Thermal Physics : Thermodynamics
Dr Stuart Reid [email protected] Royal Society of Edinburgh/Scottish Government Research Fellow room 461
8+1 lectures
Objectives (from Course Guide 2008/09) Y&F ch: 1. Temperature and Heat (3 lectures) a. Temperature and Thermal Equilibrium b. Zeroth law of thermodynamics c. Thermometric properties 17.1‐3 d. Thermal expansion of liquids and solids 17.4 e. Specific Heat Capacity f. Calorimetry and Phase Changes 17.5‐6 2. Mechanisms of Heat Transfer (2 lectures) a. Conduction b. Convection 17.7 c. Radiation, Stefan‐Boltzmann law, black body 3. Equations of state ‐ Ideal and Real Gases (2 lectures) a. Equations of state; ideal gas equation b. Van der Waals equation 18.1‐2 c. pV diagrams, phase of matter
Examples of typical applications: 1. Need for different kinds of thermometer (industrial, medical, etc.); expansion joints in bridges 2. Heat sinks for ICs; double glazing; insulation; thermal boxes; sea breezes; sunbathing
Einstein’s thoughts: “A theory is the more impressive the greater the simplicity of its premises, the more varied the kinds of things it relates and the more extended the area of its applicability. Therefore classical thermodynamics has made a deep impression upon me. It is the only physical theory of universal content which I am convinced, within the areas of the applicability of its basic concepts, will never be overthrown.”
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Thermodynamics was one of William Thomson’s (Lord Kelvin’s) legacies.
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1. Temperature and Heat (17.1‐6)
How to think: • Identify variables in a problem • Define the apparatus (the system)
‐ A system is enclosed by boundaries ‐ The rest “out there” is called the surroundings ‐ STATE: described by some variables called “STATE VARIABLES”
Goal: to reduce problems to as few variables as possible.
Examples: Dynamics: e.g. billiard balls → n balls, m, v, x, y Electricity: e.g. circuit → electrons, V, I, R Thermodynamics: cylinder & pistons → n atoms, m, P, V, T *** Things change with temperature – banana to 77 K demo ***
What is temperature? The temperature of a system is proportional to the average kinetic energy of the atoms or molecules.
Thermal Equilibrium Two systems are at thermal equilibrium when they are at the same temperature. Generally the systems are different; all they share in common is the same temperature.
Examples: Glass of cold water left in the room → approaches thermal equilibrium Microprocessor and a cold block of Al
T1 T2 thermal contact
Suppose we have a third system: e.g. the table that the glass of water is on 1 2 3 If T1 = T2 T1 T2 T3 and T2 = T3 then T1 = T3 This is the 0th law of thermodynamics – “If two thermodynamic systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other.”
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Temperature Scales
• We need reference points • We need something that varies with temperature ‐ and we need to know how it varies, ideally linearly!
END LECTURE 1 (1) Reference Points
a) Boiling water 100˚C (Celcius/Centigrade) Ice 0˚C b) Human body Fahrenheit “96” Freezing point of water “32” Coldest solution possible (water + ice+ salt) “0” c) Triple point: at the exact pressure and temperature, the combination of pure water, pure ice and pure vapour can coexist in a stable equilibrium
Boiling
Freezing
NB. In space, where the temperature approaches close to absolute zero, ice when heated will convert directly to vapour/steam without any liquid‐phase.
Triple Point = 273.16 K
Absolute zero → no molecular motion → 0 K
To convert: T (˚C) = T (K) – 273.15
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Measuring temperature
• Change in electrical resistance (convenient but not very linear) • Change in length of a bar (bimetallic strip) • Change in volume of a liquid • Change in volume of gas (very accurate but slow and bulky)
Thermal Expansion “ Hotter things become longer”
• In general, most materials expand upon being heated (but not water where between 0°C ‐> 4°C it gets smaller! Also many rubbers contract on heating).
Linear thermal expansion: ΔL = αL0ΔT *
• where α is “coefficient of linear expansion”