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 (everything 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. 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. However, the fan could be replaced by a pulley/weight system and a weight is being raised, case (b). In case (a), no weight is being raised.

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 quasi-equilibrium 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.

Thermodynamic Cycles A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning. Power cycles: The cycles that convert some heat input into a mechanical work output. Refrigeration Cycles: The cycles that cool a heat source (such as in the normal operation of a freezer) by moving heat from a cold place to a warm place using mechanical work (or a high-temperature heat source). Heat Pump Cycles: The cycles that moves heat from one location (the "source") at a lower temperature to another location (the "sink" or "heat sink") at a higher temperature using mechanical work (or a high-temperature heat source).

Power Cycles Power cycles: The cycles that convert some heat input into a mechanical work output.

Refrigeration Cycles Refrigeration Cycles: The cycles that cool a heat source (such as in the normal operation of a freezer) by moving heat from a cold place to a warm place using mechanical work (or a high-temperature heat source)

An example of ideal vapor compression refrigeration systems

Heat Pump Cycles Heat Pump Cycles: The cycles that move heat from one location (the "source") at a lower temperature to another location (the "sink" or "heat sink") at a higher temperature using mechanical work (or a high-temperature heat source).