Chapter-18 Temperature, Heat and the First Law of Thermodynamics

Chapter-18 Temperature, Heat and the First Law of Thermodynamics  Topics to be studied:  Temperature and the zeroth law of thermodynamics.  Thermometers and temperature scales.  Thermal expansion.  Temperature and heat.  Specific heat  Heat of transformation  Heat, work, and the first law of thermodynamics  Heat transfer mechanisms

Ch 18-2 Temperature  Thermodynamics: Study of application of thermal energy.  Temperature: One of the fundamental property of matter: central concept of thermodynamics; one of the seven SI base quantities  Temperature is measured in Absolute scale called Kelvin (K) Scale expressed in T  Lower limit of body temperature on kelvin scale is Zero.  Properties of many bodies such as volume, length, electrical resistance and pressure etc. Changes with temperature and can be used to measure bodies temperature.  Thermoscope: A device fitted with temperature display, display reading increasing with temperature and vice versa

Ch 18-3 Zeroth Law of Thermodynamics  Thermal contact: two or more objects in thermal contact exchange heat between them.  Thermal Equilibrium: Two bodies in thermal contact but no heat exchange takes place between them  Zeroth Law of Thermodynamics:  If bodies A and B are in thermal equilibrium with a third body T, then A and B are in equilibrium with each other.  When two bodies are in thermal equilibrium, their temperature are equal

 Body temperature measured with reference to a temperature of a standard fixed point such as freezing point or boiling point.  Triple point of water as a reference point: Liquid water, solid ice and water vapor coexist together at T 3 = K  Celsius (C) and Fahrenheit (F) Scales T C =T  T F = 9T C /5+32  Ch 18-4,5 Measuring Temperatures and its Scales

Ch 18-5 Temperatures Scales

Constant Volume Gas Thermometer A

Ch 18-6 Thermal Expansion  Thermal Expansion: Change in dimension of an object due to change in temperature  Linear expansion: Increase  L in length L due to increase  T in temperature T of an object then :  L=  L  T, where  is coefficient of linear expansion  Volume Expansion: Increase  L in length L due to increase  T in temperature T of an object then :  V=  V  T  where  is coefficient of linear expansion:  = 3 

Ch 18-7 Temperature and Heat  Heat (energy) is transferred between a system (temperature T S ) and its environment (temperature T E ) unless thermal equilibrium is achieved between them.  If Ts<T E Q is absorbed by system then Q is positive  If Ts>T E Q is lost by system then Q is negative.  If Ts=T E the system is in thermal equilibrium with its environment  One Calorie (cal) is amount of heat required to raise temperature of one gram of water from 14.5  C to 15.5  C  1 cal= Joules (J)  Joule (J) unit of energy in SI units

Ch 18-8 Absorption of Heat by Solid and Liquids  Heat Capacity C: Amount of heat required to raise temperature of an object by one degree kelvin C=Q/  T ( Joules J/K)  Specific Heat c: Amount of heat required to raise temperature of unit mass of a substance by one degree kelvin c =Q/m  T ( J/kg.K)  Molar Specific Heat: Amount of heat required to raise temperature of one mole of a substance by one degree kelvin molar specific heat =Q/n  T ( J/mol.K)  Heat of transformation: Amount of heat required to change the phase of unit mass of a substance.  Heat of Vaporization L V : for water L V =2256 kJ/kg  Heat of fusion L F : for water L F =333 kJ/kg

 Working system: gas confined to a cylinder fitted with movable piston in thermal contact with a heat reservoir to exchange heat Q:  Initial state of the system : p i, V i and T i changes to final state of the system p f, V f, T f through absorption (positive Q) or release (negative Q) of heat by the system (gas). Also work W can be done in raising the piston ( positive W) or lowering (negative W) the piston.  Ch 18-9 Heat and Work

dW=F.ds=(PA)ds=p(Ads)=pdV W=  dW=  pdV Work done represented by the area under the curve on pV diagram. Area depends upon the path taken from i to f state. Also PV=nRT For b) from i to a process volume increase at constant pressure i.e T a =T i (V a /V i ) then T a >T i. Heat Q must be absorbed by the system and work W is done a to f process is at constant V (pf>pa) then T f =T a (p f /p a ) Since T f <T a, heat Q’ must be lost by the system For process iaf total work W is done and net heat absorbed is Q-Q’

Ch The First Law of Thermodynamics  Out of quantities Q, W, Q and W are path dependent but Q-W is path independent.  Q-W represents intrinsic property called internal energy E int then   E int = E int-f - E int-I but  E int =  Q-  W First Law of Thermodynamics

Ch Some special cases of The First Law of Thermodynamics  Adiabatic Process: No heat is allowed to enter or leave the system (Q=0) then  E int =-  W  Constant Volume (Isochoric) Process: Volume remains constant (W=0) then  E int =  Q  Cylic Process: (  E int =0) then  Q=  W  Free Expansion: Adiabatic process (Q=0) in which gas expands in vacum without doing work (W=0), then  E int = Q=W= 0

Ch Heat transfer Mechanism  Three Heat Transfer mechanism:  Conduction; convection and radiation:  Conduction: Heat transfer from one end to other end via collision between the neighboring atoms only:  For a slab of face area A and length L whose faces are maintained at temperature T H and T C, the heat conduction rate P cond ( amount of energy transferred per unit time Q/t):  P cond = Q/t=  A(T H -T C )/L  where  is thermal conductivity

Ch Heat transfer Mechanism  Conduction through a composite slab: P cond = Q/t=  (T H -T C )/  (L/A)  Convection: Energy transfer to the object through direct contact of each part of the object with heat source  Radiation: Heat exchange between the object and its environment through electromagnetic radiation

Suggested problems Chapter 18