First and Second Laws of Thermodynamics

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First and Second Laws of Thermodynamics

• Class 12.1

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Class Objectives

• Understand the definitions of
• temperature, pressure, density, amount of
  substance
• states of matter and phase diagrams
• gas laws
• Understand and apply
• work, energy, reversibility, heat capacity
• First and Second Laws of Thermodynamics

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Reversibility

• Reversibility is the ability to run a process
  back and forth (backwards and forwards)
  infinitely without losses.
• Money analogy Currency conversion
• no service fee 100 ? 40, and one hour later at
  the same place, 40 ? 100
• with service fees 100 ? 38, and one hour later
  at the same place, 38 ? 90

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Reversibility and Energy

• If irreversibilities were eliminated, these
  systems would run forever.
• Perpetual machines

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Example Popping a Balloon

• A reversible process can go in either
  direction, but these processes are rare.
• Generally, the irreversibility shows up as waste
  heat

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Sources of Irreversibilities

• Friction (force drops)
• Voltage drops
• Pressure drops
• Temperature drops
• Concentration drops

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Basic Laws of Thermodynamics

• First Law of Thermodynamics
• energy can neither be created nor destroyed
• Second Law of Thermodynamics
• naturally occurring processes are directional

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First Law of Thermodynamics

• One form of work may be converted into another,
• Or, work may be converted to heat,
• Or, heat may be converted to work,
• But, final energy initial energy

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2nd Law of Thermodynamics

• We intuitively know that heat flows from higher
  to lower temperatures and NOT the other
  direction.
• i.e., heat flows downhill just like water
• You cannot raise the temperature in this room by
  adding ice cubes.
• Thus processes that employ heat are inherently
  irreversible.

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Heat/Work Conversions

• Heat transfer is inherently irreversible. This
  places limits on the amount of work that can be
  produced from heat.
• Heat can be converted to work using heat engines
• Jet engines (planes), steam engines (trains),
  internal combustion engines (automobiles)

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Heat into Work

• A heat engine takes in an amount of heat, Qhot,
  and produces work, W, and waste heat Qcold.
• Nicolas Carnot (kar no) derived the limits of
  converting heat into work.

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Carnot Equation Efficiency

• Given the heat engine on the previous slide, the
  maximum work that can be produced is governed by
• where the temperatures are absolute
  temperatures.
• Thus, as Thot ?Tcold, Wmax ? 0.
• This ratio is also called the efficiency, h.

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Pairs Exercise (5 min)

• Use Excel to create a graph showing the trend of
  work per unit heat for a heat engine whose the
  source temperature is increases from 300 K to
  3000 K and the waste heat is rejected to an
  ambient temperature of 300 K.

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Work into Heat

• Although there are limits on the amount of heat
  converted to work, work may be converted to heat
  with 100 efficiency.
• This is shown by Joules Experiment

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Joules Experiment
Joules Mechanical Equivalent of Heat
This proved 1 kcal 4,184 J
DT 1oC
m
F
Dx
1 kg H2O
E FDx 4,184 J
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Where did the energy go?

• By the First Law of Thermodynamics, the energy we
  put into the water (either work or heat) cannot
  be destroyed.
• The heat or work added increased the internal
  energy of the water.
• This is the energy stored in the atoms and
  molecules that make up the water they move faster

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Heat Capacity

• An increase in internal energy causes a rise in
  the temperature of the medium.
• Different mediums require different amounts of
  energy to produce a given temperature change.

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Heat Capacity Defined

• Heat capacity the ratio of heat, Q, needed to
  change the temperature of a mass, m, by an amount
  DT
• Sometimes called specific heat

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Heat Capacity for Constant Volume Processes (Cv)
insulation
DT
Heat, Q added
m
m

• Heat is added to a substance of mass m in a fixed
  volume enclosure, which causes a change in
  internal energy, U. Thus,
• Q U2 - U1 DU m Cv DT
• The v subscript implies constant volume

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Heat Capacity for Constant Pressure Processes (Cp)

• Heat is added to a substance of mass m held at a
  fixed pressure, which causes a change in internal
  energy, U, AND some PV work.

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Cp Defined

• Thus,
• Q DU PDV DH m Cp DT
• The p subscript implies constant pressure
• H, enthalpy. is defined as U PV,
• so DH D(UPV) DU VDP PDV DU PDV
• Experimentally, it is easier to add heat at
  constant pressure than constant volume, thus you
  will typically see tables reporting Cp for
  various materials (Table 22.2 in your text).

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Individual Exercises (5 min.)

• Calculate the change in enthalpy per unit lbm of
  nitrogen gas as its temperature decreases from
  1000 oR to 700 oR.
• Two kg of water (Cv4.2 kJ/kg K) is heated by 200
  BTU of energy. What is the change in temperature
  in K? In oF?

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Assignment 17 (Team Assignment)

• FOUNDATIONS 11.9, 11.11, 11.12
• Due1st class of week 13