Temperature, Heat and the First Law of Thermodynamics 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

Ch 18-4,5 Measuring Temperatures and its Scales 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 T3=273.16 K Celsius (C) and Fahrenheit (F) Scales TC=T-273.15 TF= 9TC/5+32

Ch 18-5 Temperatures Scales

Constant Volume Gas Thermometer

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 TS) and its environment (temperature TE) unless thermal equilibrium is achieved between them. If Ts<TE Q is absorbed by system then Q is positive If Ts>TE Q is lost by system then Q is negative. If Ts=TE 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=4.1868 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 LV: for water LV=2256 kJ/kg Heat of fusion LF: for water LF=333 kJ/kg

Ch 18-9 Heat and Work 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 : pi, Vi and Ti changes to final state of the system pf, Vf, Tf 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 Ta=Ti (Va/Vi) then Ta>Ti . Heat Q must be absorbed by the system and work W is done a to f process is at constant V (pf>pa) then Tf=Ta(pf/pa) Since Tf<Ta , heat Q’ must be lost by the system For process iaf total work W is done and net heat absorbed is Q-Q’

Ch 18-10 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 Eint then Eint= Eint-f - Eint-I but Eint=Q-W First Law of Thermodynamics

Ch 18-11 Some special cases of The First Law of Thermodynamics Adiabatic Process: No heat is allowed to enter or leave the system (Q=0) then Eint=-W Constant Volume (Isochoric) Process: Volume remains constant (W=0) then Eint=Q Cylic Process: (Eint=0) then Q=W Free Expansion: Adiabatic process (Q=0) in which gas expands in vacum without doing work (W=0), then Eint= Q=W= 0

Ch 18-12 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 TH and TC, the heat conduction rate Pcond ( amount of energy transferred per unit time Q/t): Pcond = Q/t=A(TH -TC)/L where  is thermal conductivity

Ch 18-12 Heat transfer Mechanism Conduction through a composite slab: Pcond = Q/t=(TH -TC)/(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