Thermal Physics

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Thermal Physics

• Thermal Physics
• Concepts (3 hrs)
• Properties of matter (5 hrs)
• 1/05/07

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Thermal Physics

• Temperature and Heat
• Temperature a property that determines the
  direction of thermal energy transfer between two
  bodies in thermal contact.
• Thermal energy flows from hot to cold and
  temperature is a relative measure of how hot an
  object is compared to another.

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Thermal Physics

• Temperature and Heat
• Heat the thermal energy that is absorbed, given
  up, or transferred from one object to another.
• Thermal equilibrium when two objects are at the
  same temperature.

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Thermal Physics

• Temperature and Thermometers
• A temperature scale is simply a property of a
  substance that varies linearly with temperature.
• For instance, we might measure the volume of a
  certain mass of mercury to be 10 ml and define
  that to be 10 Carlson. Then we could warm the
  mercury until it reached a volume of 15 ml and
  define that to be 20 Carlson.

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Thermal Physics

• Temperature and Thermometers
• Using the definition of Carlson on the previous
  page, we can then determine the temperature
  simply by measuring the volume of the same mass
  of mercury.
• More generally When a line is defined by the
  line graph of at least two points of property vs
  temperature, the value of the temperature for any
  measured value of the property is determined by
  the slope/intercept of the line.

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Thermal Physics

• Temperature and Thermometers
• The two most widely accepted temperature scales
  are the Kelvin (K) and Celsius (C) scales,
  (which are related by y mx b where the slope
  in this case 1)
• T(K) T (C) 273

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Thermal Physics

• Temperature and Thermometers
• On the Celsius scale, water freezes at 0 C and
  boils at 100 C. On the Kelvin scale, absolute 0
  is 0 K.
• The Fahrenheit scale is used only in the U.S. as
  the common popular temperature scale, but is not
  used in scientific circles. (Note the slopes of
  the Farenheit conversions on the next slide.)

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Thermal Physics

• Temperature and Thermometers
• Conversions
• K C 273
• C K - 273
• F 9/5(C) 32
• C 5/9(F - 32)

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Thermal Physics

• Measurement of Heat
• calorie(c) the amount of heat necessary to
  raise the temperature of 1 gram of water by 1 C.
• Kilocalorie(C) the amount of heat needed to
  raise the temperature of 1,000 grams of water by
  1 C. (Used with food.)

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Thermal Physics

• Measurement of Heat
• Food calories (C) are determined by burning the
  food and measuring the amount of energy that is
  released.
• Note British Thermal Units (BTUs) are the amount
  of heat to raise one pound of water by 1 F.
  (fergit-about-it!)

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Thermal Physics

• Measurement of Heat
• The quantity of heat (Q) that it takes to raise
  the temperature (?T) of a mass (m) is given by
  the equation
• Q mc?T

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Thermal Physics

• Measurement of Heat
• Q mc?T
• Q quantity of heat (calories)
• m mass (gram)
• c specific heat capacity (1 cal/gC)
• ?T change in temperature (C)

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Thermal Physics

• Measurement of Heat
• specific heat capacity(c) energy required to
  raise the temperature of a unit mass of a
  substance by 1 degree.
• c ? Q J/(kg K)
• (m ?T)

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Thermal Physics

• Measurement of Heat
• c ? Q J/(kg K)
• (m ?T)
• ?Q energy (Joules)
• m mass (kilogram)
• c specific heat capacity (J/kg K or J/kg C)
• ?T change in temperature (K or C)

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Thermal Physics

• Measurement of Heat
• Q mc?T Q is in calories, c is in cal/g C
• c ? Q Q is in Joules, c is in J/kg C
• (m ?T)
• Same equations, different units

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Thermal Physics

• Measurement of Heat
• Water has a high specific heat capacity, which is
  why it is used as a coolant in cars, etc.
• The Gulf Stream provides a practical example by
  warming Western Europe. It carries a huge
  quantity of heat.

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Thermal Physics

• Expansion and Contraction
• Substances expand when heated and contract when
  cooled. Gases the most, then liquids, and
  finally solids.
• This is an EXTREMELY IMPORTANT application of
  physics knowledge.

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Thermal Physics

• Expansion and Contraction
• Bridges, driveways, and sidewalks all have
  expansion joints. You can even see them in
  telephone wires and water lines.
• Coils of metal act as thermostats, and bimetallic
  strips serve as switches.

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Thermal Physics

• Expansion and Contraction
• You even have to be aware of thermal effects when
  filling your tires. On a hot day, tires should
  be filled with the max pressure. On a cold day,
  minimum tire pressure is safer.
• Fill your gas tank on cold mornings to save money
  and get more gas.

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Thermal Physics

• Expansion and Contraction
• Water contracts as it is cooled until it reaches
  4 C. From then on, cooling causes expansion.
• This is because freezing water has a crystalline
  structure.

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Thermal Physics

• Heat and Internal Energy
• Temperature scales give us a macroscopic view of
  heat.
• The microscopic view of heat is the kinetic
  theory of heat. This approach looks at heat in
  terms of the motion of atoms and molecules. (The
  mosh-pit demo.)

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Thermal Physics

• Heat and Internal Energy
• According to the kinetic theory of heat, the
  temperature of a substance is a measure of the
  average kinetic energy of the molecules of that
  substance.
• (The treatment of any difference between atoms
  and molecules is unimportant.)

