Heat and Thermodynamics

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Heat and Thermodynamics
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• All matter is in constant motion, so the atoms
  and molecules of all matter have KE.
• Gases vibrate the most b/c there are no bonds b/w
  the molecules
• Solids vibrate the least b/c there are the
  strongest bonds
• This molecular KE gives an object internal energy
  which affects its temperature. Whenever something
  becomes warmer, its because the KE of its atoms
  or molecules has increased.

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Check Your Understanding

• Does a solid desk sitting in a classroom have
  kinetic energy?
• No. The entire desk is not moving so the
  entirety of the desk does not have KE.

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Check Your Understanding

• Do the molecules in a solid desk sitting in a
  classroom have kinetic energy?
• Yes! Even though the desk itself is not moving,
  the individual molecules are constantly moving.
  We do not see the desk moving because they are
  all vibrating back and forth over VERY small
  distances.

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Temperature

• Temperature the measurement of the average
  molecular KE of an object.
• As molecules gain KE, the temperature increases
• As molecules lose KE, the temperature decreases
• SI Unit Kelvin (K)
• Ex 0C 273 K 32F

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• Temperature is the measure of the AVERAGE KE of
  the substance, not the total KE.
• Ex there is more KE in a bucketful of warm
  water than in a cupful of water, but if the
  temperature of the two samples is the same, the
  average KE of the two is the same.
• There are 3 commonly used temperature scales
• Fahrenheit
• Celsius
• Kelvin

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Temperature Scale Freezing Point of Water Boiling Point of Water Where it is Used
Fahrenheit 32F 212F In the US
Celsius 0C 100C Most of the world
Kelvin 273 K 373 K In science (SI unit)
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• The Kelvin scale and the Celsius scale are the
  same thing
• A Kelvin is the same size as a Celsius degree
• If you increase the temperature by 1C, you
  increase it by 1 K
• If you increase the temperature by 10C, you
  increase it by 10 K
• Absolute zero the lowest possible temperature.
• At this temperature all motion ceases, so the
  substance has no kinetic energy.
• SI Unit Kelvin (k)
• Ex 0 K -273C

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Check Your Understanding

• Which has a greater average KE, a hot cup of
  coffee or swimming pool full of ice cold water?
• The cup of coffee. As temperature is related to
  the average KE, the one with the higher
  temperature has the greater average KE.

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Check Your Understanding

• Which has a greater total KE?
• The swimming pool of ice cold water. Even though
  the cup of coffee has a higher average KE, the
  swimming pool has more mass and therefore a
  greater overall KE.

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Heat

• Heat The energy transfer from one object to
  another because of a temperature difference
  between them.
• Heat flows from the higher-temperature substance
  into the lower-temperature substance.
• Heat never flows on its own from a cold substance
  to a hot substance, just like water will never
  flow uphill by itself.
• Ex You feel cold holding a piece of ice because
  heat flows from your hand to the ice cube

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• Thermal Energy the energy resulting from heat
  flow.
• When heat flows from one object or substance to
  another it is in contact with, the objects are
  said to be in thermal contact.
• Ex When you hold a piece of ice, you lose
  thermal energy while the ice gains thermal energy

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Thermal Equilibrium

• Thermal Equilibrium the state of two or more
  objects or substances in thermal contact when
  they have reached a common temperature.
• Objects have reached the same temperature
• Ex when a piece of hot metal is added to a cup
  of cool water, thermal energy flows between them
  until they reach the same temperature of 10
  degrees Celsius

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Internal Energy

• Internal Energy the grand total of all energies
  inside a substance.
• A substance contains internal energy, not heat
• This energy is due to KE of the moving molecules
  as well as the PE in the bonds holding the
  molecules together.
• SI Unit Joule (J)
• Ex An iceberg has a larger internal energy than
  a small cup of coffee

