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Molecules

Molecules. Two kinds of energy: Movement and attraction Movement energy is “Heat energy” Thermometers added to substance to measure heat. They measure average molecular velocity or heat intensity levels, but not total heat or energy possessed. Four Temperature Scales. Metric Scales:

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Molecules

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  1. Molecules • Two kinds of energy: Movement and attraction • Movement energy is “Heat energy” • Thermometers added to substance to measure heat. • They measure average molecular velocity or heat intensity levels, but not total heat or energy possessed.

  2. Four Temperature Scales Metric Scales: • Celsius (Centigrade) – Relative scale. Water freezes at 0 degrees, and boils at 100 degrees • Kelvin – Absolute scale – cooled hydrogen until no detectable molecular movement: “absolute zero.” No additional heat energy could be extracted. Absolute zero = -273.16 deg C

  3. Four Temperature Scales English • Fahrenheit – Relative scale – water freezes at 32 degrees and boils at 212 degrees • Rankine – Absolute scale – 460 degrees corresponds to 0 deg F180 F = 180 + 460 = 640 deg R

  4. Conversion Formulas • Fahrenheit to Celsius: • Celsius to Fahrenheit: • Celsius to Kelvin: • Fahrenheit to Rankine:

  5. Conversion Examples • 250 deg C = deg F • 400 deg F = deg C

  6. Conversion Examples • 200 deg F = deg R • -40 deg C = deg F • 250 deg C = deg K • 460 deg K = deg F

  7. Which is the highest temp? • Which is the highest temp? The lowest? 1500 deg F, 1000 deg C, 500 deg K, 150 deg K, 75 deg C or 25 deg F? • Convert to Kelvin. • In order from lowest to highest: • 150 deg K, 25 deg F, 75 deg C, 500 deg K, 1500 deg F, and 1000 deg C.

  8. Heat Transfer • Heat transfers from warmer to cooler, so the warmer object cools and the cooler object warms, until equalization occurs. “Equilibrium Temperature” is achieved. • This equalizing occurs in three ways: conduction, convection and radiation.

  9. Conduction • Heat conduction occurs by electron and molecular collisions. • Metal nail in a flame. Molecules at the heated end move more rapidly, bumping into neighboring molecules forcing them to move. This continues until entire nail is hot.

  10. Conduction • How well an object conducts heat depends upon the bonding of molecular structure. • “looser” the outer electrons are, the better the material is at conducting heat and electricity • Metals are good conductors: Silver, Copper, Aluminum, Iron • Wood, paper, cork, Styrofoam: Poor conductors, but good insulators. • Liquids and gases are poor conductors also.

  11. Conduction • “Feeling” is an unreliable indication of temperature due to dissimilar conductivities of different materials. • Tile vs. Carpet: Same temp, tile “feels” colder.

  12. Convection • Heat transfer by the actual motion of the fluid by currents. Results from unequal temps and unequal densities. • Heating from the bottom: molecules speed up and fluid becomes less dense. Warmer fluid gets pushed up by cooler and more dense fluid taking its place at the bottom. • Convection keeps fluid stirred up. • Stove top • Warm air in room of ceiling – ceiling fans • Smoke from fire • Ventilation is helped by convection if windows are open at the top and bottom of room

  13. Radiation • Previous two types of heat transfer required two types of bodies in contact. • Electromagnetic radiation: radio waves, microwaves, infrared, visible light, uv radiation, xrays and gamma rays. • Vibration of particles results in emission of radiation. When radiations strike another body, corresponding vibrations are produced in receiving body.

  14. Radiation • This energy travels at a rate of 186,000 miles/sec. • Radiations affect matter or objects in 3 ways: • Reflection from surface. • Enters and passes through body. • Remainder is absorbed and converted into heat.

  15. Radiation • Gases transmit almost all the sun’s radiation, absorbing very little. • Black fur coat: transmits none, reflects little, and absorbs most. • Dark, rough surfaces absorb. • Bright, smooth surfaces reflect. • A body that absorbs readily, transmits readily. Dark rough body radiates more energy than bright body at same temp. Heating stoves are black.

  16. Radiation Other factors affecting rate of radiation: • Amount of surface area. • Increase in temp, increase in rate of radiation. Doubling the temp raises the radiation by 16 (proportional to the 4th power). • Focusing radiant heat: Parabolic reflectors on electric heaters.

  17. Heat Measurement in Matter • Heat is the total kinetic energy possessed by the molecules of a substance. • It is a form of energy and is only produced by converting other forms of energy or matter. • Other forms of energy can be turned into heat. Can heat be turned into energy? • Combustion of fuel: changed into ME of pistons and crankshaft.

  18. Heat Measurement Two ways: Calorie or BTU • Calorie – kilocalorie = 1000 food calories. A calorie is equal to the amount of heat required to change the temp of 1 g of water 1 deg C. 10 cal will raise temp of 1 g of water 10 deg C OR 10 cal will raise temp of 10 g of water 1 deg C

  19. Heat Measurement • BTU – British Thermal Unit. 1 BTU is the amount of heat required to change 1 pound of water 1 deg F • Conversion: 1 BTU = 252.2 cal • Formula:

  20. Heat Measurement • 100 gal of coolant at 70 deg F. What will be the temp of the coolant after absorbing 50400 BTUs? Temp = 70+60.4=130.4 deg F

  21. Heat Measurement • How much heat is given off as 100 lbs of fresh water cools from 90 deg F to 50 deg F? What if 100 lbs of sea water? Mercury? Aluminum? Will it be different? Why? Closeness or “looseness” of molecules.

  22. Specific Heat Different materials require different amounts of heat energy to cause them to change temperature. Specific heat is the quantity of heat required to change the temperature of a unit mass of the substance by 1 degree. Specific heat constants are located in tables. A small table can be found on page 260.

  23. Specific Heat Every gram of water absorbs 1 cal of heat for every 1 deg C increase in temp. So … 15 g of water requires 330 calories to raise the temperature 22 deg

  24. Example If 1 gram of aluminum is heated 1 deg C, how many calories are absorbed? Each substance has its own specific heat constant. This constant is a ratio of the efficiency of the substance to the heat capacity of water. Aluminum specific heat = 0.217

  25. Another Example 100 g of Aluminum cools from 650 deg C to 25 deg C. How much heat is released?

  26. Coefficient of Expansion When an object absorbs heat, it (usually) expands. Molecules are moving faster, colliding with neighboring molecules, pushing them farther apart. Examples: Bridges, cold glass object cracks when put in contact with heat

  27. Linear Expansion Temperature must be in Celsius as the coefficient of linear expansion is per degree C. A table of constants can be found on page 261.

  28. Example A brass rod of length at 32 deg F is heated to 120 deg C. What is its new length? Step 1: Convert length to inches: Step 2: Convert all temps to Celsius: Step 3: Plug values into the equation: Step 4: Add this change to the original length:

  29. How much expansion of a 747 that is 252 feet long when it is 40 deg F in the morning, 90 deg F on the ramp and -20 deg at cruise altitude? Two expansions! Make sure all temps are in Celsius. Watch units of linear measurement. Calculate each separately

  30. First Expansion: Second Expansion:

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