Heat and temperature

1
Heat and temperature

• Heat is energy it can do work.
• Temperature is a man-made, arbitrary scale
  indicating which direction heat is flowingis
  heat going into the system, temperature rising or
  is heat leaving the system, temperature
  declining.
• Heat is measured with an instrument called a
  calorimeter.
• Heat is NOT measured with a thermometer.
• Temperature is measured with a thermometer.
• Heat is measured in Joules.
• Temperature is measured in degrees.

2
There are 3 thermometers which measure
temperature.

• Recall that a thermometer is a man made scale
  simply indicating the direction of heat flow. It
  is also a relative measure of the motion of
  molecules in the system.
• If the molecules are moving fast with lots of KE
  then the temperature is high because a lot of
  heat is in the system.
• If the temperature is low then we may assume
  theres not as much heat in the system and we can
  expect the molecules to be moving slower with
  less KEgenerally less energy in the system.

3
Gabriel Fahrenheit and Anders Celsius

• In the briefest terms, Fahrenheit took a glass
  tube and graduated itmade marks, 1,2,3 etc. He
  used water as the standard. We often do use water
  as our standard for things, it is cheap and
  plentiful and easy to get.
• He froze some water and the mercury in the tube
  dropped to the mark he had made32 degrees. When
  water boiled the mercury in the tube rose to 212
  degrees.
• That is the thermometric scale we use in the USA.

4
Anders Celsius

• When Celsius investigated the subject of
  temperature he did something a little
  differently. He did not mark off the glass tube.
  He simply put it into frozen water and where the
  mercury landed he called that 0 degrees. When
  water was boiling and the mercury rose and
  leveled off he called that 100 degrees.
• This is the one use most around the world.
• Although in science we use a third thermometer
  when dealing with extreme temperatures.

5
Lord Kelvin

• Kelvin was interested in a more philosophical
  approach to the behavior of matter at different
  temperatures. He wondered, for example, what
  would happen if there was a condition of NO HEAT.
  At absolute zero what would the conditions be?
• At zero Kelvin, at absolute zero, all motion
  would cease since temperature is an indirect
  measure of the motion of molecules. So if there
  was no heat there would be no motion and thus no
  temperature. But a condition of absolute zero
  cannot exist since there is no absolute NO
  motion. Everything moves relative to SOME frame
  of reference. So EVERYTHING has SOME temperature.
  
• Atoms vibrate. Electrons revolve around the
  nucleus of every atom. EVERYTHING MOVES RELATIVE
  TO SOMETHING.

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CALORIES

• In our society we count calories. We measure
  the number of calories in a candy bar. We have
  weight watchers which watches calorie intake.
• So what are calories??
• It turns out that a long time ago people thought
  heat was a fluid, made of atoms, like air . They
  thought it had weight and mass and was a Thing.
  They called this substance caloric
• But, it turns out that heat is NOT a thing, not
  made of atoms, not a substance at all. But the
  name stuckheat was caloric.
• So when we talk about calories we are talking
  about energy today and the amount of energy we
  are taking in to our body. We must USE that
  energy we have taken in OR our bodies will
  convert it to the storage form of energyFAT.
• Fat is simply the bodys way of sayingdont want
  to use that energy now? OK. I will save it for
  you for later.
• Sooner or later that must be released into the
  universethat energy must be burn ed off by
  DOING WORKthat is by transferring the energy of
  your body to the world around you.
• And that will heat up the universe and thus speed
  us toward heat death or maximum entropy.

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QUANTIFICATION

• Heat and temperature can be measured and
  quantified through a series of equations.
• There are 3 thermometers, 3 temperature scales.
  We can convert from one thermometer to another
  via these equations.
• F (1.8 x C) 32
• F - 32 / 1.8 C
• K C 273

8
Quantifying heat as the temperature changes and
the heat causes a phase change.

• There are 3 common state of matter, solids
  liquids and gases.
• Whether the substance is in its solid, liquid or
  gaseous state depends on the TEMPERATURE.
• Consider water. As ice the molecules are moving
  very slowly, they have little energy and so
  electric attractive forces between molecules is
  strong enough to hold the molecules in placea
  solid. A solid has a definite shape, holds that
  shape at that temperature and it has a definite
  measurable volume at a a given temperature so
  that if we know the mass we may calculate the
  density of the sample.

