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CHAPTER 3 ENERGY, ENERGY TRANSFER AND GENERAL ENERGY ANALYSIS DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK OBJECTIVES The objectives of this chapter DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 1 CHAPTER 3 : ENERGY, HEAT AND WORK WHAT IS ENERGY? The word `ENERGY’ comes from a Greek word ἐνέργεια (Energeia) Definitions in physics field : Energy is the capacity of a physical system to perform work. Energy is a property or characteristic of matter that makes things happen (Dave Watson ) Definition in Thermodynamics : Energy can be defined as an extensive property which has an ability to change the condition (or state) of a system and its surrounding through interaction that occurs through system boundary. • Examples of changes in condition : changes in shape, volume, chemical composition pressure, temperature, density and phase changes. • Without energy, nothing would ever change, nothing would ever happen. You might say energy is the ultimate agent of change, the mother of all change agents DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK INTRODUCTION Energy terms commonly used : ENERGY and ENERGY TRANSFER ENERGY Symbol, E Unit, kJ (kJ/kg) A function of other properties E = f (T, p , V, etc) Can be stored in a body Energy can be stored within system in various form, can be converted from one form to another and can be transferred between systems Generally, in thermodynamics, energy equation can be written as E = U + KE + PE Internal Energy Kinetic Energy Potential Energy DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 2 CHAPTER 3 : ENERGY, HEAT AND WORK WORK AND HEAT ARE PROCESSES (AND FORMS OF ENERGY?) There are just two ways that energy is transferred - by work or by heat. We often use the words work and heat as if they are forms of energy. I do it all the time and I'm not sorry. But some thermodynamic text books say that work and heat are processes or methods of energy transfer, not forms of energy. When describing energy transfers in this way, we should say something like, "energy transferred during a work process". Or when talking about a heat process, we would be more correct to say, "energy transferred by heat flow". Energy transfer is not a thermodynamics property because it depends on the history of the system. e.g Heat and Work. DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK KINETIC ENERGY The energy that a system possesses as a result of its motion relative to some reference frame. When all parts of the system move with the same velocity, the kinetic energy is expressed as KE = 21 mV 2 Kinetic Energy (J) Velocity (m/s) Mass (kg) Initial State When a body with a mass m, moving from initial state vith a velocity of ⎯V1 to a final state with a velocity of ⎯V2, then 1 2 V1 m Change in Kinetic Energy, ∆KE = Or per unit mass, ∆ke = Final State (V 2 2 1 2 ( m V2 ) m V22 − V12 J ) − V12 J/kg DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 3 CHAPTER 3 : ENERGY, HEAT AND WORK POTENTIAL ENERGY The energy that a system possesses as a results of its elevation in a gravitational field and is expressed as PE = mgz (J) Mass (kg) When a work is done to a system with a mass of m, from a distance from datum z1 to a distance from datum z2, the change in potential energy can be expressed as Height (m) Gravitational Accelaration (m/s2) ∆PE = mg (z2 - z1 ) J Initial State m Final State m z2 Or per unit mass, ∆pe = g (z2- z1 ) z1 J/kg Datum DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK POTENTIAL ENERGY Hanging apple = Potential energy When it's hanging it's potential. When it's falling the potential energy is turned into kinetic energy. What happens to the energy when it hits the head? DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 4 CHAPTER 3 : ENERGY, HEAT AND WORK INTERNAL ENERGY Translation Energy • The internal energy of a denoted by U, is the total of the kinetic energy due to the motion of particles (translational, rotational, vibrational) and the potential energy associated with the vibrational and electric energy of atoms within molecules. Rotational Kinetic Energy • It includes the energy in all of the chemical bonds, and the energy of the free, conduction electrons in metals. Vibrational kinetic Energy Rotational Kinetic Energy • It is a state function of a system, and is an extensive quantity • The unit is kJ or per unit mass kJ/kg Spin Energy • The internal energy is proportional to the temperature of the system, at higher temperature, the system possess higher U, and vice versa. Spin Energy • Any gas undergoes any process which doesn’t involves any temperature different, the change in internal energy is zero. • Method to analyze