Slide 1 / 163 Slide 2 / 163 Thermal Physics www.njctl.org Slide 3 / 163 Slide 4 / 163 Thermal Physics Temperature, Thermal Equilibrium and Thermometers · Thermal Expansion · Heat and Temperature Change · Thermal Equilibrium : Heat Calculations · Phase Transitions · Heat Transfer · Temperature, Thermal Equilibrium Gas Laws · Kinetic Theory · and Thermometers Click on the topic to go to that section Internal Energy · Work in Thermodynamics · First Law of Thermodynamics · Thermodynamic Processes · Second Law of Thermodynamics · Heat Engines · Return to Table of Contents Entropy and Disorder · Slide 5 / 163 Slide 6 / 163 Temperature and Heat Temperature Here are some definitions of temperature: In everyday language, many of us use the terms temperature A measure of the warmth or coldness of an object or substance with · and heat interchangeably reference to some standard value. But in physics, these terms have very different meanings. Any of various standardized numerical measures of this ability, such · Think about this… as the Kelvin, Fahrenheit, and Celsius scale. When you touch a piece of metal and a piece of wood both · A measure of the ability of a substance, or more generally of any · resting in front of you, which feels warmer? physical system, to transfer heat energy to another physical system. When do you feel warmer when the air around you is 90°F · and dry or when it is 90°F and very humid? A measure of the average kinetic energy of the particles in a sample · In both cases the temperatures of what you are feeling is · of matter, expressed in terms of units or degrees designated on a the same. Why then are you feeling a difference? standard scale. In this unit, we will learn about temperature, heat, and the laws of thermodynamics that relate heat, mechanical work We'll consider each of these definitions in this unit. and other forms of energy.
Slide 7 / 163 Slide 8 / 163 Thermometers Thermometers and Thermal Equilibrium To measure temperature of a substance, we need... 1 . A measuring device (Thermometer) that changes visibly and is calibrated to a scale. Thermometers relate the change in a physical property of substance to temperature. Examples include: > The change of volume of a gas or liquid > The change in length of a metal strip or wire > The light or infrared radiation emitted by an object 2 . To bring the Thermometer into contact with the substance > When the thermometer has settled on a value, we say that the thermometer and the substance are in Thermal Equilibrium Slide 9 / 163 Slide 10 / 163 Temperature Scales Temperature Scales Recall that temperature can be defined as... a measure of the warmth or coldness of an object or · substance with reference to some standardized numerical measures or scales Three common scales are: CO 2 Solidifies Fahrenheit (°F) - used mainly in the USA · O 2 Liquifies Celsius (°C) - used in most of the world · Kelvin (K) - used in the physical sciences · > also known as the Absolute Temperature Scale Kelvin Celsius Fahrenheit Slide 11 / 163 Slide 12 / 163 Temperature Conversions 1 Which temperature scale does not have negative values? A Fahrenheit Celsius # Fahrenheit B Celsius C Kelvin D All the above Celsius # Kelvin E None of the above
Slide 13 / 163 Slide 14 / 163 2 Water freezes at 32°F. What temperature would this be on the 3 Water boils at 100° C. What temperature would this be on the Celsius scale? Fahrenheit scale? A 32 °C A 32 °F B 0° C B 100° F C 25° C C 0° F D 212°C D 212° F E 100° C E 180° F Slide 15 / 163 Slide 16 / 163 4 “Room temperature” is often taken to be 68 °F; what is this on the 5 The coldest temperature recorded on earth was − 89.2 °C at the Celsius scale? Soviet Vostok Station in Antarctica, on July 21, 1983. What would a Fahrenheit scale thermometer have measured? A 34 ° C B 37.78° C C 5.78° C D 20° C E 52° C Slide 17 / 163 Slide 18 / 163 Thermal Equilibrium: The Zeroth Law of 6 The coldest temperature recorded on earth was − 89.2 °C at the Soviet Vostok Station in Antarctica, on July 21, 1983. What would a Thermodynamics Kelvin scale thermometer have measured? Two objects placed in thermal contact will eventually come to the same temperature. When they do, we say they are in thermal equilibrium. The zeroth law of thermodynamics says that if two objects are each in equilibrium with a third object, they are also in thermal equilibrium with each other. T 3 T 1 T 2 That is if T 1 = T 3 and T 2 = T 3 then T 1 = T 2