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Phase change continued

• So what happens to a solid when the temperature
  goes upthat means heat is going INTO the system.
  Energy is going into the system. The molecules
  start to move faster and try to get farther
  apart. Those forces that held the molecules in
  place as a solid arent quite strong enough to
  hold the atoms and molecules in place and the
  molecules start to separates and the forces are
  weaker, more like slinky springs holding the
  atoms together. So the molecules slide around
  each othera liquid. The molecules flow past
  each other.

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Phase Change continued

• The liquid state has more energy than the solid
  state of a given substance. Energy in the form of
  heat was added to the system. Now what happens
  when even more energy is added?
• We know more energy is being added into the
  system because the temperature is rising.
  Eventually the temperature will be high enough
  that the forces that held the atoms and molecules
  together as a solid and even as a liquid arent
  strong enough to hold the atoms and molecules
  together so that they fly apart and go off
  independently, fly anywhere they like. That is a
  gas (vapor).

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So the equations are???

• Q m c T
• Now what does that mean?
• Suppose we want to know HOW MUCH HEAT is needed
  to change the temperature of a substancethis is
  the equation we use.
• Q heat. The unit of heat will be either the SI
  unit of heat energy called JOULES or it may be
  measured in Calories as well, the older unit of
  heat.
• M Mass. The amount of heat needed to change the
  temperature of a certain amount of a substance
  depends in part on HOW MUCH of the substance you
  have. Obviously you must do much more work to
  change the temperature of a ton of iron than it
  will take to change the temperature of 1 ounce of
  iron.
• c is called the specific heat of a
  substance. It turns out that every substance has
  a characteristic specific heat. We can identify
  an unknown substance IF we know its specific
  heat. So when I wish to find out how much heat is
  needed to change the temperature of a substance
  it helps to know the mass and the identity of the
  substance.
• The third piece of info needed is the amount of
  temperature change.
• So it will take more energy, more work, more heat
  to change the temperature by 50 degrees than to
  change the temperature by only 10 degrees.
• Q mcT If I know the mass and the identity of
  the substance and the amount by which we wish to
  change the temperature of a substance then we can
  calculate the amount of work, energy, heat needed
  to make the change.

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Heat can travel but Temperature does not.

• Temperature can change as heat is added or
  removed. But temperature is a man made scale.
• Heat is pure energy. It does work. It can cause
  masses to move.
• Heat can also be transferred. Recall that work is
  the transfer of energy. So heat can transfer from
  one place to another.
• Heat can travel from one place to another by 3
  ways.
• 1. Heat can travel by CONDUCTION. One object can
  cause cooler objects to heat up. Heat ALWAYS and
  ONLY travels from HOT to COLD. So via
  conduction, a hotter object can transfer the
  energy to a cooler object.
• It happens by contact. One object touches another
  and the energy of the hotter causes the molecules
  of the cooler to begin to move faster, that is
  they have more energy and thus the temperature
  goes up. Consider a metal spoon in a pan of hot
  soup. The handle would eventually get hot even
  though only the bowl of the spoon is in the soup
  so the atoms of hotter collide with atoms of
  cooler and transfer their energy.
• Conduction is the way heat travels through solids.

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Convection

• 2. The second way that Q travels from one place
  to another is via convection. Heat travels
  through fluids this way
• Let us define a fluid
• A fluid is anything that flows. Liquids flow. But
  so do gasesair masses flow in currents, air
  flows.
• When liquids and gases are heated, gain more
  energy then they become less dense. The molecules
  move faster and farther apart. Same mass in
  larger volume. As a result of less density hot
  liquids and gases rise.
• Removing energy from liquids and gases cools
  substances. The atoms and molecules have less
  energy, move slower, cannot escape gravity easily
  or for as long and so they tend to fall together
  and sink.
• Fall together means the molecules and atoms get
  closer to one another. That is the sample becomes
  more dense. The mass is the same but the volume
  is smaller. Because the molecules are more dense
  and because of that it sinks. Colder gases or
  liquids sink.

