Chapter: 02 ENERGY & ENERGY TRANSFER

Objectives Introduce the concept of energy and define its various forms. Discuss the nature of internal energy. Define the concept of heat and the terminology associated with energy transfer by heat. Discuss the three mechanisms of heat transfer: conduction, convection, and radiation. Define the concept of work, including electrical work and several forms of mechanical work. Compare Heat and Work

An important point to remember… Thermodynamics provides no information about the absolute value of the total energy. It deals only with the change of the total energy, which is what matters in engineering problems. Thus the total energy of a system can be assigned a value of zero (E =0) at some convenient reference point. The change in total energy of a system is independent of the reference point selected. The decrease in the potential energy of a falling rock, for example, depends on only the elevation difference and not the reference level selected.

ENERGY of a SYSTEM TOTAL ENERGY MICROSCOPIC FORM MACROSCOPIC FORM The microscopic forms of energy are those related to the molecular structure of a system and the degree of the molecular activity, and they are independent of outside reference frames. The macroscopic forms of energy are those a system possesses as a whole with respect to some outside reference frame. KINETIC ENERGY INTERNAL ENERGY The sum of all the microscopic forms of energy is called the internal energy of a system and is denoted by U. POTENTIAL ENERGY  The magnetic, electric, and surface tension effects are significant in some specialized cases only and are usually ignored.

ENERGY of a SYSTEM TOTAL ENERGY MICROSCOPIC FORM MACROSCOPIC FORM KINETIC ENERGY INTERNAL ENERGY POTENTIAL ENERGY  Closed systems whose velocity and elevation of the centre of gravity remain constant during a process are frequently referred to as stationary systems.

FORMS OF ENERGY Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system. Thermodynamics deals only with the change of the total energy. Macroscopic forms of energy: Those a system possesses as a whole with respect to some outside reference frame, such as kinetic and potential energies. Microscopic forms of energy: Those related to the molecular structure of a system and the degree of the molecular activity. Internal energy, U: The sum of all the microscopic forms of energy. Kinetic energy, KE: The energy that a system possesses as a result of its motion relative to some reference frame. Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational field. The macroscopic energy of an object changes with velocity and elevation.

Thermal = Sensible + Latent ENERGY of a SYSTEM Physical Insight to INTERNAL ENERGY The portion of the internal energy of a system associated with... the kinetic energies of the molecules is called the sensible energy. Internal = Sensible + Latent + Chemical + Nuclear Thermal = Sensible + Latent binding forces between the molecules of a substance. binding forces between the atoms within a molecule. binding forces and between the particles within an atom and its nucleus. 7

INTERACTION

ENERGY INTERACTION

INTERACTION HEAT INTERACTION WORK INTERACTION

ENERGY TRANSFER BY HEAT Heat: The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference. Temperature difference (∆T) is the driving force of heat transfer.

ENERGY TRANSFER BY HEAT Heat transfer per unit mass Amount of heat transfer when heat transfer rate is constant Amount of heat transfer when heat transfer rate changes with time

ENERGY TRANSFER BY HEAT During an adiabatic process, a system exchanges no heat with its surroundings.

Specifying the directions of heat and work. SIGN CONVENTION Formal sign convention: Heat transfer to a system is positive; heat transfer from a system is negative. Alternative to sign convention is to use the subscripts in and out to indicate direction. Specifying the directions of heat and work.

IDENTIFYING HEAT HEAT = HEAT TRANSFER Energy in Transit Energy is recognized as heat transfer only as it crosses the system boundary. HEAT = HEAT TRANSFER Energy in Transit

HEAT TRANSFER IN THERMODYNAMIC STATE SPCE HEAT TRANSFER is PATH function In-exact Differential

Heat = a PATH FUNCTION It is recognized at the boundaries of a system as they cross the boundaries. That is, heat is boundary phenomena. Systems possess energy, but not heat. It is associated with a process, not a state. Unlike properties, heat has no meaning at a state. It is path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states). Point to Remember: Properties are point functions have exact differentials (d ).

ENERGY TRANSFER BY WORK Work: All the energy transfers which are not associated with temperature difference is known as work / work transfer. A moving piston A rotating shaft If the energy crossing the boundary of a closed system is not heat, it must be work..

ENERGY TRANSFER BY WORK Heat is easy to recognize: Its driving force is a temperature difference between the system and its surroundings. How to recognise work transfer? Work: The energy transfer associated with a force acting through a distance. A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions

ENERGY TRANSFER BY WORK Work: The energy transfer associated with a force acting through a distance.

ENERGY TRANSFER BY WORK Work: The energy transfer associated with a force acting through a distance.

ENERGY TRANSFER BY WORK Work is said to be done by a system if the sole effect on things external to the system can be reduced to the raising of a weight.

ENERGY TRANSFER BY WORK Work is the energy transfer associated with a force acting through a distance. Work is said to be done by a system if the sole effect on things external to the system can be reduced to the raising of a weight.

ENERGY TRANSFER BY WORK Work is said to be done by a system if the sole effect on things external to the system can be reduced to the raising of a weight. Is their any work transfer?

ENERGY TRANSFER BY WORK Is their any work transfer?

