The 1st law of thermodynamics

By charnwit kheanpanya

29/4/2014,21.45

         The 1st law of thermodynamics is a version of the law of conservation of energy that internal energy change of a system equals net heat transfer minus net work done by the system. where heat and work are the methods of transferring energy for a system in thermal equilibrium. Q represents the net heat transfer—it is the sum of all heat transfers into and out of the system. Q is positive for net heat transfer into the system. W is the total work done on and by the system. W is positive when more work is done by the system than on it. The change in the internal energy of the system, ΔU, is related to heat and work by the first law of thermodynamics,                                               ΔU=Q−W.

internal energy

 

                    The relationship between internal energy and work can be understood by considering another concrete example: the tungsten filament inside a light bulb. When work is done on this system by driving an electric current through the tungsten wire, the system becomes hotter and E is therefore positive. (Eventually, the wire becomes hot enough to glow.) Conversely, E is negative when the system does work on its surroundings.

                      The sign conventions for heat, work, and internal energy are summarized in the figure below. The internal energy and temperature of a system decrease (E < 0) when the system either loses heat or does work on its surroundings. Conversely, the internal energy and temperature increase (E > 0) when the system gains heat from its surroundings or when the surroundings do work on the system.

                      

Work done in any adiabatic (Q=0) process is path independent.

 

Thermodynamic process

                 A thermodynamic process may be defined as the progress of a thermodynamic system proceeding from an initial state to a final state.   The series of states the substance or system experiences as it progresses through the process is called the path of the process.
               Typically, a thermodynamic process can be characterised, according to what system property e.g. temperature, pressure, or volume, etc., are held fixed.   Furthermore, it is useful to group these properities into pairs, in which the variable held constant is one member of the pair.   The six most common thermodynamic processes are shown below:

An isobaric process occurs at constant pressure...ΔP= 0 An isochoric process, or isometric process, occurs at constant volume...ΔV= 0 An isothermal process occurs at a constant temperature...ΔT= 0 An isentropic process occurs at a constant entropy...ΔS= 0 ( Adiabetic and reversible) An isenthalpic process occurs at a constant enthalpy...Δh= 0 An adiabatic process occurs without loss or gain of heat...ΔQ= 0 A cyclic process occurs with same initial and final states A polytropic process has the relationship pV k = constant

There are a number of thermodynamic process types encountered by engineers including non-flow, steady flow, semi-flow and unsteady flow.   These are described as follows:

• Non flow processes are those involving no flow of matter across the system boundaries
• Steady flow processes involve fluid entering and leaving the system control volume these flows do not change with time and the internal energy of the control is also fixed in the time period under consideration
• A Semi-flow process involves fluid flow into a control volume which may be rigid charging a gas bottle or flexible -blowing up a balloon
• An unsteady flow process is one with a variable internal energy- i.e changing liquid level in a boiler.

 Reversible /Irreversible Process

           If the substance or system passes through a continuous series of equilibrium states in progressing through the process as it receives or rejects energy it is referred to as a reversible process.  The path of this theoretical process is generally shown on diagrams as a full line.  If the process is reversed, in the thermodynamic sense, it would leave no trace of itself.
           In the real world there are no reversible processes..All processes are irreversible and are shown on diagrams as broken lines...Factors which make a process irreversible include friction, unrestrained gas expansion, heat transfer across finite temperature difference, mixing,chemical reactions etc. etc...

P-V diagrams

Thermodynamic cycle

Thermodynamic cycle is a sequence of processes in working fluid returns to original thermodynamic state.

Fig.1

An example of a cycle applied to a fixed mass system or closed system is presented in Fig. 2 . It a gas trapped in a piston cycle assembly undergoes four processes.

Fig.2

Thermodynamic cycle are most frequently execute by a series of flow processes.

Physiccal framework analysis

System is a specifically in idenfied fixed mass of material separated from surroundings by real or imaginary boundary.
Boundary is a real or imaginary construct indicated  between the system and surrounding





Control Volumes is a region in space separated system frome surroundings by real or imaginary boundary,and control surface is a surface that mass in system crossed boundary to surrounding

control surface

System models of thermodynamics

                   Thermodynamic system is a precisely specified macroscopic region of the universe, defined by boundaries or walls of particular natures, together with the physical surroundings of that region, which determine processes that are allowed to affect the interior of the region, studied using the principles of thermodynamics.
                   The universe outside  system is known as the surroundings,  environment, or reservoir. A system is separated from its surroundings by a boundary, which may be notional or real but by convention, delimits a finite volume. Transfers of work, heat, or matter and energy between the system and the surroundings may take  across this boundary. A thermodynamic system is classified by the nature of the transfers that are allowed to occur across its boundary, or parts of its boundary.
                                       System models of thermodynamics  
 

isolated system

An isolated system has only isolating boundary sectors. Nothing can be transferred into or out of boundary.

closed system

 A closed system has no boundary sector that is permeable to matter, but in general its boundary is permeable to energy. For closed systems, boundaries are totally prohibitive of matter transfer.

opened system

An open system has a boundary sector that is permeable to matter; such a sector is usually permeable also to energy, but the energy that passes cannot in general be uniquely sorted into heat and work components. Open system boundaries may be either actually restrictive, or else non-restrictive.

                              

Thermodynamices

By:  charnwit kheanpanya
Thermodynamics is a branch of physics which deals with the energy and study in effect of heat,work and energy on system
Laws of thermodynamics.
     Zeroth laws : if two system are in thermal equilibrium with a third system,they must be in thermal equilibrium with each other.
        such as    if system A  and system B are thermal equilibrium and system B  and system C are thermal equilibrium. Also system A  and system C are thermal equilibrium
     First laws: Is often called Low of conservation of energy.That energy can be neither created nor destroyed.That impossible a machine violate the first low.Because  flow of thermal energy from one object to another.
          The difference of internal energy of a closed system is equal to the difference of the heat supplied to the system and the work done by it : ΔU = Q - W
     Second laws:The entropy of any isolated system cannot decrease. Heat cannot be transfer from a colder to a hotter body. A result of  energy transfer must have one direction, and all natural processes are irreversible.

      Third law : if a system approaches absolute zero the entropy of the system approaches a minimum value.The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature. This law provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is the absolute entropy. Alternate definitions are, "the entropy of all systems and of all states of a system is smallest at absolute zero," or equivalently "it is impossible to reach the absolute zero of temperature by any finite number of processes".
            Absolute zero is −273.15 °C (degrees Celsius), or −459.67 °F (degrees Fahrenheit) or 0 K (kelvin).