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Chapter 1 Temperature and Heat

Prepared by Ahmed Mohamed El-Lawindy 2010-2011. Chapter 1 Temperature and Heat. Outline. * Temperature and the Zeroth Law of Thermodynamics * Temperature Scales * Thermal Expansion Heat and Mechanical Work, 1 st Law of thermodynamics Specific Heats Heat transfer Applications.

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Chapter 1 Temperature and Heat

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  1. Prepared by Ahmed Mohamed El-Lawindy 2010-2011 Chapter 1 Temperature and Heat W2

  2. Outline * Temperature and the Zeroth Law of Thermodynamics * Temperature Scales * Thermal Expansion Heat and Mechanical Work, 1st Law of thermodynamics Specific Heats Heat transfer Applications W2

  3. Objectives Understand the difference between temperature and heat Understand and apply simple equations to calculate thermal expansion coefficient Study in brief the heat energy transfer Study some medical applications related to that subject. W2

  4. Temperature and the Zeroth Law of Thermodynamics • Temperature - temperature is a measure of hotness and can be related to the kinetic energy of molecules of a substance. • Heat - Heat is the energy transferred between objects, because of a temperature difference. • Thermal contact - Objects are in thermal contact if heat can flow between them. • Thermal equilibrium - Objects that are in thermal contact, but have no heat exchange between them, are said to be in thermal equilibrium. W2

  5. Temperature and the Zeroth Law of Thermodynamics Zeroth law of thermodynamics - If objects A and B are in thermal equilibrium with object C, they are in thermal equilibrium with each other. Temperature - Temperature is the quantity that determines whether or not two objects will be in thermal equilibrium. W2

  6. Thermometers • An instrument used for measuring temperature is called a thermometer and is constructed by using one of the following principles: • the change of length, such as length of a mercury column, • the change of volume, such as volume of a fixed mass of gas at constant pressure, • the change of pressure, such as pressure of a fixed mass of gas at constant volume, • the change in electric resistance, as in a thermistor, • the flow of electricity due to Seebeck effect, as in a thermocouple, • the radiation, as in radiation pyrometers. W2

  7. Assignment Write a short account on one of thermometers Main Points: Phenomena Structure Calibration Range of measurement Field W2

  8. Temperature Scales • Celsius– water freezes at 0 C and boils at 100 C • Fahrenheit– water freezes at 32 F and boils at 212 F • Kelvin– water freezes at 273.15K and boils at 373.15K. • Absolute Zero – the lowest possible temperature: -273.15K W2

  9. Temperature Scales • Conversion between degrees Celsius and degrees Fahrenheit • TF=(9/5 oF/oC)Tc+32 oF • Conversion between degrees Fahrenheit and degrees Celsius • Tc=(5/9 oC/oF)(TF-32 oF) • Conversion between Celsius and Kelvin temperatures • TK=Tc+273.15 W2

  10. 100 212 212-32=180 0 32 100-0=100 Example Find the temperature at which its value is the same in F and C? (TF- 32)/180 = (TC- 0)/100 TF=(180/100)TC + 32=(9/5 TC + 32) oF TC=(100/180)(TF – 32)=5/9(TF -32) oC If TF=TC=T T=(9/5)T+32 (9/5 -1)T= -32 T=-32/(4/5) T=-40 W2

  11. Example • On a day when the temperature reaches 50°F, what is the temperature in degrees Celsius and in Kelvin? • Solution • Substituting TF = 50°F into Equation 1.2, we get • Tc = (5/9)(TF - 32) =(5/9) (50 - 32) = 10°C • From Equation 1.1, we find that • T=Tc+ 273.15 = 283.15 K W2

  12. W2

  13. Thermal Expansion • Most bodies expand as their temperature increase Thermal expansion joints must be included in Buildings Concrete highways Railroads tracks Bridges Electric wires To compensate for changes in dimension with temperature variations W2

  14. Thermal Expansion • Thermal expansion is related to the changes in the average separation between atoms or molecules of the substance. • If the expansion is small relative to the initial dimension, the change in any dimension is linearly proportional to temperature W2

  15. Linear Expansion • Assume an object has an initial length L • That length increases by DL as the temperature changes by DT • We define the coefficient of linear expansion as • A convenient form is DL = aLi DT W8

  16. Linear Expansion, cont • This equation, DL = aLi DT can also be written in terms of the initial and final conditions of the object: • Lf – Li = a Li (Tf – Ti) • The coefficient of linear expansion, a, has units of (oC)-1 W8

  17. Volume Expansion • The change in volume is proportional to the original volume and to the change in temperature • DV = bViDT • b is the coefficient of volume expansion • For a solid, b = 3a • This assumes the material is isotropic, the same in all directions • For a liquid or gas, b is given in the table W8

  18. Area Expansion • The change in area is proportional to the original area and to the change in temperature: • DA = 2aAi DT • DA = gAi DT, g is the area expansion coefficient • The expansion and contraction of material due to changes in temperature creates stresses and strains, sometimes sufficient to cause fracturing. W8

  19. Some Coefficients W8

  20. Thermal Expansion, Conclusion isotropic materials: The coefficient of linear expansion, α, is the same in all directions. The coefficient of volume expansion,β , β=3 α The coefficient of area expansion, g=2α b=3a DV=bVDT DA=gADT βl ›10 βs W2

  21. Example W2

  22. Gas Expansion • For a system of Gas of m, V, P, T • The relation between m, V, P, T is very complicated, and is called the • equation of state. • If P is very low (low density- Ideal gas) simple equation of state • Most gases at room T and P are Ideal gases W2

  23. Ideal gas 1. The number of molecules in the gas is large, and the average separation between them is large compared with their dimensions. This means that the molecules occupy a negligible volume in the container. This is consistent with the ideal gas model, in which we imagine the molecules to be point-like. 2. The molecules obey Newton’s laws of motion, but as a whole they move randomly. By “randomly” we mean that any molecule can move in any direction with any speed. At any given moment, a certain percentage of molecules move at high speeds, and a certain percentage move at low speeds. 3. The molecules interact only by short-range forces during elastic collisions. This is consistent with the ideal gas model, in which the molecules exert no long range forces on each other. 4. The molecules make elastic collisions with the walls. 5. The gas under consideration is a pure substance; that is, all molecules are identical. W2

  24. Ideal gas I mole= mass of the substance that contains a specific number of molecules, NA NA=6.022X1023 molecules/mole= number of 12C atoms in 12 g. The number of moles, n, of a substance is related to its mass m, through the relation; Where M is the molecular weight of the substance W2

  25. Equation of state of Ideal gas N is number of molecules k is called Boltzman’s constant W2

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