Fluids and Thermal Physics

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Fluids and Thermal Physics
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Fluids
OK, cool, I was kind of sick of mechanics

• Fluids (statics and dynamics)
• What is a fluid?
• You probably think of a fluid as a liquid, but a
  fluid is simply anything that can flow. This
  includes liquids, but gases are fluids too. We
  live on the only planet in the solar system
  covered mostly by a liquid.If we were a little
  closer to the sun, the oceans would turn to vapor
  (evaporate or boil), a little farther away and
  the oceans would be a solid (ice)..Its a good
  thing we are where we are!
• Mass density
• When we talk about density it's usually mass
  density we're referring to. The mass density of
  an object is simply its mass divided by its
  volume. The symbol for density is the Greek
  letter rho
• Density depends on a few basic things. On a
  microscopic level, the density of an object
  depends on the weight of the individual atoms and
  molecules making up the object, and how much
  space there is between them. On a large-scale
  level, density depends on whether the object is
  solid, hollow, or something in between.
• In general, liquids and solids have similar
  densities, which are of the order of 1000 kg /
  m3. Water at 4 C has a density of exactly this
  value very dense materials like lead and gold
  have densities which are 10 - 20 times larger.
  Gases, on the other hand, have densities around 1
  kg / m3, or about 1/1000 as much as water. Look
  at Table 11.1 on page 301.
• Densities are often given in terms of specific
  gravity. The specific gravity of an object or a
  material is the ratio of its density to the
  density of water at 4 C (this temperature is
  used because this is the temperature at which
  water is most dense). Gold has a specific gravity
  of 19.3, aluminum 2.7, and mercury 13.6. Note
  that these values are at standard temperature and
  pressure objects will change size, and therefore
  density, in response to a change in temperature
  or pressure.
• Example 1 page 301

I am surrounded by fluids!
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Fluids

• Fluids (statics and dynamics)
• Pressure
• Density depends on pressure, but what exactly is
  pressure? Pressure is simply the force
  experienced by an object divided by the area of
  the surface on which the force acts. Note that
  the force here is the force acting perpendicular
  to the surface.
• The unit for pressure is the pascal, Pa. Pressure
  is often measured in other units (atmospheres,
  pounds per square inch, millibars, etc.), but the
  pascal is the unit that goes with the MKS
  (meter-kilogram-second) system.
• When we talk about atmospheric pressure, we're
  talking about the pressure exerted by the weight
  of the air above us. The air goes up a long way
  (about 11km up contains about ¾ of the atmosphere
  mostly nitrogen and oxygen), so even though it
  has a low density it still exerts a lot of
  pressure
• On every square meter at the Earth's surface the
  atmosphere exerts about 1.0 x 105 N of force.
  This is very large, but it is not usually noticed
  because there is generally air both inside and
  outside of things, so the forces applied by the
  atmosphere on each side of an object balance.
• It is when there are differences in pressure on
  two sides that atmospheric pressure becomes
  important. A good example is when you drink using
  a straw you reduce the pressure at the top of
  the straw, and the atmosphere pushes the liquid
  up the straw and into your mouth.
• Example 2 page 302

Im under pressure! Tremendous pressure!
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Fluids

• Fluids (statics and dynamics)
• Pressure and depth in a static field
• The pressure at any point in a static fluid
  depends only on the pressure at the top of the
  fluid and the depth of the point in the fluid. If
  point 2 lies a vertical distance h below point 1,
  there is a higher pressure at point 2 the
  pressure at the two points is related by the
  equation
• Note that point 2 does not have to be directly
  below point 1 it is simply a vertical distance
  below point 1. This means that every point at a
  particular depth in a static fluid is at the same
  pressure.
• Example 4 page 305

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Fluids

• Pascals Principle
• Pascal's principle
• Pressure applied to an enclosed fluid is
  transmitted undiminished to every part of the
  fluid, as well as to the walls of the container.
• Pascal's principle can be used to explain how
  hydraulic systems work. A common example of such
  a system is the lift used to raise a car off the
  ground so it can be repaired at a garage. In a
  hydraulic lift, a small force applied to a
  small-area piston is transformed to a large force
  at a large-area piston. If a car sits on top of
  the large piston, it can be lifted by applying a
  relatively small force, the ratio of the forces
  being equal to the ratio of the areas of the
  pistons.
• Even though the force can be much less, the work
  done is the same. Work is force times the
  distance, so if the force on the large piston is
  10 times larger than the force on the smaller
  piston, the distance it travels is 10 times
  smaller.
• Example 7 page 309

A2
A1
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Fluids

• Archimedes Principle
• Eureka! (or why a ship floats.)
• According to legend, this is what Archimedes'
  cried when he discovered an important fact about
  buoyancy, so important that we call it
  Archimedes' principle (and so important that
  Archimedes allegedly jumped from his bath and ran
  naked through the streets after figuring it out).
  
