《物理化学 Physical Chemistry》课程教学资源（英文版）The First Law（The Concepts）

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