Thermodynamics (part-2)

Heat depends on the path

We have seen that, in general, the work done on or by a system is not a function of the coordinates of the system but depends on the path by which the system is brought from the initial to the final state. Exactly the same is true of the heat transferred to or from a system. The heat Q is not a function of thermodynamic coordinates but depends on the path. An infinitesimal amount of heat, i.e. dQ is not an exact differential.  

Impossibility of perpetual motion of the first kind

perpetual motion

When a system is carried through a cyclic process change of internal energy, i.e. du is equal to zero and so dQ=dW. That is, the net heat flowing into the system is equal to the net work done by the system. This means that it is impossible to construct a cycles, will put out more energy in the form of work than is absorbed in the form of heat. A machine which would create energy out of nothing is called a perpetual motion machine of the first kind. The first law is sometimes stated, “A perpetual motion machine of the first kind is impossible”. 

Reversible and irreversible processes

reversible process

(1)    A reversible process is one which can be retraced in the opposite direction so that the working substance passes through exactly the same states in all respects as in the direct process. Moreover, the thermal and mechanical effects at each stage should be exactly reversed. That is, if heat is absorbed by a substance in the direct process to produce external work, the substance will give out an equal quantity of heat in the reverse process when the same amount of external work is done on it. In practice, no change is completely reversible, but changes which occur slowly are normally almost so. The following examples will clarify the process.

(a)    When heat is added to a given mass of a gas at constant pressure, it expands and performs some external work. If the same amount of work be done on the gas, it will give out the same quantity of heat. It is assumed that were is no friction to be overcome during the process as work done in overcoming friction is wasted. The process must be slow otherwise oscillations and eddy currents will be setup and energy wasted in producing them is not recoverable. It is also to be noted that no heat must be lost by conduction, convection and radiation during the operation; as such losses cannot be reversed.

(b)    In a reversible isothermal operation, heat is absorbed by the gas as it expands and does external work and is given out when it is compressed by the same amount and work is done on the gas in the reverse process.

(c)    A given mass of ice changes to water when a certain amount of heat is absorbed by it and the same mass of water changes to ice when the same quantity of heat is removed from it.

(d)    Evaporation is reversible, as a water changes to steam on absorbing heat and steam changes to water on losing heat.

(e)    All isothermal and adiabatic changes are reversible when performed slowly. When a gas is compressed isothermally its volume decreases and on releasing the pressure the gas regains is original volume if there is no friction.

irreversible process

(2)    An irreversible process is such that it cannot be retraced in the opposite direction by reversing the controlling factors. All changes which occur suddenly like the explosion etc. may be considered as irreversible. Some examples of irreversible process are:

(a)    sudden unbalanced expansion of a gas, either isothermal or adiabatic,

(b)    Joule-Thomson expansion,

(c)    Heat produced by friction,

(d)    Heat generated when a current flows through an electrical resistance,

(e)    Exchange of heat between bodies at different temperatures by conduction or radiation.