The first law The Concepts C Stephen L. Davis, 2003
The First Law The Concepts © Stephen L. Davis, 2003
System and surroundings System-the part of the world in which we have a special interest Surroundings -where we make our system observations surroundings
System and Surroundings • System - the part of the world in which we have a special interest. • Surroundings - where we make our observations. system surroundings
ypes of systems Open -exchanges matter and Matter energy with surroundings Energy Closed-exchanges onl Energy energy with surroundings Isolated-no interchange with surroundings
Types of Systems • Open - exchanges matter and energy with surroundings. • Closed - exchanges only energy with surroundings. • Isolated - no interchange with surroundings. Energy Energy Matter
Heat Heat is the energy transferred between a hot object and a cold object ° Temperature is the degree of hotness of high an object
Heat • Heat is the energy transferred between a hot object and a cold object. • Temperature is the degree of “hotness” of an object. q Thigh Tlow
Isothermal vs. adiabatic An adiabatic process in one in which no heat is exchanged between the system and Its surroundings An isothermal process in one in which the initial and final temperatures are the same Isothermal processes are not necessarily adiabatic
Isothermal vs. Adiabatic • An adiabatic process in one in which no heat is exchanged between the system and its surroundings. • An isothermal process in one in which the initial and final temperatures are the same. • Isothermal processes are not necessarily adiabatic
Work Work is any energy exchange that is not heat meter Work is any interaction that could have as the sole effect the raising of a weight k g Work force. distance
Work • Work is any energy exchange that is not heat. • Work is any interaction that could have as the sole effect the raising of a weight. • Work = force · distance 1 kg 1 meter
State Function vs. Path Function A state function is a state B property of a system that depends only on Its current state and not on how that state was reached path I A path function state A depends on how the state was reached
State Function vs. Path Function • A state function is a property of a system that depends only on its current state and not on how that state was reached. • A path function depends on how the state was reached. state A state B path 1 path 2
The first law ·dU=dw+dq state B defines the internal energy change of a system The internal energy is a function of state Corollary: Energy is state A conserved in an isolated system
The First Law • dU = dw + dq defines the internal energy change of a system. • The internal energy is a function of state. • Corollary: Energy is conserved in an isolated system. state A state B w q w q
Work of expansion/Compression ex Per external pressure ·A= piston area dz= displacement Z ·d=-Adz= volume change for the gas ·ah=Fdz gas ex A
Work of Expansion/Compression gas dz pex • pex = external pressure • A = piston area • dz = displacement • dV = - A dz = volume change for the gas • dw = F dz dw = - pex dV A
Reversible Changes a reversible change is one that can be reversed by an infinitesimal modification of a variable In a reversible expansion or compression, Pex pgas
Reversible Changes • A reversible change is one that can be reversed by an infinitesimal modification of a variable. • In a reversible expansion or compression, pex = pgas pex pgas