• Archimedes principle An object that is partly
  or completely submerged in a fluid will
  experience a buoyant force equal to the weight of
  the fluid the object displaces.
• The buoyant force applied by the fluid on the
  object is directed up. The force comes from the
  difference in pressure exerted on the top and
  bottom of an object. For a floating object, the
  top surface is at atmospheric pressure, while the
  bottom surface is at a higher pressure because it
  is in contact with the fluid at a particular
  depth in the fluid, and pressure increases with
  depth. For a completely-submerged object, the top
  surface is no longer at atmospheric pressure, but
  the bottom surface is still at a higher pressure
  because it's deeper in the fluid. In both cases,
  the difference in pressure results in a net
  upward force (the buoyant force) on the object.
• CJ6 Goodyear Blimp example

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Fluids

• Fluids in Motion
• Fluid dynamics is the study of how fluids behave
  when they're in motion. This can get very
  complicated, so we'll focus on one simple case,
  but we should briefly mention the different
  categories of fluid flow.
• Fluids can flow steadily, or be turbulent. In
  steady flow, the fluid passing a given point
  maintains a steady velocity. For turbulent flow,
  the speed and or the direction of the flow
  varies. In steady flow, the motion can be
  represented with streamlines showing the
  direction the water flows in different areas. The
  density of the streamlines increases as the
  velocity increases.
• Fluids can be compressible or incompressible.
  This is the big difference between liquids and
  gases, because liquids are generally
  incompressible, meaning that they don't change
  volume much in response to a pressure change
  gases are compressible, and will change volume in
  response to a change in pressure.
• Fluid can be viscous (pours slowly) or
  non-viscous (pours easily).
• Fluid flow can be rotational or irrotational.
  Irrotational means it travels in straight lines
  rotational means it swirls.
• we'll focus on irrotational, incompressible,
  steady streamline non-viscous flow.

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Fluids

• Fluids in Motion
• The equation of continuity
• The equation of continuity states that for an
  incompressible fluid flowing in a tube of varying
  cross-section, the mass flow rate is the same
  everywhere in the tube. The mass flow rate is
  simply the rate at which mass flows past a given
  point, so it's the total mass flowing past
  divided by the time interval. The equation of
  continuity is

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Fluids

• Fluids in Motion
• Bernoullis Equation
• The pressure, speed, and height (y) at two points
  in a steady-flowing, non-viscous, incompressible
  fluid are related by the equation
• Some of these terms probably look familiar
• the second term on each side looks like kinetic
  energy
• the third term looks like gravitational potential
  energy
• If the equation was multiplied through by the
  volume, the density could be replaced by mass,
  and the pressure could be replaced by force x
  distance, which is work. Looked at in that way,
  the equation makes sense the difference in
  pressure does work, which can be used to change
  the kinetic energy and/or the potential energy of
  the fluid.

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Fluids

• Fluids in Motion
• Bernoullis Equation
• There are two ways to make fluid flow through a
  pipe.
• tilt the pipe so the flow is downhill, in which
  case gravitational kinetic energy is transformed
  to kinetic energy.
• make the pressure at one end of the pipe larger
  than the pressure at the other end. A pressure
  difference is like a net force, producing
  acceleration of the fluid.
• As long as the fluid flow is steady, and the
  fluid is non-viscous and incompressible, the flow
  can be looked at from an energy perspective. This
  is what Bernoulli's equation does, relating the
  pressure, velocity, and height of a fluid at one
  point to the same parameters at a second point.
  The equation is very useful, and can be used to
  explain such things as how airplanes fly, and how
  baseballs curve.

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Fluids

• Fluids in Motion
• Curveball (why a curveball curves)
• The figure below shows a baseball, as viewed from
  the top, moving to the right with no spin. Since
  the air flows with the same speed above and below
  the ball, the pressure is the same above and
  below the ball. There is no net force to cause
  the ball to curve in any particular direction
  (except for gravity which results in the usual
  parabolic trajectory).
• If the ball is given a spin that is
  counterclockwise when viewed from the top, as
  shown below, the air close to the surface of the
  ball is dragged with the ball. In accord with
  Bernoulli's equation, the air on the right side
  of the ball is "speeded up" (lower pressure),
  while that on the left side of the ball is slowed
  down (higher pressure).
• Because of the pressure difference, a deflection
  force is generated that is directed from the
  higher pressure side of the ball to the lower
  pressure side of the ball. Therefore, the ball
  curves to the left on its way to the plate.