internal energy depends on the type of the working fluid (vapor or gas) DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK ENERGY TRANSFERRED BY HEAT n nitio Defi Fl ow Di re ct io n Heat Transfer Mode He at A form of energy transferred between two bodies (two systems or a system and its surroundings) by virtue of a temperature difference HEAT TRANSFER Direction of heat Transfer is always from the higher Temperature body to The lower temperature body Symbol Q (kJ) , q (kJ/kg) & (kW) = Q (kJ) Q Heat is Rejected Q negatif ∆t (s) Conduction Convection Radiation System Surrounding Heat is Supplied, Q positif A process without heat transfer is called adiabatic process. i.e the system is well insulated or both the system and the surrounding are at the same temperature DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 5 CHAPTER 3 : ENERGY, HEAT AND WORK ENERGY TRANSFERRED BY WORK WORK ion nit Work sign fi De The energy transfer associated with a force acting through a distance Symbol W (kJ) , w (kJ/kg) & (kW) = W (kJ) W ∆t (s) (Power) Work done by system W positive System Surrounding Work done on system W negative DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK ENERGY TRANSFERRED BY WORK δW = Fdx F W12 = x2 ∫x Work to Lift a Mass F dx Pulley 1 dx Work to Push A Mass Initial State F Datum x=0 m Final state m Initial State m Final State F m z2 z1 Datum W12 = mg(z2 - z1) x1 x2 W12 = F(x2 - x1) DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 6 CHAPTER 3 : ENERGY, HEAT AND WORK FORMAL SIGN CONVENTION OF HEAT AND WORK Heat transfer to a system and work done by a system are positive (Positive machine) Heat transfer from a system and work done on a system are negative (Negative machine) Heat is transferred from system Work done by system System System Surrounding Work done on system Heat is transferred to system Similarities between heat transfer and work • Both are recognized at the boundaries of a system as they cross the boundaries • System possess energy but not heat or work • Both associated with a process not a state • Both are path function Surrounding Path Function • Have inexact differentials, differential amount of heat or work is represented by δQ or δW instead of dQ or dW • Work done during process 1-2 = W12 • Heat transferred during process 1-2 = Q12 2 ∫1 δW = W12 (not W2 - W1 or ∆W ) 2 ∫1 δQ = Q 12 (not Q 2 - Q1 or ∆Q) DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK EXAMPLE 2-3 pg 64 DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 7 CHAPTER 3 : ENERGY, HEAT AND WORK EXAMPLE 2-4 pg 64 No heat is transferred to the system Heat is transferred to potato through boundary DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK EXAMPLE 2-5/2-6 pg 64 DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 8 CHAPTER 3 : ENERGY, HEAT AND WORK ELECTRICAL WORK DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK SHAFT WORK DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 9 CHAPTER 3 : ENERGY, HEAT AND WORK EXAMPLE 2-7 DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK SPRING WORK DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 10 CHAPTER 3 : ENERGY, HEAT AND WORK THE FIRST LAW OF THERMODYNAMICS The 1st Law of Thermodynamics also known as the conservation of energy The 1st Law of Thermodynamics state that energy can be either created nor destroyed during a process, it can only change forms PE1 =100 kJ KE1 = 0 kJ Total Energy = 100 kJ PE2 = 50 kJ KE2 = 50 kJ Total Energy = 100 kJ PE3 = 0 kJ KE3 = 100 kJ Total Energy = 100 kJ z/2 z Other ways to say it: Energy doesn't pop into existence from nowhere. Energy doesn't pop out of existence into nowhere. "Energy in" equals "energy stored" plus "energy out" (if no energy is stored, then "Energy in" equals "Energy out"). The net change in the total energy of system during a process is equal to the difference between the total energy entering and the total energy leaving the system DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 CHAPTER 3 : ENERGY, HEAT AND WORK THE FIRST LAW OF THERMODYNAMICS ⎛ Total energy ⎞ ⎛ Total energy ⎞ ⎛ Change in the total ⎞ ⎜⎜ entering the system ⎟⎟ − ⎜⎜ leaving the system ⎟⎟ = ⎜⎜ energy of the system ⎟⎟ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ E in − E out = ∆E system Energy change = Energy at final state – Energy at initial state ∆Esystem = ∆Efinal - ∆Einitial = E2 – E1 Where ∆E = ∆U + ∆KE ∆PE = m(u2 − u1 ) + 1 2 (V 2 2 ) − V12 + mg(z 2 − z 1 ) For closed system KE and PE are very small compared to internal energy and can be neglected, so DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 11 CHAPTER 3 : ENERGY, HEAT AND WORK EXAMPLE 2-10 DESIGNED AND PREPARED BY : MOHD KAMAL ARIFFIN/2003 DAH HABIS DA……. 12