First Law of Thermodynamics

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Chapter 10

• Section 2

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First Law of Thermodynamics

• As we mentioned before if we take internal energy
  into account then energy is conserved.
• This idea of energy conservation of internal
  energy, mechanical energy, heat, and work is
  known as the first law of thermodynamics.
• We can express this idea with the following
  equation
• ?U Q - W

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Signs Of Q W

• The signs of Q and W mean different things for a
  system.
• If Q gt 0 then energy is added as heat.
• If Q lt 0 then energy is removed as heat.
• If Q 0 then no energy is transferred as heat.
• If W gt 0 then work is done by the system (gas
  expansion)
• If W lt 0 then work is done on the system (gas
  compression.
• W 0 then no work is done.

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First Law of Thermodynamics and the Processes

• If we apply the first law of thermodynamics to
  the processes we studied last section we can see
  exactly what happens.
• The first law of thermodynamics helps us to
  analyze each one of these processes and to see
  how they differ.

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Isovolumetric Process

• No work is done so ?U Q.
• This gives us two possible situations.
• Energy is added to the system as heat which
  increases the systems internal energy.
• Energy is removed from the system as heat which
  decreases the systems internal energy.

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Isothermal Process

• Since there is no change in temperature Q W.
• This also gives us two possible results.
• Energy is added to the system as heat is removed
  from the system as work done BY the system.
• Energy is removed from the system as heat and
  added to the system as work done ON the system.

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Adiabatic Process

• Since there is no energy transfer due to heat
  then ?U -W.
• This again creates two possibilities.
• Work done on the system increases the systems
  internal energy.
• Work done by the system decreases the systems
  internal energy.

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Isolated System

• An isolated system is one where there is no
  interaction with the environment so no heat or
  work is transferred.
• Since there is no work or heat transferred in
  either direction then the internal energy cannot
  change.
• This causes the initial internal energy to equal
  the final internal energy.

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Example Problem 3

• A gas is trapped in a small metal cylinder with a
  movable piston and is submerged in a large amount
  of ice water so that the initial temperature of
  the gas is 0 C. A total of 1200 J of work is
  done by a force that slowly pushes the piston
  inward.
• A. Is this process isothermal, adiabatic, or
  isovolumetric.
• B. How much energy is transferred as heat
  between the gas and the ice water?
• a. Isothermal, b. Q 1200 J

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Example Problem 4 (Book Problem)

• A total of 135 J of work is done on a gaseous
  refrigerant as it undergoes compression. If the
  internal energy of the gas increases by 114 J
  during the process, what is the total amount of
  energy transferred as heat? Has energy been
  added to or removed from the refrigerant as heat?
• Q -21 J, heat is transferred from the
  refrigerant

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Cyclic Processes

• Some processes have final conditions that are the
  same as the initial conditions which means there
  is no change in internal energy.
• These processes are known as cyclic processes.
• The amount of energy transferred by heat must
  equal the work.

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Heat Engines

• Engines that do work by converting heat to work
  are known as heat engines.
• Heat engines utilize cyclic processes.
• A combustion engine in your car is a heat engine.
• However, this is not an ideal cyclic process
  since it is not 100 efficient.

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Problems to Work

• Pg 346 Practice B
• Pg 349 Section Review