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## Molecules

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**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: • 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**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**Conversion Formulas**• Fahrenheit to Celsius: • Celsius to Fahrenheit: • Celsius to Kelvin: • Fahrenheit to Rankine:**Conversion Examples**• 250 deg C = deg F • 400 deg F = deg C**Conversion Examples**• 200 deg F = deg R • -40 deg C = deg F • 250 deg C = deg K • 460 deg K = deg F**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.**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.**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.**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.**Conduction**• “Feeling” is an unreliable indication of temperature due to dissimilar conductivities of different materials. • Tile vs. Carpet: Same temp, tile “feels” colder.**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**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.**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.**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.**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.**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.**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**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:**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**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.**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.**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**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**Another Example**100 g of Aluminum cools from 650 deg C to 25 deg C. How much heat is released?**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**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.**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:**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**First Expansion:**Second Expansion: