Parul Institute of Technology Subject Code : Name Of Subject : Engineering Thermodynamics Topic :FIRST LAW OF THERMODYNAMICS Name of Faculty : ASHISH PRAJAPATI 1 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

2 FIRST LAW OF THERMODYNAMICS Will discuss internal energy, the first law of thermodynamics, and applications of the first law The first law of thermodynamics describes systems in which the only energy change is that of internal energy. The transfers of energy are by heat and work. Will consider work done on deformable systems

Law of Conservation of Energy Law of Conservation of Energy The total energy of the universe is constant and can neither be created nor destroyed; it can only be transformed from one state to another. internal energy The internal energy, U, of a sample is the sum of all the kinetic and potential energies of all the atoms and molecules in a sample. i.e. it is the total energy of all the atoms and molecules in a sample 3 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

4 The first law of thermodynamics is an expression of the conservation of energy principle. Energy can cross the boundaries of a closed system in the form of heat or work. Energy transfer across a system boundary due solely to the temperature difference between a system and its surroundings is called heat. Work energy can be thought of as the energy expended to lift a weight. Closed System First Law A closed system moving relative to a reference plane is shown below where z is the elevation of the center of mass above the reference plane and is the velocity of the center of mass. Heat Work z Closed System Reference Plane, z = 0 For the closed system shown above, the conservation of energy principle or the first law of thermodynamics is expressed as Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

5 or According to classical thermodynamics, we consider the energy added to be net heat transfer to the closed system and the energy leaving the closed system to be net work done by the closed system. So Where Normally the stored energy, or total energy, of a system is expressed as the sum of three separate energies. The total energy of the system, E system, is given as Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

6 Recall that U is the sum of the energy contained within the molecules of the system other than the kinetic and potential energies of the system as a whole and is called the internal energy. The internal energy U is dependent on the state of the system and the mass of the system. For a system moving relative to a reference plane, the kinetic energy KE and the potential energy PE are given by The change in stored energy for the system is Now the conservation of energy principle, or the first law of thermodynamics for closed systems, is written as Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

7 If the system does not move with a velocity and has no change in elevation, the conservation of energy equation reduces to We will find that this is the most commonly used form of the first law. Closed System First Law for a Cycle Since a thermodynamic cycle is composed of processes that cause the working fluid to undergo a series of state changes through a series of processes such that the final and initial states are identical, the change in internal energy of the working fluid is zero for whole numbers of cycles. The first law for a closed system operating in a thermodynamic cycle becomes Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Systems & Surroundings In thermodynamics, the world is divided into a system and its surroundings A system is the part of the Universe under study, separated from the rest of the Universe by a well-defined boundary. The surroundings consist of everything else outside the system – rest of the universe. SYSTEM CLOSED OPEN ISOLATED System Surroundings Types of system 8 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

OPEN SYSTEM: can exchange both matter and energy with the surroundings (e.g. open reaction flask, rocket engine), across its boundaries CLOSED SYSTEM: can exchange only energy with the surroundings (matter remains fixed) e.g. a sealed reaction flask. Closed systems have impermeable diathermal and moveable boundariesthat permit the transfer of heatand work between system and surroundings but prevent transfer of matter. ISOLATED SYSTEM: can exchange neither energy nor matter with its surroundings (e.g. a thermos flask) …entirely removed from environmental influences 9 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Thermodynamic properties of a System/Matter Extensive : Additive, and depends on the total mass of the system. Example: Volume Intensive : Independent of the amount of matter present in the system. Example: pressure, temperature, density Change of state: when the properties of the system change it is called “change of state”. Example: when a quantity of gas in a cylinder is compressed by moving a frictionless piston, the system undergoes a “change of state”….State Variables 10 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

INTERNAL ENERGY (U)-very imp concept Internal energy changes when energy enters or leaves a system  U = U final - U initial  U change in the internal energy Heat and work are 2 equivalent ways of changing the internal energy of a system 11 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