Slide 19 / 163 Slide 20 / 163 Thermal Conductors and Insulators 7 Three objects A, B, and C initially have different temperatures T A >T B >T C . Objects A and B are separated by an insulating plate but they are in contact with object C through a conducting plate. Which of the following is true when objects A and B reach thermal equilibrium with object C? Conductors - materials that allow heat to flow easily (metals) Insulators - materials that slow or block heat flow (wood, plastic, fiberglass) A The temperatures of all three objects do not change B Object A has a higher temperature than Object B and Object C C Object C has a higher temperature than Object A and Object B D Object B has a higher temperature than Object A and Object C E All three objects have the same temperature Slide 21 / 163 Slide 22 / 163 Thermal Expansion Most materials expand when their temperatures increase. · Liquids expand in a thermometer. · A tight metal jar lid can be loosened by running it in hot water. · These are examples of thermal expansion. · Thermal Expansion We consider two types of thermal expansion: · > Linear > Volume Return to Table of Contents Slide 23 / 163 Slide 24 / 163 Linear Expansion Linear Expansion Suppose a rod composed of some substance has a length L 0 at Substance an initial temperature of T 0 . Coefficient of Linear Exansion α (×10 -6 / °C) If the temperature is changed by ∆T, the length changes by ∆L. Aluminum 23.1 If ∆T is not too large, ∆L is directly proportional to ∆T. Diamond 1 L 0 Copper 17 T 0 Glass 8.5 T 0 +∆T L = L 0 +∆L Iron 11.8 The change in length is Gold 14 ∆L = α L 0 ∆T Steel 13.2 where α is the coefficient of linear expansion Ice 51
Slide 25 / 163 Slide 26 / 163 8 A steel rod measures 10 meters at 0° C. Given that the coefficient 9 A simple pendulum is made of a steel string supporting a brass sphere. The temperature in a room with the pendulum is increased from 15° C to 30° C. of linear expansion of steel is 1.2 x 10 -5 per °C, what will the rod Which of the following is true about the period of oscillations? measure at 75°C? A the period doubles B the period does not unchanged C the period slightly increases D the period slightly decreases E the period increases by √2 Slide 27 / 163 Slide 28 / 163 Volume Expansion Volume Expansion Suppose a volume of some substance (gas, liquid or solid) has Substance a length V 0 at an initial temperature of T 0 . Coefficient of Volume Exansion β (×10 -6 / °C) If the temperature is changed by ∆T, the volume changes by Gasoline 950 ∆V. Glycerine 485 If ∆T is not too large, ∆V is directly proportional to ∆T. Water 207 Aluminum 69 The change in volume is ∆V = βV 0 ∆T Diamond 3 Glass 9.9 where β is the coefficient of volume expansion. Steel 32.4 Gold 42 Slide 29 / 163 Slide 30 / 163 Volume Expansion of Water 10 A glass flask is filled to the brim with glycerin, both at the same initial temperature. When the temperature of the flask and glycerine is increased by a few degrees, which of the following Water above 4°C water expands occurs? when heated. The coefficients of volume expansion are: β glycerin = 49x10 -5 K -1 and BUT in the temperature range from β glass = 2x10 -5 K -1 . to 0°C to 4°C, the volume decreases as temperature increases. A the level of the glycerine in the flask goes down This means that 4°C water is more dense than 0°C water. B the level of the glycerin in the flask remains unchanged Hence water has its greatest density at 4°C. C the glycerine overflows 4°C water will sink to the bottom of a lake, so ice will D there isn't enough information given to answer this question form in the 0°C water floating on top. This protects life in bodies of freshwater since the water on the bottom will be at worst 4°C; not freezing!
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