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Thermal Physics

• Heat and Internal Energy
• If two substances can be said to have the same
  temperature, their molecules have the same
  average kinetic energy.
• Remember
• KE ½ mv2

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Thermal Physics

• Heat and Internal Energy
• Molecules in a substance can have two kinds of
  kinetic energy, translational and rotational.
• Molecules also have potential energy in the form
  of intermolecular forces.
• Together these are internal energy.

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Thermal Physics

• Kinetic Theory
• Molecules are arranged in different ways
  depending on the phase of the substance.
• Phases Solid
• Liquid
• Gas

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Thermal Physics

• Kinetic Theory
• Solids Have a fixed volume and shape.
• Molecules held in place by bonds.
• Molecules vibrate in place.
• Higher temperature More vibration

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Thermal Physics

• Kinetic Theory
• Liquids Have a fixed volume.
• Shape may change.
• Molecules vibrate in place.
• Molecules can move around each other.

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Thermal Physics

• Kinetic Theory
• Gases Have no fixed volume or shape.
• Expands to fill container.
• Molecules are independent.
• Collisions can increase heat.

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Thermal Physics

• Heat and Work
• Macroscopic
• Work Force x Distance
• Microscopic
• Work Heating of a substance

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Thermal Physics

• Thermal Energy Transfer
• Conduction
• Convection
• Radiation

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Thermal Physics

• Thermal Energy Transfer
• Conduction thermal energy is transferred along
  a substance without any bulk movement within the
  substance.
• Insulators substances that are poor conductors.

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Thermal Physics

• Thermal Energy Transfer
• Metals tend to be good conductors.
• Gases and (most) liquids tend to be poor
  conductors.
• Examples coat, thermos, cork, spoon, pot

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Thermal Physics

• Thermal Energy Transfer
• Convection thermal energy moves between two
  points because of a bulk movement of matter.
  This can occur only in a liquid or a gas, NOT a
  solid.

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Thermal Physics

• Thermal Energy Transfer
• When heated, a fluid expands, which reduces its
  density and causes it to rise. This sets up a
  convection current.
• Examples gliders, sea breezes, fireplaces

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Thermal Physics

• Thermal Energy Transfer
• Radiation the transfer of thermal energy by
  electromagnetic waves, mostly infra-red. Energy
  is absorbed and radiated.
• Increasing the temperature of an object increases
  the frequency of the radiation.

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Thermal Physics

• Thermal Energy Transfer
• An object that remains at a constant temperature
  is absorbing and radiating thermal energy at the
  same rate.
• Good radiators are also good absorbers.
• Examples sun, fireplace, heat lamp, oven

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Thermal Physics

• Phases of Matter and Latent Heat
• Remember, a substance may be found in one of
  three states or phases.
• Solid
• Liquid
• Gas

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Thermal Physics

• Phases of Matter and Latent Heat
• When a substance changes phase, the temperature
  of the substance may remain constant, even though
  energy is still being transferred.
• The energy does not increase kinetic energy but
  rather breaks molecular bonds.

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Thermal Physics

• Phases of Matter and Latent Heat
• latent heat - the amount of energy required to
  complete a phase change.
• specific latent heat - the amount of energy per
  unit mass absorbed or released by a phase change.

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Thermal Physics

• Phases of Matter and Latent Heat
• specific latent heat L ? Q J/kg
• m
• ?Q energy (Joules)
• m mass (kilogram)

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Thermal Physics

• Fusion and Vaporization
• fusion a phase change from a solid to a liquid.
• vaporization a phase change from a liquid to a
  gas.

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Thermal Physics

• Evaporation
• evaporation a phase change from a liquid to a
  gas that takes place at the surface of the
  liquid. The temperature of the liquid is below
  the liquids boiling point.
• Depends on temperature of liquid, surface area,
  humidity, pressure and convection.

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Thermal Physics

• Evaporation
• Evaporation is a statistical phenomena. For any
  substance with a certain average kinetic energy,
  some of the molecules will be above average and
  some below. The hotter molecules can escape
  the surface, leaving the cooler ones behind.

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Thermal Physics

• Condensation
• condensation the changing of a gas to a liquid.
• When water vapor molecules collide with a cold
  surface, they give up kinetic energy and cannot
  remain in the gaseous state.

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Thermal Physics

• Condensation
• Water vapor molecules may also collide with a
  liquid surface and give up enough kinetic energy
  to become liquid.
• sublimation the changing of a solid directly to
  a gas.

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Thermal Physics

• Condensation
• saturated the limit to the amount of water
  vapor in the air at a particular temperature.
• relative humidity a ratio of the amount of
  water vapor currently in the air to the amount in
  the air if it were saturated.

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Thermal Physics

• Condensation
• At 100 saturation, the air cannot support more
  water vapor and condensation is occurring.
• The higher the temperature, the more water vapor
  that air can support.

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Thermal Physics

• Condensation
• Clouds are water vapor, as is fog.
• A ground fog happens when warm moist air from the
  ocean or a lake moves over cooler land. The
  small size of the droplets keep a fog from
  falling.

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Thermal Physics

• End of Thermal Physics

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Thermal Physics

• Notes