THERMODYNAMICS CH 15

THERMODYNAMICS Thermodynamics is the study of processes in which energy is transferred as heat and as work System is any object or set of objects that we wish to consider Environment is everything else in the universe

OPEN AND CLOSED SYSTEMS Closed system is one for which no mass enters or leaves (but energy may be exchanged with the environment) Ex: idealized systems studied in physics Open system is one for which mass may enter or leave (as well as energy) Ex: plants and animals

Isolated System Isolated system is a closed system where no energy in any form passes across its boundaries

1ST Law of Thermodynamics The change in internal energy of a closed system, ∆U, will be equal to the sum of the energy transferred across the system boundary by heat (Q) and the energy transferred by work (W) ∆U = Q + W

Sign Convention Energy of any kind that goes into the system is + Energy of any kind that comes out of the system is –

First Law of Thermodynamics is conservation of energy It is one of the great laws of physics Its validity rests on experiments (such as Joule’s) in which no exceptions have been seen Internal energy is the sum total of all the energy the molecules of the system. It is a property of a system like pressure, volume and temperature Work and heat are not properties of a system

First Law of Thermodynamics applied to some simple systems Isothermal Process is an idealized process that is carried out at constant temperature (∆T = 0) An ideal gas in a cylinder fitted with a movable piston

PV diagram for an ideal gas undergoing isothermal processes If the temperature is to remain constant, the gas must expand and do an amount of work W on the environment (it exerts a force on the piston and moves it through a distance) ∆U = 3 N k∆T = 0 (since the 2 temperature is kept constant) U = Q + W = 0 W = -Q (the work done by the gas in an isothermal process equals the heat lost to the environment)

Adiabatic Process Adiabatic process is one in which no heat is allowed to flow into or out of the system: Q = 0 It can occur if the system is extremely well insulated, or the process happens so quickly that the heat-which flows slowly- has no time to flow in or out.

PV diagram for adiabatic process Since Q = 0, ∆U = W (The internal energy decreases if the gas expands) The temperature decreases as well since ∆U = 3 N k ∆T 2 In an adiabatic compression work is done on the system so U and T increases

Isobaric Process Isobaric process is one in which the pressure is kept constant, so the process is represented by a straight line on the PV diagram

Isochoric Process Isochoric or isovolumetric process is one which the volume does not change

PV diagram for Different processes

Work done in volume changes W = F d = Pad W = - P ∆V Work done by a gas is equal to the area under the PV curve

The Second Law of Thermodynamics The First Law of Thermodynamics states that energy is conserved Some processes in nature do not occur in reverse even though they wouldn’t violate the First Law Ex: broken glass back to be together spontaneously

Essential Question Which processes occur in nature and which do not? On the second half of xix century scientists came to formulate a new principle known as the second law of thermodynamics It is a statement about which processes occur in nature and which do not

The second law of thermodynamics It is stated in a variety of ways A general statement is based on the study of heat engines

Heat Engines A heat engine is any device that changes thermal energy into mechanical work Ex: steam engines ( most electricity today is generated with steam turbines) Car engines (internal combustion engine)

Heat Engine Diagram A heat input QH at a high Temperature TH is partially transformed into work W and partially exhausted as heat QL at a lower temperature TL QH = W + QL (Conservation of energy)

Diagram of Reciprocating type of steam engine

Diagram of Turbine steam Engine

Internal Combustion Engine In an internal combustion engine, the high temperature is achieved by burning the gasoline-air mixture in the cylinder itself (ignited by the spark plug)

Why a ∆T is needed to run a heat engine? Same temperature would mean that the pressure of the gas being exhausted would be the same as that on intake. Work would be done by the gas on the piston when it expanded but the same amount of work would be done by the piston to force the gas to exhaust. No net work!

Efficiency of a Heat Engine A net amount of work is obtained only if there is a difference in temperature. QH = W + QL (Conservation of energy) The efficiency, e, of any heat engine can be defined as the ratio of the work it does, W, to the heat input, QH e = W = QH – QL = 1 - QL QH QH QH

Carnot Engines It is an ideal engine, investigated by the French scientist Sadi Carnot (1796-1832) to see how to increase the efficiency of a real engine No Carnot engine actually exists, but as a theoretical engine it played an important role in the development of thermodynamics

PV Diagram of Carnot cycle 1-2: Isothermal expansion 2-3: Adiabatic expansion 3-4: Isothermal compression 4-1: Adiabatic compression

Ideal x Real Process Ideal- the process is reversible, that is, is done so slow that can be considered a series of equilibrium states, so the whole process can be done in reverse with no change in the magnitude of work done or heat exchanged. Real- the process is done more quickly so there is heat lost because of friction and turbulence, so the process cannot be done precisely in reverse, the process is then called irreversible.

Carnot efficiency For ideal engines Q ˜T (T in kelvin) eideal=TH – TL= 1-TL TH TH Real engines that are well designed reach 60 to 80 percent of the Carnot efficiency

Entropy It was not until the latter half of the nineteenth century the second law of thermodynamics was finely stated in a general way in terms of a quantity called entropy. Entropy is a measure of order or disorder of a system Entropy is a function of state of a system Like potential energy, it is the change in entropy during a process that is important not the absolute amount

Entropy The change in entropy ∆S of a system when an amount of heat Q is added to it by a reversible process at constant temperature: ∆S = Q (T in Kelvin) T

Entropy The entropy of an isolated system never decreases. It can only stay the same (ideal, reversible processes) or increase (real processes) ∆S > 0 If the system is not isolated: ∆S = ∆SS + ∆Senv ≥ 0

Second Law of thermodynamics (General Statement) The total entropy of any system plus that of its environment increases as a result of any natural process Or Natural processes tend to move toward a state of greater disorder

Order to Disorder The second law introduces a new quantity, S, that is not conserved in natural processes, it always increase in time. Entropy concept is very abstract. To get a better feel of it, we can relate it to order and disorder Entropy of a system is a measure of the disorder of the system

Which process occur in nature and which not? The normal course of events is an increase of disorder (entropy) Ex: a solid coffee cup is a more “orderly” object than the pieces of a broken cup Cups break when they fall, but they do not spontaneously mend themselves

Thermodynamocs Laws 0th: A equilibrium B » A equilibrium C B equilibrium C 1st: Conservation of energy: ∆U = Q – W 2nd: Which processes occur in nature and which do not (Entropy). Natural processes tend to move toward a state of greater disorder (∆S ≥ 0) 3rd: From careful experimentation absolute zero is unattainable.

Diagram of energy transfer for a heat pump The operating principle of refrigerators, air conditioners, and heat pumps is just the reverse of a heat engine. Each operates to transfer heat out of a cool environment into a warm environment

Heat engine Diagram

Heat Pump Refrigerator Heat pump or air conditioner