+= Change in internal energy Energy supplied to system as heat Energy supplied to system as work  U = q (heat) + w (work) q w q w U U like reserves of a bank: bank accepts deposits or withdrawals in two currencies (q & w) but stores them as common fund, U. 12 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Work Done by An Expanding Gas Gas expands slowly enough to maintain thermodynamic equilibrium. Increase in volume, dV +dV Positive Work (Work is done by the gas) -dV Negative Work (Work is done on the gas) Energy leaves the system and goes to the environment. Energy enters the system from the environment. 13 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Pressure as a Function of Volume Work is the area under the curve of a PV-diagram. Work depends on the path taken in “PV space.” 14 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Total Work Done To evaluate the integral, we must know how the pressure depends (functionally) on the volume. Definition: Work is defined as a quantity that flows across the boundary of a system during a change in its state and is completely convertible into the lifting of a weight in the surroundings. Definition: Work is defined as a quantity that flows across the boundary of a system during a change in its state and is completely convertible into the lifting of a weight in the surroundings. Work is the transfer of energy that takes place when an object is moved against an opposing force 15 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Heat Q is not a “state” function --- the heat depends on the process, not just on the initial and final states of the system Sign of Q : Q > 0 system gains thermal energy Q < 0 system loses thermal energy The term Heat (Q) is properly used to describe energy in transit, thermal energy transferred into or out of a system from a thermal reservoir. QU BUT, the quantity Q - W does not depend on the path taken; it depends only on the initial and final states. Q - W internal energy. 16 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Heat and work are forms of energy transfer and energy is conserved. The First Law of Thermodynamics  U = Q + W on work done on the system change in total internal energy heat added to system or  U = Q - W State Function Path Functions Positive Q ->heat added to the system Positive W -> work done by the system statement of energy conservation for a thermodynamic system 17 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

First Law of Thermodynamics What this means: The internal energy of a system tends to increase if energy is added via heat (Q) and decrease via work (W) done by the system.... and increase via work (W) done on the system. 18 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Enthalpy (H) On integrating between the limits initial and final stages, we can write from 1 st Law, Or, For constant pressure, Therefore, and Replacing by H i, This H is called enthalpy 19 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

First Law of Thermodynamics: Energy is Conserved ΔU = U final - U inital = q - w q = heat absorbed by the system from the surroundings w = work done by the system on the surroundings heat is random molecular motion while work is force times distanced moved under its influence Exothermic Processes release heat and have q<0 Endothermic Processes absorb heat and have q>0 Energy: The SI unit is joule (J) although we will frequently use calorie ; 1 cal = 4.2 J 20 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Engineering View of First Law Any machine which violates first law is called as Perpetual Motion Machine of first kind. A PMM-1 is a control mass which works continuously in a cycle and – generates only work or – consumes only work or – accepts only heat or – rejects only heat. It is impossible to construct A Perpetual Motion Machine of first kind (PMM – 1). 21 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Applications of the Steady Flow Energy Equation Flow through a rocket nozzle A liquid bi-propellant rocket consists of a thrust chamber and nozzle and some means for forcing the liquid propellants into the chamber were they react, converting chemical energy to thermal energy. NOZZLE 22 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Once the rocket is operating we can assume that all of the flow processes are steady, so it is appropriate to use the steady flow energy equation. Also, for now we will assume that the gas behaves as an ideal gas, though in general this is a poor approximation. There is no external work, and we assume that the flow is adiabatic. Then we can write the First Law as w hich becomes therefore 23 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

If we assume quasi-static, adiabatic expansion then so Where Tc and pc are conditions in the combustion chamber (set by propellants), and pe is the external static pressure 24 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

TurbineS If we neglect the changes in kinetic and potential energies as fluid flows through an adiabatic turbine having one entrance and one exit, the conservation of mass and the steady-state, steady-flow first law becomes 25 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

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27 Heat exchangers Heat exchangers are normally well-insulated devices that allow energy exchange between hot and cold fluids without mixing the fluids. The pumps, fans, and blowers causing the fluids to flow across the control surface are normally located outside the control surface. Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

Compressors and fans Compressors and fans are essentially the same devices. However, compressors operate over larger pressure ratios than fans. If we neglect the changes in kinetic and potential energies as fluid flows through an adiabatic compressor having one entrance and one exit, the steady- state, steady-flow first law or the conservation of energy equation becomes 28 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS

THANK YOU 29 Sub : ET Topic : FIRST LAW OF THERMODYNAMICS