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Thermal Expansion

• When the temperature of a substance increases
  this means the average kinetic energy of the
  substance increases, so the molecules move faster
  and further apart.
• This results in an expansion of the substance.
• Almost all forms of matter expand when they are
  heated and contract when they are cooled

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• Thermometers work off of this concept
• As the mercury or alcohol heats up, it expands up
  the thermometer
• When the temperature decreases, the liquid
  contracts down the tube
• Not all objects have the same rate of thermal
  expansion.
• Some objects expand more than others
• Liquids tend to expand more than solids

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Bimetallic strip

• A bimetallic strip is two metals welded together,
  steel and copper
• When the strip is heated, it curves
• That is b/c the steel side expands at a different
  rate compared with the copper side

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Check Your Understanding

• Why is it advisable to allow telephone wires to
  sag when stringing between poles in summer?
• Because if the lines were pulled tight in the
  summer, when they contract in the winter, they
  would snap.

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Expansion of Water

• Water is the exception to thermal expansion
• Water at the temperature of melting ice contracts
  when the temperature is increased, and will
  continue to contract until it reaches 4C.
• Water is most dense at 4C
• This is why ice floats in water
• The ice is less dense than the water

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Check Your Understanding

• What would happen if you put a can of soda in the
  freezer and leave it there overnight?
• It will explode. As the water goes away from
  4C, the volume increases. As the pressure
  builds from the excess volume, the can will
  eventually explode.

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Heat Transfer

• Remember that heat is the transfer of energy
• There are 3 types of heat transfer
• Conduction
• Convection
• Radiation
• More than one type of heat transfer can occur at
  the same time, but usually one is more dominate
  than the rest

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Conduction

• Conduction energy transfer from one object to
  another when the two are in direct contact.
• The materials MUST be touching.
• Heat moves from the warmer object to the cooler
  object.
• Ex a hot piece of metal burning your hand your
  hand melting a piece of ice

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• Materials that conduct (move) heat well are
  called heat conductors.
• Objects will change their temperature easily
• Ex metals, glass
• Wood would be considered a good insulator. An
  insulator delays the transfer of heat.
• Objects do NOT change their temperature easily
• Ex Styrofoam, wood, air
• A poor conductor is a good insulator.
• Liquids and gases in general are good insulators.
  Porous materials having many small air pockets
  are good insulators as well (the secret behind
  feathers, furs, and wools perceived warmth).

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Ice HotelIce Hotels and Igloos keep people warm
because ice and snow are very good insulators or
heat.
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Check Your Understanding

• Touch a piece of metal and a piece of wood in you
  immediate vicinity. Which one feels colder?
• Since wood is a poor conductor, the metal feels
  colder b/c it is a better conductor heat easily
  moves out of your warmer hand into the cooler
  metal.

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Check Your Understanding

• Which is really colder?
• If the wood and metal are in the same general
  area, they should have the same temperature (room
  temperature) thus neither is colder.

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Check Your Understanding

• Why can fire walkers walk on red-hot coals
  without getting injured?
• The coals are good insulators, and therefore poor
  conductors, of heat. Even though the coals are
  red hot, they give up very little heat in their
  brief contact with a cooler surface (your foot).
  Things would be different if they were walking
  over red-hot metal shards.

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Convection

• Convection heat transfer by movement of the
  atoms themselves from place to place
• In convection, heating occurs by currents in a
  fluid.
• Ex heat rises from downstairs to upstairs
  bubbles in boiling water rises to the top and
  rolls back to the bottom of the pot

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• Convection occurs in all fluids, liquids and
  gases alike.
• Convection will NEVER occur in a solid!
• When the fluid is heated, it expands, becomes
  less dense, and rises. Cooler fluid then moves to
  the bottom and the process continues.

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Check Your Understanding

• You can hold your fingers beside a flame without
  harm, but not above the flame. Why?
• Heat travels upward by air convection. Since air
  is a poor conductor, very little heat travels
  sideways.

