First law of thermodynamics

1
First law of thermodynamics

• first law of thermodynamics
• heat added to a system goes into the internal
  energy of the system and/or into doing work
• heat in work change in internal energy Q
  W ?U
  
• is different formulation of energy conservation
• for isolated system
• no heat flow ? if work done, must reduce internal
  energy
• historical note
• 1st law quoted as a law in its own right because
  it took a long time to realize that heat is a
  form of energy. Until around 1800, heat was
  considered a fluid called caloric that is
  contained in materials, can be soaked up by
  materials,..
• It took about 50 years to replace this with the
  new paradigm that heat is a form of energy, and
  that total energy, including thermal energy, is
  conserved.
• Milestones on path to first law
  Experiments and observations by Benjamin
  Thompson, James Prescott Joule, Julius Robert
  Mayer,.. and conjectures by Mayer, Hermann
  Helmholtz, Rudolf Clausius,..
  

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HEAT ENGINES

• heat engine
• is a device that converts heat into work
• principle heat input, some of it used to do
  work, some of it discarded
• operate in cyclical process, i.e. at end of an
  engine cycle, engine must be in same state as
  before
• for a cyclical process ?Unet 0, ? Q W,
  i.e. work done net heat
  input (heat in) - (heat out)
• heat engine operates between two reservoirs
• reservoir system from which heat may be readily
  extracted and into which heat can be deposited at
  given temperature
• heat engine takes heat from high temperature
  reservoir, converts some of it into work, and
  ejects rest of heat into low temperature
  reservoir
• example car engine
• hot reservoir cylinder in which air-fuel
  mixture is exploded
• cold reservoir environment to which waste heat
  is expelled
• thermal efficiency of a heat engine ratio of
  work output to heat input
• ? W/Qin W/ Qh 1 - (Qc /Qh )

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heat engines and refrigerators

• engine
• refrigerator

4
CARNOT ENGINE

• Nicolas Léonard Sadi Carnot (1796 -1832)
  (Réflexions sur la puissance motive de la
  chaleur, 1824)
• constructed idealized method for extracting work
  with greatest possible efficiency from an engine
  with heat-flow from one substance at higher
  temperature to another substance at lower
  temperature, - the Carnot cycle
• Carnot cycle is reversible process - can run in
  either direction
• Carnot engine
• container with piston
• can be brought into thermal equilibrium with two
  heat reservoirs, one at high temperature Th, one
  at low temperature Tc
• or can be isolated from outside world (i.e. no
  heat-flow to or from container)
• isothermal process temperature constant
• adiabatic process isolated ? no heat exchange
• efficiency of Carnot engine ? 1 - (Tc
  /Th)
  (note temperature here is measured in Kelvin)
• Carnot engine is the most efficient engine
  possible (2nd law of thermodynamics).

5
Second law of thermodynamics

• several different formulations of 2nd law
• all can be shown to be equivalent
• law of heat flow Heat (thermal energy) flows
  spontaneously (i.e. without external help) from
  region of higher temperature to region of lower
  temperature. By itself, heat will not flow from
  cold to hot body.
• Kelvin formulation No process is possible whose
  sole result is the removal of heat from a source
  and its complete transformation into work.
• Clausius formulation No process is possible
  whose sole result is the transfer of thermal
  energy from a body at low temperature to a body
  at high temperature.
• heat engine formulation No heat engine can be
  more efficient than the Carnot engine.
• consequence of 2nd law
• the quality of thermal energy (its ability to do
  work) depends on the temperature
• thermal energy at low temperature less useful
  than thermal energy at high temperature
• using energy does not mean destroying it
  (cannot be destroyed) it means converting it
  into work and thermal energy at lower temperature
  than before ? degradation of energy

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ENTROPY

• Entropy
• when heat Q at temperature T enters a system, the
  system's entropy S changes by ?S Q/T
• for the Carnot cycle Qh taken from reservoir at
  temperature Th , Qc given to reservoir at
  temperature Tc ?S Qh/Th - Qc/Tc 0,
  i.e. for the Carnot cycle,
  the change in entropy is
  0.
• for other cyclical processes 2nd law of
  thermodynamics ? efficiency smaller than that of
  Carnot process
• entropy formulation of 2nd law of thermodynamics
• For any process, the total entropy of all the
  participants either increases or stays the same
  it cannot decrease.
• entropy related to the degree of disorder,
  to the probability of a state
• order is less probable than disorder (there are
  many more ways of having disorder than there are
  of having order)
• some systems (e.g. living things and beings)
  decrease their entropy, but at the cost of
  increasing the entropy of the rest of the
  universe.
• the total entropy of the universe keeps
  increasing.