Chapter 21: Temperature, heat, and expansion

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Chapter 21 Temperature, heat, and expansion
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Do Now 4/17/09

• 1.) What is potential energy? List some objects
  that have potential energy.
• 2.) What is kinetic energy? List some objects
  that have kinetic energy.

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Temperature

• The quantity that tells how hot or cold something
  is temperature.
• Temperature can be measured in degrees Celsius or
  degrees Fahrenheit.
• Nearly all Matter expands when its temperature
  increases and contracts when its temperature
  decreases.

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Temperature and Kinetic Energy

• Molecules in matter have potential and kinetic
  energy.
• They have the potential to contract when cooled
  and expand when heated. They also have kinetic
  energy when the molecules are actually
  contracting or expanding.
• When you touch a hot surface, there is a transfer
  of kinetic energy from the molecules in the hot
  surface to the molecules in your hand. If you
  touch ice, energy from the molecules in your hand
  to the molecules in the ice.

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Heat

• Energy is always transferred from a warmer
  substance to a cooler substance.
• The energy that is transferred is called heat.
  Matter does not contain heat matter contains
  energy, that when transferred from one substance
  to the next turns into heat.
• When energy is transferred from one substance to
  the next, the substances are said to be in
  thermal contact.

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Transfer of Energy

• Heat will flow from the higher temperature
  substance to the lower temperature substance, not
  necessarily from a substance with more molecular
  kinetic energy (kinetic energy of molecules) to a
  substance with less molecular kinetic energy.
• Energy will only be transferred when there is a
  difference in temperature, not a difference in
  kinetic energy.

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Do Now 4/20

• 1.) Does heat flow from a higher temperature
  substance to a lower temperature substance OR
  from a lower temperature substance to a higher
  temperature substance.
• 2.) When two substances are in contact, what type
  of energy is transferred from one substance to
  the other?
• 3.) What is the freezing point for water in
  degrees Celsius? What is the boiling point for
  water in degrees Celsius?
• 4.) When are two substances in thermal contact?

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Thermal Equilibrium

• After two objects are in thermal contact with
  each other, they reach the same temperature and
  eventually no heat flows between them.
• When two substances reach the same temperature
  after being in contact with each other, we say
  that they are in thermal equilibrium.

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A thermometer shows its own temperature

• A thermometer will reach thermal equilibrium with
  the substance that it is in contact with.
• If you put a thermometer in a glass of water, the
  thermometer and the water will reach thermal
  equilibrium.

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Internal Energy

• Molecules in matter have potential and kinetic
  energy.
• The total of all the energies present in matter
  is called internal energy.
• Matter does not contain heat it contains energy,
  in particular internal energy.

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Calorie

• We can determine the amount of heat that is
  transferred from one warm substance, such as a
  hot piece of iron, to a cooler substance, such as
  water, by measuring the temperature change of the
  water that absorbs the heat.
• A certain amount of heat must be necessary to
  produce a temperature change in substances. The
  unit for the amount of heat necessary to produce
  a specific temperature change in a substance is
  known as the calorie.

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Calorie

• The calorie is the amount of heat required to
  raise the temperature of 1 gram of water by 1C.
  
• The calorie is a unit of energy, just like the
  joule.
• 1 calorie 4.184 J
• Therefore, 4.184 joules of energy raises the
  temperature of 1 gram of water by 1C.

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Kilocalorie

• 1 kilocalorie 1000 calories
• The kilocalorie is most commonly referred to as
  the Calorie.
• This is the Calorie that you know from nutrition
  labels.
• The Calories of foods are determined by burning
  the food and measuring the energy that is
  releases as heat. Calories tell you how much
  energy the food gives off when it is burned.

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Slice of cheese pizza 230 Calories
Cheeseburger 400 Calories
Since the cheeseburger contains more Calories,
the cheeseburger gives off more heat when it is
burned.
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Banana 170 Calories
Apple 80 Calories
The BANANA gives off more heat when it is burned.
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Specific Heat Capacity

• Different materials require different amounts of
  heat to raise the temperature of the material by
  a certain number of degrees.
• The specific heat capacity of any substance if
  defined as
• The quantity of heat required to raise the
  temperature of a unit mass of the substance by 1
  degree.

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Specific Heat Capacity

• 1 calorie of energy is required to raise 1 gram
  of water by 1C, but it takes only about 1/8 of a
  calorie of energy to raise 1 gram of iron by 1C.
  
• Therefore, water has a higher specific heat
  capacity because it absorbs more heat per gram
  than iron does for the same change in
  temperature.

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• Which has a higher specific heat capacity
  water or sand?
• Sand heats very quickly in the sun and cools
  very quickly at night. Water does not heat has
  quickly and it takes much longer for warm water
  to cool down. For that reason, water has a
  higher specific heat capacity, because it
  requires much more heat to raise its temperature
  than sand. Water also takes longer to cool.

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Heat Transfer

• In order to find the amount of heat (Q) needed to
  raise the temperature of a substance by a certain
  number of degrees, we need to know the specific
  heat capacity (c) of the substance, the mass (m)
  of the substance that is undergoing the change in
  temperature, and the actual change in temperature
  (?T).
• Q mc?T
• heat transferred (number of calories) mass x
  specific heat capacity x change in temperature

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Q mc?T

• 1.) Suppose we want to know the number of
  calories needed to raise the temperature of 1
  liter of water by 15C. The specific heat
  capacity for water, c, is 1 cal/gC and the mass
  of 1 liter of water is 1 kilogram or 1000 grams.
• 2.) Calculate the number of calories of heat
  needed to change 500 grams of water by 50 Celsius
  degrees.

