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Heat

Heat. Heat is a form of Energy Transfer. Temperature. Temperature is defined as a measure of the average kinetic energy of the molecules. This is often called the internal energy of an object. 06-01 Temperature and Thermal Expansion. 06-01 Temperature and Thermal Expansion.

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Heat

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  1. Heat • Heat is a form of Energy Transfer Herriman High Physics

  2. Temperature • Temperature is defined as a measure of the average kinetic energy of the molecules. This is often called the internal energy of an object. Herriman High Physics

  3. 06-01 Temperature and Thermal Expansion

  4. 06-01 Temperature and Thermal Expansion • Convert 30°C to K • 303 K

  5. 06-01 Temperature and Thermal Expansion • Heat always flows from hotter object to colder object until thermal equilibrium • Zeroth Law of Thermodynamics • If A and B are in equilibrium, and B and C are in equilibrium, then A and C are in equilibrium

  6. 06-01 Temperature and Thermal Expansion • Normal Solids • How do you open a glass jar if the metal lid is too tight? • Run it under hot water • The lid expands as the temperature increases

  7. 06-01 Temperature and Thermal Expansion • Even small changes in length can have serious consequences • Have you ever seen expansion joints in bridges?

  8. 06-01 Temperature and Thermal Expansion • A steel bridge is 2 km long. If the temperature when it was built was 21°C (70°F), what length expansion joints are needed to prevent buckling at 43°C (110°F)? • ΔL = 0.528 m

  9. 06-01 Temperature and Thermal Expansion • Bimetallic Strip • Made from two strips of metal that have different coefficients of linear expansion • One side expands more than the other causing the strip to bend • Used in automatic switches in appliances and thermostats

  10. 06-01 Temperature and Thermal Expansion

  11. 06-01 Temperature and Thermal Expansion • Why do fluids in the car usually have a reservoir tank (radiator, brake fluid, power steering fluid, oil)? • As the fluids heat, the volume increases • There needs to be some place for the extra fluid to go

  12. 06-01 Temperature and Thermal Expansion • Water • Water is unique • The volume of water decreases from 0°C to 4°C • Then water expands from 4°C and up • Water is the densest (least expanded) at 4°C • As the weather gets cold, the lake water cools and sinks because it becomes more dense pushing the warmer water up • After all the water is 4°C, the top starts to freeze • Because the 0°C water is less dense than the 4°C water, it floats • The ice floats and provides insulation for the warmer water underneath so it does not freeze

  13. 06-01 Homework • Expand your mind with these questions • Read 13.3, 13.4

  14. 06-04 Heat and Temperature Change • Heat is energy that flows from a higher-temperature object to a lower-temperature object because of the difference in temperatures • Unit: Joule (J), calorie (cal), kilocalorie (kcal or Cal) • If an object feels hot, the heat is flowing into you • If an object feels cold, the heat is flowing out of you • Mechanical Equivalent of Heat • Since heat is energy, other types of energy can make the same effect as heat • 1.000 kcal = 4186 J

  15. 06-04 Heat and Temperature Change

  16. 06-04 Heat and Temperature Change • Measuring the change in temperature of different heated objects (usually water and an unknown) inside a thermos can be used to measure the specific heat capacity of the object • This can be used to identify the unknown material

  17. 06-04 Heat and Temperature Change

  18. Chapter 16 Heat Transfer Herriman High Physics

  19. Conduction • Heat transfer through physical contact • In order for conduction to occur the two objects must begin at different temperatures. Objects at the same temperature will not transfer heat and are said to be at thermal equilibrium. • Each material has an innate ability to absorb or give off heat – specific heat • The amount of heat an object can transfer depends upon three things: • The mass of the object, m (in Kg) • The specific heat of the object, Cp( in J/g°C) • The temperature change of the object, t (in ºC) • Heat is transferred according to the equation: Q = mCpt The heat transferred is measured in a unit called a calorie. Herriman High Physics

  20. Convection • Unlike conduction which transfers energy without transferring mass, convection requires a current – or mass movement of molecules in a fluid. • Keep in mind that a fluid in physics is either a liquid or a gas • Convection occurs because when fluids are heated they become less dense and rise past the colder, denser molecules – this creates the current which allows for heat transfer Herriman High Physics

  21. Convection • Wind is a form of convection • Using the furnace in your house is a form of convection • Boiling water is also a form of convection Herriman High Physics

  22. Radiation • Radiation is the transfer of energy via an electromagnetic wave. • Electromagnetic waves include radio waves, microwaves, and light • Packets of energy called photons are carried on a wave front Herriman High Physics

  23. The Greenhouse Effect • A car parked in the sun can get significantly hotter than the outside temperature, this is the greenhouse effect. It occurs because the glass windows let heat in as an electromagnetic wave (sunlight), but then don’t let the resulting heat waves back out. Herriman High Physics

  24. Solar Power • Solar Power is the rate at which the Earth receives energy from the sun. • Solar power can be used by either passive solar heating (greenhouse effect) or by using photovoltaic cells, also called solar panels, to convert the solar energy to electricity. Herriman High Physics

  25. Change of Phase Chapter 17 Herriman High Physics

  26. Phases Changes • Phase changes require that a substance absorb energy or release energy to occur. • There is NO Change in Temperature associated with a phase change! • Different words are used to denote direction when dealing with a phase change. Herriman High Physics

  27. Vocabulary of a Phase Change • Freezing – change from liquid to solid. • Energy is released! • Melting – change from solid to liquid • Energy is absorbed! Both of these changes happen at the same point; 0° C for water! Herriman High Physics

  28. Vocabulary of a Phase Change • Condensing – change from gas or vapor to a liquid. • Energy is released! • Boiling – change from liquid to gas or vapor • Energy is absorbed! Vapor is the gas phase of any substance which is normally a liquid at room temperature! Herriman High Physics

  29. Energy Required for a Phase Change • Just like specific heat is a set amount of energy for each substance – the amount of energy required for a phase change is also substance specific. • Heat of Fusion – ΔHf , is the amount of energy absorbed or released when a substance melts or freezes! Herriman High Physics

  30. Energy Required for a Phase Change • Heat of Vaporization – ΔHv , is the amount of energy absorbed or released when a substance boils or condenses! • Mathematically: Q = mΔHffor Fusion or Q = mΔHv for Vaporization Herriman High Physics

  31. Sample Problem How much heat is required to melt 5 kg of ice at 0° C? Solution Q = mΔHf = (5000 g)(80 calories/g) = 400,000 calories or 400 Kilocalories Herriman High Physics

  32. Phase Diagram 120 100 0 -5 Herriman High Physics

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