14
Radiation

• Radiation is the way heat can travel through a
  vacuum, through empty space.
• Heat travels through nothing on something called
  an electromagnetic wave.
• This is how heat reaches us from the sun.
• We will study more about EMR as we study light
  later on down the line.
• Radiation, waves, shouldnt be confused with
  harmful radiation, that is waves of energy
  produced when the nucleus of an atom falls apart
  and radioactivity is released.
• This is just heat moving through space in
  something called an electromagnetic wave.
• Matter can react to EMR in two ways. The heat
  waves can be absorbed. The energy is taken in ,
  the molecules start to move faster and the
  temperature goes up.
• OR.
• Matter can reflect the heat waves. That is the
  object does not absorb the energy but simply
  allows it to bounce back like throwing a ball
  against a wall.

15
Phase Change Diagram

• The phase change diagram is just a visual way to
  describe and calculate all the heat needed to
  change the temperature of a sample through the 3
  phases of matter, solid to liquid to gas.
• You need two equations. QMCT as we have just
  discussed and H m Hv.
• Or
• H m Hf.
• Heat mass times the heat of vaporization and
  mass times the heat of fusion.
• It turns out the Hv, and the Hf are both
  characteristic of a substancewe could use to
  identify an unknown if we knew that value.
• Heat of vaporization, Hv, is the amount of heat
  needed to boil / condense 1 gram of your
  sample.
• Heat of fusion, Hf, is the amount of heat needed
  to freeze/melt 1 gram of your sample.

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Melt-FreezeBoil-Condense

• It turns out that the temperature at which a
  solid melts to a liquid, whether ice or iron,
• Is characteristic of the material. Aluminum melts
  at 660 C
• Tin melts at 232C
• Water(ice) at 0C
• Iron melts at 1535 C
• It turns out that the temperature at which water
  freezes to a solid is also 0C
• Molten iron will freeze at 1535 C
• Liquid Tin will freeze at 232 C
• Etc. etc. etc.
• What do you notice about the temp. at which a
  substance melts or freezes
• It is the same!

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So lets consider water

• Water freezes to ice at 0 C. Ice melts to water
  at 0 C.
• So AT 0 C what do you have?
• Slush.
• Ice is melting and water is freezing and ice is
  melting and water is freezing and on and on so
  long as the temperature remains AT 0 C.

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The same thing is true at the other end

• The temperature at which a liquid turns into a
  vapor (gas) is characteristic of the substance.
• A liquid boils at a certain temperaturewater for
  example boils at 100 C.
• The temperature at which a vapor (gas) turns into
  a liquidcoolsis also characteristic and say
  for example water, that temperature is also 100C.
• So AT that temperature, water would be boiling
  and gas would be condensing back to a liquid.
• And different substances boil and condense at
  their own characteristic temperature we could
  look up in the HANDBOOK OFR PHYSICS AND CHEMISTRY.

19
We will

• We will use worksheets etc to practice solving
  some heat and temperature problems and
  conversions.
• We will put a phase change diagram on the board
  or overhead for you to examine for concepts
  involved in tracing the heat transfer as
  something hearts up or cools off through changes
  of state.
• Lets solve a few now.

20
Some problems.

• 1. The specific heat of water is 4184 J/kg K -
  odd units - but good for this problem.
• How many Joules of energy will be needed to raise
  the temp. of 50 grams of water from 21 C to 37
  C.
• 2. How much heat must be added to MELT 5 grams
  of ice at 0 C to liquid water. Answer in
  calories. Then convert that to Joules. 1 cal is
  4.184 J.