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Temperature, Heat, and Expansion

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  1. Temperature, Heat, and Expansion

  2. Temperature • Moving molecules have kinetic energy. We can change the kinetic energy by adding heat. • Warmer = more KE • We can tell how hot or cold something is by temperatures

  3. Temperature Scales • We use 3 scales • Celsius – foreign countries and the sciences • 0 = freezing • 100 = boiling • Farenheit – US (Standard) • 32 = freezing • 212 = boiling • Kelvin Scale – Used in science as a standard • 273K = freezing • 373K = Boiling • -273C = 0 = absolute zero in Kelvin • Absolute zero is when there is no KE to be given up. Basically molecules stop moving.

  4. Scales can be compared to cold blooded animals • Low temp = low movement • High temp = high movements In an ideal gas, temperature is proportional to the average KE.

  5. Heat • Heat – the direction of energy transfer is always from a warmer substance to a cooler substance. • The temperature difference between the two objects is called heat. • Once it is transferred it becomes internal energy.

  6. Thermal Contact • When heat flows from one substance to another. Always high to low. • Parents are always telling you to close the door because your are heating the outside. • Thermal Equilibrium • When two objects are in contact and no heat is transferred. • Typically temperature is not affected by the thermometer. • Small drop of water would be affected by the thermometer, but not a cup of water.

  7. Internal Energy – the grand total of all energy inside a substance. • Made up of – • Translational KE • Rotational KE • KE due to internal movements of atoms within molecules • Potential Energy

  8. Quantity of heat – when substance absorbs heat the temperatures change depends on type of material and quantity of material. • The most commonly used unit of heat is the calorie. • Calorie = Amount of heat required to raise the temp of 1g of water by 1o C • Kilocalorie = 1000 calories • Amount of heat to raise 1 kg of water by 1o C • Kilocalorie is actually what food is measured in. We just call it calorie. To distinguish calorie to food calorie, we write food calorie with C.

  9. The SI unit for heat is joule. • 1 Calorie = 4.187 J • The energy of food is determined by the burning of it and measuring the total internal energy. • Same concept with fuel. When burned, how much heat is given off? • In gasoline, pure gas burns better as a far as you get more mileage because it adds more energy. When detergents and other fuels are mixed the amount of internal energy decreases. You get fewer miles to the gallon.

  10. Specific Heat Capacity • Different substances have different capacities for storing internal energy. • High specific heat – holds more heat longer • Water • Apple pie filling • Heats up slow – cools down slow • Low specific heat – holds heat briefly • Aluminum Foil • Heats up fast – cools down fast

  11. The specific heat capacity of any substance is defined as the quantity of heat required to raise the temperature of a unit of mass of a substance by 1o. • Why is water used as a cooling agent? • A small amount of water absorbs a great deal of heat for a correspondingly small temperature rise. Water also takes longer to cool.

  12. Why do climates change? • West coast – warm water moving across the land and bringing the heat. • East Coast – air moves from land to water, and land does not have as high specific heat as water. • Why is the temperature on the coast within 10o day and night, while Colorado is usually a 40o change between day and night?

  13. Thermal Expansion • Solids, liquids, and gases expand when they are heated and contract when they are cooled. P308-9 • That is why we see sidewalks in strips, expansion joints on bridges, parking garages. • Bumps in the road in winter versus summer. • A dentist used filling material that has the same expansion rate as teeth. • Bimetallic Strip • Two metals riveted together which expand at different rates.

  14. Thermostat • The cold causes one metal to contract. It will then contact a metal piece of the thermostat and close a circuit which will turn on the furnace. Once it is warm enough, the metal goes back to normal and expands. The contact is lost and the switch turns off. • Where else do we see this? • Refrigerators • Electric Toaster • Automatic chokes on carburetors

  15. Expansion of water • Water is most dense at 4o C. Anything below 4o C and water expands to crystal structure. P313