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Radiation

• Heat from the sun warms the Earths surface after
  passing through the atmosphere.
• As air is a poor conductor, conduction cannot be
  responsible for this warming. Neither does it
  pass via convection, as convection begins only
  after Earth is warmed.
• Both forms need molecules to transfer heat.

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• Radiation heat transfer by electromagnetic
  waves.
• Radiation does NOT require a medium to transfer
  heat
• Ex The sun warms the earth
• Types of EM waves
• Radio waves
• Microwaves
• Infrared waves
• Visible light
• Ultraviolet waves
• X-rays
• Gamma rays

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Absorption of Radiant Energy

• Absorption and reflection are opposite processes.
• A good absorber of radiant energy reflects very
  little radiant energy, including light.
• Because of this, a good absorber appears dark.
• A perfect absorber reflects no radiant energy and
  appears perfectly black. No visible light is
  reflected.
• Ex Your eyes pupil

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Check Your Understanding

• If you paint the inside of a box white and cut a
  hole on one side to see inside, the hole appears
  black. Why?
• Radiant energy that enters an opening has little
  chance of leaving before it is completely
  absorbed. Thats why holes appear black.

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Check Your Understanding

• Why are most coffee mugs painted white on the
  inside?
• B/c white is good reflector of radiant energy
  (light). You want the radiant energy to be
  reflected back into your coffee, keeping it
  hotter for a longer time.

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Emission of Radiant Energy

• Good absorbers are also good emitters
• poor absorbers are poor emitters
• poor absorbers are good reflectors
• Light colored objects in contact with dark
  colored objects eventually reach thermal
  equilibrium. The warmer, darker object must give
  up (emit) radiant energy to the cooler, lighter
  object.

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Check Your Understanding

• Is it more efficient to paint a heating radiator
  black or silver?
• Black. The radiator painted silver would be a
  poor emitter and poor absorber or energy. Black
  would increase the contribution of heat from the
  radiator.

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Thermodynamics

• Thermodynamics the study of heat and its
  transformation into mechanical energy.
• The foundation of thermodynamics is the law of
  conservation of energy and the fact that heat
  flows from a warmer object to a cooler object

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1st Law of Thermodynamics

• The First Law of Thermodynamics whenever heat
  is added to a system, it transforms to an equal
  amount of some other form of energy.
• Conservation of energy energy cannot be created
  nor destroyed, just change from one form to
  another
• Ex Food changes from chemical energy to body
  heat and/or kinetic energy

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Object/ Process Energy Conversions
Automobile Engine Chemical Kinetic
Heater/Furnace Chemical Heat
Hydroelectric Gravitational Potential Electrical
Solar Optical Electrical
Nuclear Nuclear Heat, Kinetic, Optical
Photosynthesis Optical Chemical
Food Chemical Heat, Kinetic
Battery Chemical Electrical
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• When energy is added to a system (any group of
  atoms, molecules, particles, or objects), this
  energy does one or both of two things
• increase the internal energy of the system if it
  remains in the system
• does external work if it leaves the system
• Equation
• heat increase in external work done
• added internal by the system
• energy

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Check Your Understanding

• If 10 J of energy is added to a system that does
  no external work, by how much will the internal
  energy of that system be raised?
• 10J
• If 10J of energy is added to a system that does
  4J of external work, by how much will the
  internal energy of that system be raised?
• 10J 4J 6J

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Second Law of Thermodynamics

• The second law of thermodynamics heat will
  never flow by itself from a cold object to a hot
  object.
• In order for heat to flow from a cold object to a
  hot object, work must be put into it
  (refrigerators work off of this idea)
• Ex heat flows from your hand to a piece of ice
  (from hot to cold)

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Entropy

• Natural systems tend to proceed toward a state of
  greater disorder.
• Entropy the measure of the amount of disorder.
  
• As disorder increases, entropy increases.
• The second law states that for natural processes,
  in the long run, entropy always increases.
• Ex liquids evaporating, ice melting, a messy
  room
• You cant get more energy out of a system then
  what you put in it.