Presentation on theme: "Outline Topic 3 Thermal physics [11hr] 3.1 Thermal concepts 3.2 Thermal properties of matter 3.3 Ideal gases (covered in next PowerPoint)"— Presentation transcript:
Outline Topic 3 Thermal physics [11hr] 3.1 Thermal concepts 3.2 Thermal properties of matter 3.3 Ideal gases (covered in next PowerPoint)
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 constant Describe mechanisms by which thermal energy is transferred.
3.1 Temperature Temperature – Useful idea to compare two bodies internal energy –This is measured using a thermal property of a substance like: Linear expansion (mercury)mercury Electrical resistance (thermocouple)thermocouple Emitted radiation (Infrared)Infrared
3.1 Temperature Scales Fahrenheit - Daniel Gabriel Fahrenheit ( ) 0 o is the temperature of a ice, water, salt mixture, 32 o is freezing water, 96 o was body temperature.Daniel Gabriel Fahrenheit Celsius - Anders Celsius ( ) 0 o is freezing water, 100 o was the boiling point of water at 1 ATM. Later related to Kelvin scale.Anders Celsius Kelvin - William Thomson, 1st Baron Kelvin ( ) defined by two points: absolute zero, and the triple point of specially prepared water.William Thomson, 1st Baron Kelvinabsolute zerotriple point
3.1 Heat & Internal Energy James 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.
3.1 Absolute Temperature Measure 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.
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
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 mole In other words, there are 6.02 x atoms of hydrogen.
Example How many grams are there in a quantity of oxygen containing 1.20 x molecules? Solution: The number of moles is 1.20 x / 6.02 x = mol Since the molar mass is 32 (periodic table) x 32 = 638 g = kg
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.
3.1 Conduction Equation Also called “heat current” Q / t is the energy per unit time, Joules sec -1 – thermal conductivity A – cross sectional areacross sectional area T – temperature x – length of material
Example Two 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 o C? 100 o C 0 o C
Solution Imagine 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 o C.
3.1 Convection Since 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.”
3.1 Radiation Radiation requires no medium (matter) to transfer energy. Every body at some absolute temperature T radiates away energy as electromagnetic waves. P (power) AT 4 IB Note: this equation is not tested Infrared picture of house
3.1 Radiation Good emitters of radiation are dark and dull, think charcoal. At low temperatures emission is small but increase by T 4 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.
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