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Thermal Energy

Thermal Energy. Temperature. Temperature is the amount of average kinetic energy of a substance Temperature is a property that determines the direction of thermal energy transfer between two bodies in contact Temperature is measured in degrees Celsius ( o C) or Kelvin (K)

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Thermal Energy

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  1. Thermal Energy

  2. Temperature • Temperature is the amount of average kinetic energy of a substance • Temperature is a property that determines the direction of thermal energy transfer between two bodies in contact • Temperature is measured in degrees Celsius (oC) or Kelvin (K) • Where Temp in K = Temp in oC + 273 • K is known as the absolute temperature

  3. Temperature Units and Conversions • Temperature is measured in degrees Celsius (oC) or Kelvin (K) • Where Temp in K = Temp in oC + 273 • K is the SI unit Temperature Conversions Celsius-Fahrenheit: Tf=9/5t +32 F-C: t= 5/9 (Tf -32) K-C: T=t+273

  4. Thermal Equilibrium • When 2 bodies are placed in contact • Heat will flow from the warmer body to the colder body • Until the two objects reach the same temperature • They will then be in Thermal Equilibrium • This is how a thermometer works

  5. Thermometers • A temperature scale is constructed by taking two fixed, reproducible temperatures • The upper fixed point is the boiling point of pure water at atmospheric pressure • The lower fixed point is the melting point of pure ice at atmospheric pressure

  6. These were then given the values of 100 oC and 0 oC respectively, and the scale between them was divided by 100 to give individual degrees

  7. Heat • The term heat represents energy transfer due to a temperature difference • Occurs from higher to lower temperature regions

  8. Methods of Heat Transfer • Heat can be transferred from one body to another by: • Conduction • Convection • Radiation • Undesired heat transfers can be reduced with insulators: • R value

  9. Specific Heat • Defined as the amount of thermal energy required to produce a change in temperature mass of the MATERIAL • Unit mass is normally 1kg, and unit temperature rise is normally 1K • Specific Heat = Q=cmT • in J/kg K • Q is the amount of energy require to heat up • where m is the mass of the material • C is the specific heat • T is the change in temperature • The specific heat of water is 4186 J/Kg K (water has a high specific heat –metals have very low specific heats)

  10. Phases (States) of Matter • Matter is defined as anything that has mass and occupies space • There are 4 states of matter • Solids, Liquids, Gases and Plasmas • Most of the matter on the Earth in the form of the first 3 • Most of the matter in the Universe is in the plasma state

  11. Macroscopic Characteristics

  12. Fluids • Liquids • Gases • are both fluids • Because they FLOW

  13. Arrangement of Particles - 1 • Solids • Closely packed • Strongly bonded to neighbours • held rigidly in a fixed position • the force of attraction between particles gives them PE

  14. Arrangement of Particles - 2 • Liquids • Still closely packed • Bonding is still quite strong • Not held rigidly in a fixed position and bonds can break and reform

  15. Arrangement of Particles - 3 • Gases • Widely spaced • Only interact significantly on closest approach or collision

  16. Thermal Properties of Gases • Gasses exhibit a measure of • Pressure • Volume • Temperature • Observations of gasses have led to the following gas laws. • Boyle’s Law • Charles Law • Bournoulli’s Principle • Archimedes Principle

  17. Changes of State • A substance can undergo changes of state or phase changes at different temperatures • Pure substances have definite melting and boiling points which are characteristic of the substance

  18. Changes of State - 2 • The kinetic theory of matter can be used to explain the microscopic behavior of these phase changes • When the solid is heated the particles of the solid vibrate at an increasing rate as the temperature is increased • The vibrational KE of the particles increases

  19. Changes of State -3 • At the melting point a temperature is reached at which the particles vibrate with sufficient thermal energy to break from their fixed positions and begin to slip over each other • As the solid continues to melt more and more particles gain sufficient energy to overcome the forces between the particles and over time all the solid particles are changed to a liquid • The PE of the system increases as the particles move apart

  20. Changes in State - 4 • As the heating continues the temperature of the liquid rises due to an increase in the vibrational energy of the particles • At the boiling point a temperature is reached at which the particles gain sufficient energy to overcome the inter-particle forces and escape into the gaseous state. PE increases. • Continued heating at the boiling point provides the energy for all the particles to change

  21. Temp / oC Time /min Heating Curve Liquid - gas phase change Gas Solid - liquid phase change Liquid Solid

  22. SOLID LIQUID GAS Changes of State Thermal energy added sublimation melting vaporisation condensation Freezing/solidification Thermal energy given out

  23. Evaporation • The process of evaporation is a change from the liquid state to the gaseous state which occurs at a temperature below the boiling point • The Moving Particle (Kinetic) theory can be applied to understand the evaporation process

  24. Explanation • A substance at a particular temperature have an average range of particle energy • So in a liquid at any instant, a small fraction of the particles will have KE considerably greater than the average value

  25. So • If these particles are near the surface of the liquid, they will have enough KE to overcome the attractive forces of the neighbouring particles and escape from the liquid as a gas • This energy is needed as gases have more PE than liquids.

  26. Cooling • Now that the more energetic particles have escaped • The average KE of the remaining particles in the liquid will be lowered • Since temperature is related to the average KE of the particles • A lower KE infers a lower temperature

  27. Cool • This is why the temperature of the liquid falls as an evaporative cooling takes place • A substance that cools rapidly is said to be a volatile liquid • When overheating occurs in a human on hot days, the body starts to perspire • Evaporation of the perspiration cools the body

  28. Factors Affecting The Rate • Evaporation can be increased by • Increasing temperature • (more particles have a higher KE) • Increasing surface area • (more particles closer to the surface) • Increasing air flow above the surface • (gives the particles somewhere to go to)

  29. Heat Engines • Use the expansion of chemicals (in the liquid and/or gas state) to move mechanical parts. • The expansion is can be caused by actively heating or through chemical reaction (ex. Combustion, or other energy releasing reaction)

  30. Heat Movers • Refrigerators, Heat pumps, Air Conditioners are known as heat movers.

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