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THERMAL ENERGY

THERMAL ENERGY. Temperature, Heat, Specific Heat, & Heat Transfer. Thermal Energy. “Kinetic-Molecular Theory” Hot object – particles move faster, & have more KE When average KE of atoms & molecules increases, they need more room to move around Objects expand when they get hot

williamgribble
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THERMAL ENERGY

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  1. THERMAL ENERGY Temperature, Heat, Specific Heat, & Heat Transfer

  2. Thermal Energy • “Kinetic-Molecular Theory” • Hot object – particles move faster, & have more KE • When average KE of atoms & molecules increases, they need more room to move around • Objects expand when they get hot • If average KE decreases, object contracts • Examples?

  3. Thermal Energy • “Thermal Energy” – the overall energy of motion of the particles that make up an object • Hot object = more thermal E, & vice versa • How do we determine “hotness” of an object? • TEMPERATURE! • Body heat or heat lamps are part of the infrared spectrum.

  4. Temperature • Temperature – a measure of the AVERAGE molecular KE • 3 commonly used temp scales: • Fahrenheit (United States) (98.6 ° F = avg body temp) • Celsius (most of the rest of the world) • Kelvin (scientific community – i.e., us!) • Both C and K scales are “metric” scales because Freezing Point and Boiling Point are 100 degrees apart on each

  5. Kelvin Temperature Scale • K = °C + 273 • “Absolute Zero” = 0 K • i.e., no molecular motion, therefore no KE on the molecular level • 0 K is the lowest you can go! You can’t stop motion more than 0

  6. Temperature

  7. Heat & Temperature • Heat (Q) and temperature are NOT the same thing! • Heat – refers to total amount of thermal energy in a substance • Temp – refers only to average kinetic energy • Which has more heat: 2L of boiling water or 1L of boiling water? • 2L has more heat, even though temp is the same! • Can a cup of hot water have the same heat as a cup of cold water? • YES! If you have a larger mass of cold water and a smaller mass of hot

  8. Specific Heat (c) • Heat Units: Q can be measured in Joules • Specific Heat (c) – the amount of energy needed to raise the temp of a substance • 1 calorie = amount of Q needed to raise 1g (1mL) of water by 1°C • 1 Calorie = amount of Q needed to raise 1 kg (1L) of water by 1°C • The higher the specific heat of a substance, the longer it takes to change temperature

  9. Specific Heat • The higher the specific heat of a substance, the longer it takes to change temperature • High specific heat  Slow temp change • Low specific heat  Fast temp change • Water has a very HIGH specific heat (c = 4186 J / kg K) • It is good to use as a coolant!

  10. Specific Heat - Examples • Examples • Beach sand – LOW specific heat • Hot in the day and cold at night • Ocean water – HIGH specific heat • Temp at day and night roughly the same • Desert – huge swings in temp due to low specific heat of sand • British Isles – temperate climate because the water surrounding them holds in the heat • Weather in Austin more temperate than Dallas because they are closer to the coast

  11. Heat Transfer • Heat Transfer • Heat will transfer between objects if they are not in thermal equilibrium (at the same temp), and if they are in thermal contact (touching). • Ex: thermometer, metal spoon in a bowl of hot soup

  12. Heat Transfer • How is heat transferred? • Warmer molecules have higher KE – they collide with colder molecules and transfer some of their KE to the colder molecules. • “Cold” is not a physical quantity! • “Cold” cannot move from one object to another – only “heat” (thermal energy) moves!

  13. Methods of Heat Transfer • Conduction • Convection • Radiation

  14. Conduction • Conduction – Q transferred through collisions of electrons, atoms, & molecules • i.e., through direct contact between objects • Ex: Thermometer – particles in your body hit the glass, transferring their KE to the mercury & raising its KE

  15. Conduction, cont’d • Conductor – good conductors of Q are also good conductors of electricity (ex: most metals) • Insulators – bad conductors of Q • Examples: styrofoam, wood, rubber, GASES (many times it is the air trapped in the substance that is actually doing the insulating!)

