of the molecules.
Such processes are referred to as quasi-static (semi-equilibrium) processes.
Work is not a property of a system.
Work is a transitory phenomenon used to transfer energy.
Work exists only during the process that is causing it.

1.5.3 HEAT TRANSFER
Heat transfer is another possible form of energy transfer across system boundaries. It is an interaction
that occurs across the boundary due to temperature differences.

1.5.3.1 Definition of heat (transfer)
Heat (transfer) is the interaction between systems which occurs by virtue of temperature
differences.
• Heat interactions are associated with energy transfers.
• Heat interactions are transitory phenomena.
• Heat is energy usually being transmitted across a boundary.
• Heat is not a quantity of energy contained within a system.
• If two bodies at different temperatures are brought into contact then heat is the energy
transferred; the process ceases when the temperatures are equal (zeroth law).
• If the energy of a system is increased by heat interaction then the heat transfer is positive; if the
energy is decreased then the heat transfer is negative.