2 Outline Topic 3 Thermal physics [11hr] 3.1 Thermal concepts 3.2 Thermal properties of matter3.3 Ideal gases (covered in next PowerPoint)
3 3.1 Thermal Concepts Objectives: Understand how a temperature scale is constructed.Understand heat is energy that is exchanged by systems at different temperatures.Understand internal energy.Understand absolute temperature is a measure of average kinetic energy.State the meaning of the mole and the Avogadro constantDescribe mechanisms by which thermal energy is transferred.
4 3.1 TemperatureTemperature – Useful idea to compare two bodies internal energyThis is measured using a thermal property of a substance like:Linear expansion (mercury)Electrical resistance (thermocouple)Emitted radiation (Infrared)
5 3.1 Temperature ScalesFahrenheit - Daniel Gabriel Fahrenheit ( ) 0o is the temperature of a ice, water, salt mixture, 32o is freezing water, 96o was body temperature.Celsius - Anders Celsius (1701 - 1744) 0o is freezing water, 100o was the boiling point of water at 1 ATM. Later related to Kelvin scale.Kelvin - William Thomson, 1st Baron Kelvin (1824 - 1907) defined by two points: absolute zero, and the triple point of specially prepared water.
6 3.1 Heat & Internal EnergyJames Prescott Joule ( ) determined the mechanical equivalent of heat experimentally.Heat is defined as energy transfer resulting in a temperature difference.Internal energy – the total kinetic energy of the molecules of a substance, plus any potential energy between the molecules.
7 3.1 Absolute TemperatureMeasure of the average kinetic energy of the molecule in a substance.This is directly proportional to the Kelvin temperature.Sometimes thermal energy is used in reference to internal energy.
8 3.1 The Atomic Model Three phase of ordinary matter. Solid: High density with molecules in a fixed position.Liquid: Lower density with molecules further apart and free to change position.Gas: Lowest density with molecules even further apart and free to move.Water at triple point
9 3.1 Moles Avogadro’s number tells the molar mass Hydrogen has a molar mass of 2 g mol-1, so 2 g of hydrogen represents 1 moleIn other words, there are x 1023 atoms of hydrogen.
10 ExampleHow many grams are there in a quantity of oxygen containing 1.20 x 1025 molecules?Solution:The number of moles is 1.20 x 1025 / 6.02 x 1023 = molSince the molar mass is 32 (periodic table) x 32 = 638 g = kg
11 3.1 Heat Transfer There are three ways energy is transferred: Conduction is movement of molecular kinetic energy through collisions between molecules.Good conductors of heat are usually good electric conductors.This includes most metal as their valance electron are free to move and “bump” into things.
12 3.1 Conduction Equation Also called “heat current” Q / t is the energy per unit time, Joules sec-1k – thermal conductivityA – cross sectional areaT – temperaturex – length of material
13 ExampleTwo rods of the same length and cross-sectional area are joined together. The left rod has a higher k than the rod at right. The ends of the rod are kept at a fixed temperature as shown. In which rod is the rate of heat transfer the largest? Is the temperature at the joining point lower or higher than 50 oC?100 oC0 oC
14 SolutionImagine a vertical line through any part of the rod. The heat entering the line must be equal to the heat leaving it (conservation of energy). Hence, the rate of heat transfer is the same everywhere. A much larger temperature difference can be maintained across a bad conductor of heat. Thus, the temperature at the joint will be higher than 50 oC.
15 3.1 ConvectionSince fluid molecules are free to move “hotter” fluid is less dense than “colder” fluid. In hot fluids the molecules are further apart, thus less dense. This less dense fluid rises in the presence of colder fluid and create a “convection current.”
16 IB Note: this equation is not tested 3.1 RadiationRadiation requires no medium (matter) to transfer energy.Every body at some absolute temperature T radiates away energy as electromagnetic waves.P (power) AT4IB Note: this equation is not testedInfrared picture of house
17 3.1 RadiationGood emitters of radiation are dark and dull, think charcoal.At low temperatures emission is small but increase by T4 as temperature rises.These are also good absorbers, think black shirt on a hot summer day.Shiny surfaces are reflectors, therefore are poor absorbers and emitters.AT room temperature objects emit wavelengths in the infrared spectrum.
18 Cross Sectional AreaCross sectionClick to go back