  16. Convection • Convection – Warm, less-dense fluid (gas or liquid) rises and cool, more-dense fluid sinks • “Convection Current” • Examples: • Sea breeze & land breeze • Heating & cooling vents in your home

  17. Radiation • Radiation • Infra-red electromagnetic wave that can travel through empty space • Created by vibrating electrons • Examples: • Night vision goggles • Infra-red satellite images • “Heat-seeking” missiles

  18. “Conduction, Convection, or Radiation?” wkst

  19. Now what do you think? • Suppose you have two cups of water. One is hot and the other is cold. • How is the cold water different from the hot water? • Describe the motion of the molecules in each. • What changes would occur if the hot water was changed into steam? • What are the common scales used to measure temperature? • When is each scale generally used? • All scales use degrees to measure temperature. Which scale has the largest degrees? Explain.

  20. What do you think? • Internal energy is the energy due to the kinetic and potential energy of the particles. • Does the ice water or an equal quantity of hot chocolate have greater internal energy? Why? • Which has more internal energy, a gallon of cold water or a drop of hot chocolate? • How will the internal energy of the water and hot chocolate change over time? • How will this change occur?

  21. Internal Energy and Heat • Internal Energy (U) is the energy contained within the particles of a substance. • Heat (Q) is the internal energy transferred between objects. • Heat always moves from a higher-temperature object to a lower-temperature object. • The rate of transfer depends on the difference in temperature. • The greater the temperature difference, the greater the rate of energy transfer (if other factors are equal).

  22. Internal Energy and Heat • Which way does heat flow if you place a warm canned beverage in cold water? • How does this occur, on a molecular level?

  23. Temperature and Heat Click below to watch the Visual Concept. Visual Concept

  24. Heat Transfer • In what three ways can internal energy be transferred from a hot object to a colder object? • Conduction is the transfer of heat through a substance by molecule to molecule contact. • Metals are good conductors. • Styrofoam is a good insulator. • Convection is the transfer of energy by the movement of a fluid. • Hot air in a room rises and cold air moves in to replace it. • Radiation is the transfer of energy by electromagnetic waves. • No matter is transferred, only energy.

  25. Comparing Convection, Conduction, and Radiation Click below to watch the Visual Concept. Visual Concept

  26. Heat and Work • Work can be changed into internal energy. • Rub your hands together and you’ll feel the increase in internal energy produced by your work. • Pull a nail from a piece of wood and the nail is hot. • Mechanical energy (PE + KE) is conserved when there is no friction. • Total energy, including internal energy, is always conserved.

  27. Now what do you think? • Does the ice water or an equal quantity of hot chocolate have greater internal energy? Why? • Which has more internal energy, a gallon of cold water or a drop of hot chocolate? • How will the internal energy of the water and hot chocolate change over time? • How will this change occur?

  28. What do you think? • What property of water makes it so useful as a coolant in automobiles, nuclear reactors, and other machinery? • How does it differ from other liquids regarding its ability to cool substances? • Why do you feel cool when getting out of a warm swimming pool on a hot day? • How do you feel if it is windy out? Why? • How do you feel if it is an indoor pool? Why?

  29. Specific Heat Capacity • Specifc heat capacity (cp) measures the amount of heat required to raise the temperature by 1°C for 1 kg of a substance. • It is different for every substance. • SI Units: J/kg•°C • Alternate form of this equation: Q = cpmT

  30. Suppose each metal shown above absorbs 100 J of energy. • Which will show the greatest increase in temperature? the least? • How does water compare to iron with regard to heat capacity?

  31. Latent Heat • Latent heat is heat gained or lost during phase changes. • When substances melt, freeze, boil, condense, or sublime, the temperature does not change during the phase change. • Heat absorbed changes the potential energy of the particles.

  32. Latent Heat Click below to watch the Visual Concept. Visual Concept

  33. Latent Heat • Heat of fusion (Lf) is the heat required to melt 1 kg of a substance. • Also equals the heat released when 1 kg freezes • Which graph segment represents this? • Heat of vaporization (Lv) is the heat required to change 1 kg of a substance from a liquid to a gas. • Which graph segment represents this?

  34. Latent Heats of Fusion and Vaporization

  35. Now what do you think? • What property of water makes it so useful as a coolant in automobiles, nuclear reactors and other machinery? • How does it differ from other liquids regarding its ability to cool substances? • Why do you feel cool when getting out of a warm swimming pool on a hot day? • How do you feel if it is windy out? Why? • How do you feel if it is an indoor pool? Why?

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