Energy interaction (Transition)
An energy interaction or transition is said to occur or to exist between two systems when one system influences a sustains the state of the other system. Thermodynamics mainly studies the interactions between heat and work and associated property change of the system
In otherwords
Energy Interaction
energy is a General term embracing energy in transition and stored energy. The stored energy of a substance may be in the form of mechanical energy and internal energy (other forms of stored energy may be chemical energy and electrical energy). Part of the Stored energy metre the form of either potential energy (which is the gravitational energy due to height above a chosen datum line) or kinetic energy due to velocity. The balance part of the energy is known as internal energy. In a non flow process usually there is no change of potential or kinetic energy and hence change of mechanical energy will not enter the calculations. In a flow process however there may be changes in both potential and kinetic energy and these must be taken into account while considering the changes of stored energy. Heat and work are the form of energy in transition. these are the only forms in which energy can cross the boundaries of a system. Neither heat nor work and exist as stored energy.
Heat
Heat is defined as the form of energy that is transferred between two systems due to temperature difference between them. Thus temperature difference is the driving force or potential for heat transfer. Heat is not stored or passed by the system. It is identified only when it crosses the boundary of a system.Thus heat is energy in transit due to temperature gradient the transfer of heat is called heat addition and transfer of heat is called heat rejection
In otherwords
Heat
heat is something which appears at the boundary when a system changes its state due to a difference in temperature between the system and its surroundings.
Sign convention :
The magnitude of a heat flow is represented by the symbol, Q and the following sign convention is usually adopted.
Heat transfer to the system (heat addition) is positive while the heat transfer from the system (heat rejection) is negative sign convention for heat transfer is illustrated in figure
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| Fig 1. Sign Convention for Heat |
Work
Work is defined as the energy in transition which is caused by a property gradient other than the temperature difference. The work transfer results due to change in the boundary under influence of force or pressure. The the driving force for potential for work transfer is the pressure gradient across the boundary. Work is not a energy stored or passed by a system and it is identified when it crosses the boundary of a system.
In otherwords
Work
Work is said to be done when a force moves through a distance. If a part of the boundary of a system undergoes a displacement under the action of a pressure ,the work done W is the product of the force (pressure × area) and the distance it moves in the direction of the force.Fig.1 illustrates this with the conventional Piston and cylinder arrangement, the heavy line defining the boundary of the system. Fig.2 illustrates another way in which work might be applied to a system. A force is exerted by the paddle as it change the momentum of the fluid and since this force moves during rotation of the paddle room work is done.
Work is a transient quality which only appears at the boundary while change of state is taking place within a system. Work is ‘something’ which appears at the boundary when a system changes its state due to the movement of a part of the boundary under the action of a force
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| Fig.1 |
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| Fig. 2 |
Sign convention
The work transfer is represented by symbol, W and sign convention adopted for transfer in opposite due to that of heat transfer. Work done by the system (expansion) is positive while work done on the system (compression) is negative.
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| Fig 2. Sign Convention for Work |
It follows that if the work is done by the system it is done on the surroundings and vice versa. In otherwords when a system does a positive work, its surroundings do an equal quantity of negative work and vice versa. It can be represented as
Wsystem + Wsurroungings = 0
Comparison between heat and work
From the above the discussion it is clear that both heat and work are interactions between a system and its surroundings and they have the following similarities.
Both heat and work are boundary phenomenon and recognise only as they cross the boundary of a system.
Heat and work are transit energies and they are not stored or possessed by a system i.e., they are not the energies in state.
Heat and work are depend on the path followed by the system during a process they are independent of states
The main difference of heat and work is that the driving force for heat transfer is temperature difference whereas for work transfer is unbalanced mechanical forces.
In otherway
Similarities
Both are path functions and inexact differentials
Both are boundary phenomenon i.e., both are recognised at the boundaries of the system as they cross them
Both are associated with a process not a state. Unlike properties work or heat has no meaning as a state.
Systems passes, energy but not work or heat
Dissimilarities
In heat transfer temperature difference is required
in a stable system there cannot be work transfer however there is no restriction for the transfer of heat.
The sole effect external to the system could be reduced to rise of a weight but in the case of a heat transfer other effects are also observed.
Reversible work
Let us consider an ideal frictionless fluid container in a cylinder behind a piston. Assume that the pressure and temperature of the fluid are uniform and that there is no friction between the Piston and cylinder walls
Let A = cross-sectional area of the piston
p = pressure of the fluid at any instant
(p – dp)A = restraining force exerted by the surroundings on the piston
dl = the distance moved by the Piston under the action of the force exerted.
Then work done by the fluid on the piston is given by force times the distance moved,
i.e., Work done by the fluid
= (pA) × dl = pdV
(where dV = a small increase in volume)
Or considering unit mass
Work done = pdv (where v = specific volume)
This is only true when
a) the process is frictionless and
b)the difference in pressure between the fluid and its surroundings during the process is infinitely small. Hence when a reversible process takes place between state 1 to sates 2 and we have
Work done by the unit mass of fluid = ₁ʃ² pdv
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| Fig.3 |
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| Fig.4 |
When a fluid undergoes a reversible process a series of state can be joined up to form a line on a diagram of properties. The work done by the fluid during any reversible process is therefore given by the area under the line of process plotted on a p-v diagram Fig.4.
Work done = Shaded Area on Fig.4
= ₁ʃ² pdv.
When p can be expressed in terms of v then the integral, ₁ʃ² pdv ,can be evaluated.