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Periods 2, 3, 7

• 1.) Calculate the number of calories of heat
  needed to change 2 kg of water by 24 Celsius
  degrees.
• 2.) What mass of water will give up 300 calories
  when its temperature drops from 100C to 85C?
• 3.) A 45 gram piece of iron is heated to 100C
  and then dropped into cool water where the irons
  temperature drops to 40C. How many calories
  does it lose to the water if the specific heat
  capacity of iron is 0.11 cal/gC?

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Period 4

• 1.) Calculate the number of calories of heat
  needed to change 200 grams of water by 24 Celsius
  degrees.
• 2.) How many calories are needed to raise the
  temperature of 550 grams of water by 20C?
• 3.) What mass of water will give up 300 calories
  when its temperature drops from 100C to 85C?

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Do Now 4/21

• 1.) Does a thermometer show its own temperature?
• 2.) What is thermal equilibrium?
• 3.) If you drop a hot rock into a pail of water,
  the temperature of the rock and the water will
  change until both are equal. The rock will cool
  and the water will warm. Does the same principle
  hold true if the rock is dropped into a large
  lake? Explain.
• 4.) What is specific heat capacity?

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Water has a HIGH specific heat capacity

• Water has a much higher capacity for storing
  energy than most materials.
• A relatively small amount of water absorbs a
  great deal of heat. Water also takes longer to
  cool.
• Which would be better a 10 kilogram iron brick
  or a 10 kilogram jug of hot water at the same
  temperature?
• The jug of water because it has a higher specific
  heat capacity and takes longer to cool than the
  iron. It will stay warmer for much longer.

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Thermal Expansion

• When the temperature of a substance increases,
  the molecules of the substance tend to move
  farther apart.
• With a few exceptions, almost all forms of matter
  solids, liquids, and gases expand when they
  are heated and contract when they are cooled.
• Gases generally expand or contract more than
  liquids, and liquids expand or contract more than
  solids.

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Thermal Expansion

• When concrete heats up, it expands.
• Tar separations allow the concrete to expand on
  warm days. Without those separations, the
  concrete would crack as a result of expansions
  and contractions during different seasons.
• Why is it advisable to allow telephone lines to
  sag when stringing them between poles in the
  summer?
• Telephone lines are longer in the summer, when
  they are warmer, and shorter in the winter
  (contract) when they are cooler. They sag more
  on hot summer days than in winter. If the
  telephone lines are not strung with enough sag in
  the summer, they might contract too much and snap
  during the winter.

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Bimetallic Strip

• A bimetallic strip is a strip of two different
  metals, i.e. one of brass and one of iron, that
  are welded together. When the strip is heated,
  the metals expand in different ways.
• If the brass expands much more than the iron, the
  brass part of the strip will become longer than
  the iron strip, causing it to bend towards the
  iron side. Since the brass expands more than the
  iron when heated, it will also contract more than
  the iron when cooled.
• As the bimetallic strip is cooled, the brass
  contracts more than the iron, causing the strip
  to become shorter than the iron strip and bend
  towards the brass side.

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On loose leaf

• How does a thermostat use a bimetallic strip to
  function?
• When a room becomes too cold, the bimetallic
  strip will curve toward the brass side, it will
  curve upward, and in doing so closes an electric
  switch that turns on the heat. When the room
  becomes too warm, the bimetallic strip will curve
  toward the iron side, downwards, which opens the
  switch and turns off the heat.

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Water is a special case!

• Water contracts when its temperature increases.
• As water is heated and its temperature rises, it
  continues to contract until it reaches a
  temperature of 4C. But once it reaches 4C it
  will start to expand as the temperature
  increases.
• Water is the most dense at 4C.
• Large density small volume therefore, water
  has its smallest volume at 4 degrees Celsius.
  Water has its smallest density and largest volume
  when it turns into ice. Ice is less dense than
  water, which is why ice floats on liquid water.

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Test Review (track 2)

• The temperature at which water freezes and boils
  (in degrees Celsius and Fahrenheit
• Kinetic energy is transferred between molecules
  when temperature of a substance changes.
  Potential energy is the stored energy of the
  molecules (before they actually expand and
  contract)
• Thermal contact and Thermal equilibrium
• The temperature at which water is the most dense
• Which contracts/expands the most when temperature
  decreases/increases solids, liquids, or gases?
• Specific heat capacity
• Thermal expansion how is the breaking of
  bridges and concrete sidewalks prevented?
• Matter does not contain heat. It contains
  internal energy.
• Heat flow
• Bimetallic strips how does a thermostat use a
  bimetallic strip to function?
• Heat transfer (Q mc?T)

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Test Review (track 3)

• The temperature at which water freezes (in
  degrees Celsius and Fahrenheit)
• Kinetic energy is transferred between molecules
  when temperature of a substance changes.
  Potential energy is the stored energy of the
  molecules (before they actually expand and
  contract)
• When temperature increases, a substance expands,
  which means the molecules move farther apart.
  When temperature decreases, molecules move closer
  together.
• Thermal contact and Thermal equilibrium
• The temperature at which water is the most dense
• Which contracts/expands the most when temperature
  decreases/increases solids, liquids, or gases?
• Specific heat capacity (water has a high specific
  heat capacity substances that have low specific
  heat capacities heat up quickly cool down
  quickly)
• Thermal expansion how is the breaking of
  bridges and concrete sidewalks prevented?
• Matter does not contain heat. It contains
  internal energy.
• Heat flows from warmer substance to cooler
  substance.
• Bimetallic strips how does a thermostat use a
  bimetallic strip to function?
• Heat transfer (Q mc?T)