Page 26 - Advanced thermodynamics for engineers

1.5 INTERACTIONS BETWEEN SYSTEMS AND SURROUNDINGS 9

The specific heat at constant volume is a property of the system because it is a derivative of the
two properties. The units of c v in SI are J/kg K, although in engineering it is usual to quote the values in
kJ/kg K.

1.5.6 THE FIRST LAW OF THERMODYNAMICS IN THE ABSENCE OF KINETIC
AND POTENTIAL ENERGY EFFECTS
If the kinetic and potential energy terms are removed from Eqn (1.13) then the First Law equation
becomes
dQ dW ¼ mu 2 mu 1 (1.17)
which in turn can be simplified to
dQ ¼ dU þ dW (1.18)
Most of the applications of the First Law will be based on the formulations in Eqns (1.17) and (1.18).

1.5.6.1 The First Law of Thermodynamics and the conversion of work to heat
The First Law contains no information on the direction of transferability of heat to work and work to
heat. This means that work can be completely converted to heat, and also implies that heat can be
completely converted to work. The First Law only states that energy can only be converted from one
form to another.

1.5.6.1.1 Perpetual motion machine of the first kind (PMM 1)
A perpetual motion machine of the first kind (PMM 1) is one which operates in a cycle and generates a
work output without any other interaction with the surroundings. This is impossible by the First Law.

1.5.6.1.2 Perpetual motion machine of the second kind (PMM 2)
A perpetual motion machine of the second kind (PMM 2) does not disobey the First Law, because the
heat transfer to the cyclical system is transmuted into an equal and opposite work transfer. Hence,
energy is conserved. It is not until the Second Law of Thermodynamics has been introduced that it can
be shown that a PMM 2 is impossible.
1.5.6.2 Application of First Law of Thermodynamics to three general processes
1.5.6.2.1 Constant volume process
Two processes for a constant volume (isochoric) system have the same end states, one is brought about
by work transfer and the other by heat transfer, and hence the change in internal energy is the same in
both cases.
The heat transfer to the system, dQ, is equal to the work transfer to the system, LdW.
1.5.6.2.2 Constant pressure process
It is easier to ensure that a system undergoes a constant pressure (isobaric) process than a constant
volume one; this can be achieved by a piston cylinder arrangement.

(a) Heating process
The First Law gives
dQ ¼ dU þ pdV (1.19)