Thermal Physics Dr Rob Wi3enmyer Lecture 1 – Temperature Thermal Physics Lecture 1 – Temperature Textbook reference: 19.1-­‐19.2, 19.4 Dr Rob Wi3enmyer Office: Old Main 130, in Astrophysics area rob@phys.unsw.edu.au h3p://phys.unsw.edu.au/~rob Difficult because: • Less intuiQve than mechanics • Need to use approximaQons • Experiments harder to conduct ObjecQve: We will use as much fire and liquid nitrogen as possible to help make this topic clear! Some definiQons Thermal Contact: Two objects are in thermal contact with each other if energy can be exchanged between them in the form of heat or electromagneQc radiaQon. Energy is exchanged due to a temperature difference. Thermal contact does not necessarily mean physical contact. Thermal Equilibrium: Thermal equilibrium occurs when two objects would not exchange any net energy if they were placed in thermal contact. Zeroth Law of Thermodynamics If objects A and B are separately in thermal contact with a third object C, then A and B are in equilibrium with each other. Zeroth Law of Thermodynamics If objects A and B are separately in thermal contact with a third object C, then A and B are in equilibrium with each other. Temperature Two objects in thermal equilibrium are said to be at the same temperature. But, our sense of temperature is not reliable: The cushion of your seat is touching the metal, so they will be at the same temperature. However, the metal bits feel cooler. (try it for yourself) Quick Quiz: Two objects, with different sizes, masses, and temperatures, are placed in thermal contact. In which direcQon does the energy travel? 1.
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Energy travels from the larger object to the smaller object. Energy travels from the object with more mass to the one with less mass. Energy travels from the object at higher temperature to the object at lower temperature. Quick Quiz: Two objects, with different sizes, masses, and temperatures, are placed in thermal contact. In which direcQon does the energy travel? 1.
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Energy travels from the larger object to the smaller object. Energy travels from the object with more mass to the one with less mass. Energy travels from the object at higher temperature to the object at lower temperature. The direc)on of the transfer of energy depends only on the temperature and not on the size of the object or on which object has more mass. Temperature measurement Physical properQes that change with temperature • The volume of a liquid • The dimensions of a solid • The pressure of a gas at constant volume • The volume of a gas at constant pressure • Colour change • The voltage of a thermocouple Temperature measurement Physical properQes that change with temperature As temperature increases the volume of a liquid will usually increase. Microscopically this is because the parQcles that make up the liquid start moving about more quickly. Solids will generally expand when heated. The pressure of a gas will also increase with temperature. YouTube “exploding aerosol can” for examples…. Temperature measurement Physical properQes that change with temperature Thermometers: How do they work? Temperature Scales Temperature Scales • Thermometers must be calibrated – usually with water (ice point, steam point). • Some problems with liquid-­‐in-­‐glass thermometers: • Alcohol and mercury thermometers may only agree at calibraQon points. • Discrepancies between then can be large when far from the calibraQon points. • Limited range of values measurable • Mercury cannot be used under -­‐30oC – why? • Alcohol cannot be used above 85oC Measuring Temperature An absolute temperature scale would be useful… Constant Volume Gas Thermometer •  The physical change exploited is the variaQon of pressure of a fixed volume gas as its temperature changes •  The volume of the gas is kept constant by raising or lowering the reservoir B to keep the mercury level at A constant Absolute Zero •  The thermometer readings are virtually independent of the gas used •  If the lines for various gases are extended, the pressure is always zero when the temperature is –273.15o C •  This temperature is called absolute zero Measuring Temperature Some Examples of Absolute Temperatures •  The figure at right gives some absolute temperatures at which various physical processes occur •  The scale is logarithmic •  The temperature of absolute zero cannot be achieved –  Experiments have come close Quick Quiz: Consider the following pairs of materials. Which pair represents two materials, one of which is twice as hot as the other? 1.
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boiling water at 100°C, a glass of water at 50°C boiling water at 100°C, frozen methane at -­‐50°C an ice cube at -­‐20°C, flames from a circus fire-­‐eater at 233°C none of these pairs Quick Quiz: Consider the following pairs of materials. Which pair represents two materials, one of which is twice as hot as the other? 1.
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boiling water at 100°C, a glass of water at 50°C boiling water at 100°C, frozen methane at -­‐50°C an ice cube at -­‐20°C, flames from a circus fire-­‐eater at 233°C The phrase “twice as hot” refers to a ra)o of temperatures. When the given temperatures are converted to Kelvins, only those in part 3 are in the correct ra)o (T=506K & 253K). none of these pairs Temperature and physical changes Thermal Expansion: It works Hot water on jar lid  expands the metal, makes it easier to open. Expansion joint on bridge, so it will not buckle on very hot days. D Demo Unit Hb1 & Hb2: Thermal Expansion of a Ball and Ring •  A ball will pass through a ring at room temperature but not if the ball is heated, or if the ring is cooled. Thermal Expansion: It works Watch your units! Kelvin/oC osen a source of errors here. Expansion rate depends on material Bimetallic Strip •  Each substance has its own characterisQc average coefficient of expansion •  In bimetallic strip the brass expands more than the steel for the same temperature rise –  Strip bends •  An applicaQon is the thermostat Liquid A warm day > 4 oC Warm water remains on surface as it is less dense warm ΔV = β Vi ΔT A cool day < 4 oC Water sinks when it reaches 4oC: When it is all 4oC cooler water floats < 4oC = 4oC cooler Water’s Unusual Behavior: Why ice floats •  As the temperature increases from 0oC to 4oC, water contracts! –  Its density increases •  Above 4oC, water expands with increasing temperature –  Its density decreases •  The maximum density of water (1.000 g /cm3) occurs at 4oC –  CriQcal for the existence of marine life!