Intro to Thermodynamics

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Intro to Thermodynamics

• 3/12/07
• Temperature and Heat

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Objectives for today

• At the end of this lesson you should be able to
  answer the following questoins
• What is need to change an objects temperature?
• Whats the difference between temperature and
  internal energy?
• When are objects in thermal equilibrium?
• What are three scales used to measure
  temperature?
• What is specific heat?

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Where should we start?

• Thermody-whatsit?
• Thermodynamics is the study of work, heat and
  energy in a system.
• There are three main laws used to study
  thermodynamics (0th, 1st and 2nd)
• Well start with the 0th (zeroth) law.

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Who uses this stuff?

• Geochemists, geophysicists
• Nuclear physicists
• Geologists
• Engine designers (planes, trains and automobiles)
• Home builders (heating and cooling specialists)
• Engineers
• Bioengineers
• Etc

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Temperature

• We feel hot and cold.
• Hot things have high temp, cold things have low
  temp
• Physicists describe temperature by the average
  kinetic energy of molecules.
• TEMP AVERAGE KINETIC ENERGY!

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Kinetic Energy (1/2mv2)

• The kinetic energy of the molecules is called the
  INTERNAL ENERGY (U) of a substance.

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Thermal equilibrium

• What will happen to the ice cubes?
• What will happen to the tea?

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Equilibrium

• When two objects (solid, liquid or gas) come in
  contact, their ave. KE will become equal.
• One object will lose KE the other will gain.
• Objects in thermal equilibrium have the same
  temperature.
• Will the ices KE be high or lower than
  initially?
• What will happen to the teas KE?

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Measuring Temperature

• Thermal expansion
• Scales (Fahrenheit, Celsius, Kelvin)

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HEAT

• Turn the heat on
• The stove has a lot of heat
• The sun gives off heat
• Dont leave the door open, youll let all the
  heat out

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Heat (continued)

• The energy (kinetic) itself is what we will call
  heat.
• Energy is transferred between objects as heat.
• When things are at equilibrium the net transfer
  of heat is zero, but there is still a
  temperature.

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Hot, hot

• The standard units of heat are the same as for
  the other types of energy that weve studied, the
  Joules (J).
• KE kinetic energy
• PE potential
• U internal energy
• Q heat

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Changing Temperature

• Why dont fire walkers burn their feet?

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Specific Heat Capacity

• Objects react differently when heat is added.
• Some metals get warm very quickly. Water takes a
  long time to boil (and a lot of heat).
• Every substance gains a certain amount of
  temperature for a given amount of heat added.
• This is called specific heat capacity.
• Cp Q/(mT)
• - Cp specific heat
• - Q heat transferred
• - m mass of substance
• - T change in temperature

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Determining Cp

• Calorimeters

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Q Q (Duh)

• In calorimeters, the heat absorbed by the water
  is equal to the heat released by the object
  dropped in.
• Qw Qx
• CpwmwTw CpxmxTx

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Example