ENERGY TRANSFER BY WORK What is the type of work transfer?

ENERGY TRANSFER BY WORK What is the type of work transfer?

ENERGY TRANSFER BY WORK An Important point to note: Work can be identified when it crosses the boundary of the system.

POWER Power is the work done per unit time (kW)

Specifying the directions of heat and work. SIGN CONVENTION Formal sign convention: Work done by a system is positive; work done on a system is negative. Alternative to sign convention is to use the subscripts in and out to indicate direction. This is the primary approach in this text. Specifying the directions of heat and work.

** TYPES OF WORKDONE ** Mechanical form of work : Movement of boundary with the application of force. Shaft work : The power transmitted through the shaft. Moving boundary work : The expansion and compression work in a piston-cylinder device. Electrical work : When N coulombs of electrical charge move through a potential difference V

MECHANICAL FORMS OF WORK There are two requirements for a work interaction between a system and its surroundings to exist: there must be a force acting on the boundary. the boundary must move. When force is not constant Work = Force  Distance

A force F acting through a moment arm r generates a torque T Shaft Work A force F acting through a moment arm r generates a torque T

Shaft Work Shaft work Power = 2πNT

Electrical Work Electrical power Electrical work As electrons cross the system boundary do electrical work on the system. In an electric field, electrons in a wire move under the effect of electromotive forces, doing work. When N coulombs of electrical charge move through a potential difference V, the electrical work done is... Electrical power Electrical work

Paddle Wheel / Stirring Work A paddle wheel is a form of waterwheel or impeller in which a number of paddles are set around the periphery of the wheel.

Spring Work When the length of the spring changes by a differential amount dx under the influence of a force F, the work done is Substituting and integrating yield For linear elastic springs, the displacement x is proportional to the force applied x1 and x2: the initial and the final displacements k: spring constant (kN/m) Elongation of a spring under the influence of a force. The displacement of a linear spring doubles when the force is doubled.

Work Done on Elastic Solid Bars Work Associated with the Stretching of a Liquid Film

Work Done to Raise or to Accelerate a Body The work transfer needed to raise a body is equal to the change in the potential energy of the body. The work transfer needed to accelerate a body is equal to the change in the kinetic energy of the body. Nonmechanical Forms of Work Electrical work: The generalized force is the voltage (the electrical potential) and the generalized displacement is the electrical charge. Magnetic work: The generalized force is the magnetic field strength and the generalized displacement is the total magnetic dipole moment. Electrical polarization work: The generalized force is the electric field strength and the generalized displacement is the polarization of the medium.

** MOVING BOUNDARY WORK ** Moving boundary work: The expansion and compression work in a piston-cylinder device. Displacement Work / pdV work

** Simultaneous Work Transfer **

MOVING BOUNDARY WORK in Thermodynamic state space The area under the process curve on a p-V diagram is equal, in magnitude, to the work done during a quasi-equilibrium expansion or compression process of a closed system.

** MOVING BOUNDARY WORK ** Point to Note: Quasi-equilibrium process: A process during which the system remains nearly in equilibrium at all times.

** MOVING BOUNDARY WORK ** The work done during a process depends on the path followed as well as the end states. In-exact Differential

Work is a PATH FUNCTION It is recognized at the boundaries of a system as they cross the boundaries. That is, both work is boundary phenomena. Systems possess energy, but not work. It is associated with a process, not a state. Unlike properties, work has no meaning at a state, i.e., Work is not a property. It is path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states). Properties are point functions have exact differentials (d ). Path functions have inexact differentials ( )

** MOVING BOUNDARY WORK ** How are the different paths possible between same states?

During actual expansion and compression processes of gases, pressure and volume are related by … Polytropic processes Schematic and P-V diagram for a polytropic process.

Polytropic processes n = 0 P-V diagram for a Isobaric process. ISOBARIC PROCESS # p = C n = 0 P-V diagram for a Isobaric process.

Polytropic processes n = ∞ P-V diagram for a Isobaric process. ISOCHORIC PROCESS # p = C n = ∞ P-V diagram for a Isobaric process.

It is ISOTHERMAL process for Ideal Gas. Polytropic processes HYPERBOLIC PROCESS # pV = C n = 1 It is ISOTHERMAL process for Ideal Gas. P-V diagram for a Hyperbolic process.

n = γ Polytropic processes P-V diagram for a Hyperbolic process. ADIABATIC PROCESS n = γ ADIABATIC Wall P-V diagram for a Hyperbolic process.

Polytropic, Isothermal, and Isobaric processes

Polytropic, Isothermal, and Isobaric processes

Expression of Work-done for various Polytropic processes

Polytropic, Isothermal, and Isobaric processes Polytropic process: C, n (polytropic exponent) constants Polytropic process Polytropic and for ideal gas When n = 1 (isothermal process) Constant pressure process

Summary Forms of energy Macroscopic = kinetic + potential Microscopic = Internal energy (sensible + latent + chemical + nuclear) Energy transfer by heat Energy transfer by work Mechanical forms of work Moving boundary work Wb for an isothermal process Wb for a constant-pressure process Wb for a polytropic process

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