Download The first law for a process

Chapter 2: Energy and the First Law of
Thermodynamics
Work is defined in Mechanics as: A force F acting through a
displacement x. That is
If F is a constant, W = Fx
In Thermodynamics, we define work as follows:
Work is done by a system if the sole effect on the surroundings
(every external to the system) could be the raising of a weight
This definition is, following the tradition of Thermodynamics, is a
natural extension of the concept of work in Mechanics, and the two
definitions are equivalent.
Definition of Work
• The raising of a weight is in effect a force acting through a distance.
The definition herein covers the definition of Mechanics, but it is more
general.
• Notice that our definition does not state that a weight was actually
raised or that a force actually acted through a given distance, but that
the sole effect external to the system could be the raising of a weight.
In case (a), no weight is
being raised.
However, the fan could be
replaced by a pulley/weight
system and a weight is
being raised, case (b).
Based on the work definition of
Mechanics, the battery is not doing
work. However, based on the work
definition of Thermodynamics, the
battery is doing work, as its effect
on the motor/pulley system is the
raising of a weight.
Units of work
The unit for work in SI is called joule (J)
1 J = 1 N-m ~ one Newton acting through one meter.
Work done at the moving boundary of a simple
compressible system in a quasi-equilibrium process
p and V are the pressure
and volume of the gas,
properties of the gas .
Work done in a process
Work done in a process
For a fluid, work
can be done only
by expansion,
while work is
received through
contraction.
Work done in a process
Polytropic processes
A polytropic
process is a quasiequilibrium process,
which consists of a
series of equilibrium
states.
Some other systems in which work is done at
a moving boundary
The work here is the
work done by the
system, the wire or
film. Therefore, it is
negative!
Some other systems in which work is done at
a moving boundary
• Work can be identified only at the
boundaries of the system
Definition of Heat
Units of heat
Like work, heat is a form of energy transfer to or from a system. SI : Joule
Calorie: The amount of heat required to raise 1 gram of water from
14.5 C to 15.5 C
First Law of Thermodynamics - The first law
for a control mass undergoing a cycle
Gas
Gas
First Law of Thermodynamics - The first law for a control
mass undergoing a cycle
The concept of energy
conservation and conversion
Heat is a form of energy/In a
cycle, mechanical energy is
converted into thermal energy
The first law for a process
The first law for change in
state of a control mass or
system
The first law for a process
A property is a function of the
state, independent of
processes
where E1, and E2 are the initial and final values of energy E of the system
The first law for a process
E represents all the energy of the system in the given state.
(Kinetic or potential energy, energy associated with the motion
and position of molecules, atomic energy, chemical energy ... )
In thermodynamics, it is convenient to separate the bulk kinetic
and potential energy with the other energy of the control mass:
E = Internal energy + kinetic energy + potential energy
E = U+ KE +PE
The first law for a process
The first law for a process
From state 1 to state 2 with constant g
The first law for a process
Three observations:
• Energy E is a property. It is more convenient to consider
internal energy and kinetic and potential energies separately.
• The first law is a statement of energy conservation and
conversion. The energy cannot be destroyed nor created, but can be
converted from one form to another
• The above two equations can give only changes in internal
energy, kinetic energy, and potential energy. The absolute
values of these quantities depend on the reference status.
The kinetic energy of a body with zero velocity relative to the
earth is assumed zero. The value of the potential energy is
assumed zero when the body is at some reference elevation, the
reference value of internal energy is based on applications and
convenience.