Chapter-18

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

2. 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

3. 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

4. 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

5. 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

6. Ch 18-5 Temperatures Scales

7. A Constant Volume Gas Thermometer

8. 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

9. 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

10. 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 kelvinC=Q/T ( Joules J/K) • Specific Heat c: Amount of heat required to raise temperature of unit mass of a substance by one degree kelvinc =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 kelvinmolar 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 fusionLF: for water LF=333 kJ/kg

11. 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.

12. 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’

13. 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

14. 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

15. 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

16. 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

17. Suggested problemsChapter 18