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Thermal Physics

Thermal Physics. IDS Physics: Unit 04. Temperature. There are many ways to measure temperature Common stable temperatures were first used as reference points (Fahrenheit) Fixed temperatures of phase transitions were then used (Centigrade and Celsius)

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Thermal Physics

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  1. Thermal Physics IDS Physics: Unit 04

  2. Temperature • There are many ways to measure temperature • Common stable temperatures were first used as reference points (Fahrenheit) • Fixed temperatures of phase transitions were then used (Centigrade and Celsius) • Absolute scales were later used as we understood the nature of temperature better (Kelvin and Rankin)

  3. Temperature

  4. Temperature

  5. Converting Temperature

  6. Example 1 • A child has a fever of 40.3°C. Convert this to the more familiar Fahrenheit scale. • 104.5°F

  7. Thermal Energy • All atoms are in motion. • They all have kinetic energy • Translating (moving) • Rotating (spinning) • Oscillating • Kinetic energy associated with molecular motion is called thermal energy

  8. Definitions • The chemical potential, thermal, and nuclear energy all combined is called the internal energy of the substance • When thermal energy is transferred from one object to another, we called the transferred energy heat • Temperature is a measure of the thermal energy. It is proportional to the kinetic energy of the moving molecules.

  9. A cold Balloon • What will happen to a balloon if you take it outside where it is cold? • How small will it get? • As the kinetic energy of the molecules decreases, so will the temperature. • If the molecules stop moving, there is no KE, so then T must be zero.

  10. A new Temperature • We need a new temperature scale that reads ‘zero’ only when all molecules have stopped moving. • This new ‘zero’ is called absolute zero.

  11. Absolute Zero • -273 °C is the coldest anything can ever get. • Kelvin is an absolute temperature scale based on the Celsius Scale • The absolute temperature is a measure of the average thermal (kinetic) energy of a substance.

  12. Think and Explain • Which has a greater amount of thermal energy? • A: 300 kg snowball • B: 300 g. cup of hot tea? • C: 3000 ton iceburg

  13. Thermal Expansion • Demo: heating a copper ball • As the temperature of a substance increases, the average distance between the molecules increase. • This is true for most substances (Water is the notable exception )

  14. Linear Expansion

  15. Example • Find the change in the width of a 3.0 meter square of ordinary glass as the temperature changes from 40. F down to -10. F • Note: you must convert temperatures to Celsius before computing T

  16. Applications • Bridges, Roads and Railways • Filling teeth • Thermostats

  17. Thermostats

  18. The Blachly Scale • Mr. Blachly invents a new temperature scale: • 100 B is the temperature at which a Twix Bar starts to melt. (30C) • 40 B is the temperature at which a Twix Bar is perfect for eating. (5C) • What is room temperature, 22 C, in B

  19. The Blachly Scale

  20. Pizza Capacity

  21. Heat Capacity • Demo: the candle and the balloon

  22. Heat Capacity • All substances will change temperature as they gain thermal energy. • The magnitude of the chance depends on the substance and the mass.

  23. Heat Capacity

  24. Heat Capacity

  25. Example • In order to fill a small tub to wash a toddler, you need to warm up 33.75 kg. of water from 65 F to 105 F. How much energy will you need? (Answer in MJ) • If energy is cheap because you generate it from your wind turbine on your roof, and costs $0.036 for a kW-hr ( 3.6 MJ ), what will it cost to heat the water for the baby’s bath? (Answer in cents).

  26. Applications • Feel the leg of your chair and the leg of the table. Are they the same temperature? • Why is the hard floor cold and the carpet warm? • Why do some parts of the pizza burn your mouth and other parts do not?

  27. Example #2 • 1.05 MJ of energy is needed to bring a pot of water, starting at room temperature, to its boiling point. What is the mass of the water?

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