Thermodynamics 1

1
Thermodynamics 1

• Heat is Internal Energy

2
The Story of Heat

• Heat was once thought to be a fluid called
  caloric
• James Joule proved that heat was a form of energy
  in the 18th century

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Heat is Energy

• Temperature of the water rises if either
• heat is added
• work is done
• Since work is energy, heat must be energy as well

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The First Law of Thermodynamics

• The work done on a system plus the heat added to
  a system must equal the change in total energy of
  the system
• Won Qto ?E
• ? represents the change in something
• Q is a common symbol for heat

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Negative Work?

• Work is positive if it is done on a system
  (energy is added to the system)
• Work is negative if it is done by a system
  (energy is extracted from the system)
• Example
• Pumping a Supersoaker--work is positive
• Squirting a Supersoaker--work is negative

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Example

• When a cylinder is compressed, work is done on
  the cylinder
• No heat added (Q 0)
• W Fd ?E
• The change in energy is positive and results in
  an increased temperature (T2 gt T1)

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Heat and Temperature

• Temperature is a measure of the average energy of
  the individual particles of a system
• Temperature does not indicate the quantity of
  heat in a system
• However, adding energy to a system does increase
  the temperature

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Calories and Btus

• Definition A calorie is the amount of heat
  required to raise 1 gram of water 1 Celsius
• Definition A Btu (British thermal unit) is the
  amount of heat required to raise 1 pound of water
  by 1 Fahrenheit
• Joule showed that 1 calorie 4.18 Joules

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Some Common Temperatures
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The Kelvin Temperature Scale

• Scientists prefer Kelvin to degrees Celsius in
  measuring temperature
• degrees Celsius Kelvin - 273
• Example 25 C 298 K (Kelvin)
• Kelvins are useful because no object in nature
  can ever have a temperature lower than 0 K
  (absolute zero)
• 0 K -273 C

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Specific Heat

• The specific heat of a substance is the number of
  Joules necessary to raise the temperature of one
  gram by 1 Celsius
• The specific heat of water is 4180 J/kg/C
• The relation between temperature and heat is Q
  mc?T
• m is the mass in kilograms
• c is the specific heat

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Specific Heats of Common Substances
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Example

• Energy used to take a bath How much energy is
  required to heat 200 kg of water from 20C to
  50C?
• Answer Q (200)(4,180)(50-20)
• ? 25,000,000 J or 7 kwh

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Latent Heat

• Sometimes, adding heat to a system does not
  result in an increase in temperature
• This occurs during a change of phase
• There are three phases of matter
• solid
• liquid
• gas

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Latent Heat of Vaporization

• At 100 C, adding heat to water causes a phase
  change (to steam) rather than a rise in
  temperature
• Water requires 2260 kJ of heat per kilogram to
  change from water to steam

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Latent Heat of Fusion

• At 0 C, adding heat to ice causes a phase change
  (to water) rather than a rise in temperature
• Water requires 335 kJ of heat per kilogram to
  change from ice to water

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Liberating Energy as Heat

• Many systems liberate chemical energy as heat
  before converting it to work
• Examples gasoline engines, fossil fuel power
  plants, wood-burning stoves
• The amount of heat that can be liberated by
  burning is the energy density (food calories are
  measured this way)
• Fuel cells attempt to eliminate burning by
  converting chemical energy directly into
  electricity

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Energy Density of Common Fuels