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Fluids

• Fluids in Motion
• Airfoils/wings (why planes fly, and why racing
  cars dont)

Thanks to the Bernoulli effect, air will lift my
plane up once I get moving!
The wings on this racing car are designed to push
down, rather than lift up
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Fluids

• Fluids in Motion
• How would an engineer or scientist simulate and
  design better wings, more aerodynamic cars (i.e.
  less air resistance), etc? Fluid dynamics is used
  in industries including aerospace, automotive,
  chemical processing, power generation, heating,
  ventilation, air conditioning, biomedical, oil
  and gas, marine and many others
• Computational Fluid Dynamics (CFD)
• Fluid dynamics equations that are known are
  programmed into computers.
• The computers provide solutions to the problem of
  external airflow over vehicle shapes. The body of
  the configuration and the space surrounding it
  are represented by clusters of points, lines and
  surfaces fluid dynamics equations are solved at
  these points. CFD is divided into three steps.
  Grid generation, numerical simulation and
  post-process analysis.

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Thermal Physics
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Thermal Physics

• Temperature Scales
• We'll shift gears in the course now, moving from
  the physics of fluids to thermal physics.
• Temperature scales
• In the USA, the Fahrenheit temperature scale is
  used. Most of the rest of the world uses Celsius,
  and in science it is often most convenient to use
  the Kelvin scale.
• The Celsius scale is based on the temperatures at
  which water freezes and boils. 0C is the
  freezing point of water, and 100 C is the
  boiling point. Room temperature is about 20 C, a
  hot summer day might be 40 C, and a cold winter
  day would be around -20 C.
• To convert between Fahrenheit and Celsius, use
  these equations
• The two scales agree when the temperature is
  -40. A change by 1.0 C is a change by 1.8 F.
• The Kelvin scale has the same increments as the
  Celsius scale (100 degrees between the freezing
  and boiling points of water), but the zero is in
  a different place. The two scales are simply
  offset by 273.15 degrees. The zero of the Kelvin
  scale is absolute zero, which is the lowest
  possible temperature that a substance can be
  cooled to. Several physics formulas involving
  temperature only make sense when an absolute
  temperature (a temperature measured in Kelvin) is
  used, so the fact that the Kelvin scale is an
  absolute scale makes it very convenient to apply
  to scientific work.
• Measuring temperature
• A device used to measure temperature is called a
  thermometer, and all thermometers exploit the
  fact that properties of a material depend on
  temperature. The pressure in a sealed bulb
  depends on temperature the volume occupied by a
  liquid depends on temperature the voltage
  generated across a junction of two different
  metals depends on temperature, and all these
  effects can be used in thermometers.

Id say this is HOT stuff! Or is it cold stuff?
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Thermal Physics

• Temperature Scales

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Thermal Physics

• Thermal Expansion
• Linear thermal expansion
• The length of an object is one of the more
  obvious things that depends on temperature. When
  something is heated or cooled, its length changes
  by an amount proportional to the original length
  and the change in temperature
• The coefficient of linear expansion depends only
  on the material an object is made from.
• If an object is heated or cooled and it is not
  free to expand or contract (it's tied down at
  both ends, in other words), the thermal stresses
  can be large enough to damage the object, or to
  damage whatever the object is constrained by.
  This is why bridges have expansion joints in
  them. Even sidewalks are built accounting for
  thermal expansion.
• Holes expand and contract the same way as the
  material around them.
• Bimetallic strip (electrical switches)

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Thermal Physics

• Thermal Expansion
• Linear thermal expansion
• Bimetallic strip (electrical switches)

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Thermal Physics

• Temperature, internal energy, and heat
• Temperature
• The temperature of an object is a measure of the
  energy per molecule of an object. To raise the
  temperature, energy must be added to lower the
  temperature, energy has to be removed. This
  thermal energy is internal, in the sense that it
  is associated with the motion of the atoms and
  molecules making up the object.
• When objects of different temperatures are
  brought together, the temperatures will tend to
  equalize. Energy is transferred from hotter
  objects to cooler objects this transferred
  energy is known as heat.
• Specific heat capacity
• When objects of different temperature are brought
  together, and heat is transferred from the
  higher-temperature objects to the
  lower-temperature objects, the total internal
  energy is conserved. Applying conservation of
  energy means that the total heat transferred from
  the hotter objects must equal the total heat
  transferred to the cooler objects. If the
  temperature of an object changes, the heat (Q)
  added or removed can be found using the equation
  
• where m is the mass, and c is the specific heat
  capacity, a measure of the heat required to
  change the temperature of a particular mass by a
  particular temperature. The SI unit for specific
  heat is J / (kg C).
• This applies to liquids and solids. Generally,
  the specific heat capacities for solids are a few
  hundred J / (kg C), and for liquids they're a
  few thousand J / (kg C). For gases, the same
  equation applies, but there are two different
  specific heat values. The specific heat capacity
  of a gas depends on whether the pressure or the
  volume of the gas is kept constant there is a
  specific heat capacity for constant pressure, and
  a specific heat capacity for constant volume. We
  will discuss this when we move to Thermodynamics
• Example 10 page 350

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Thermal Physics

• Heat Units
• Q can be in Joules (unit of energy)
• Q can be in calories amount of heat needed to
  raise the temperature of 1 gram of water by 1
  degree Celsius
• Q can be in Kcalories or Calories, this is the
  amount of heat needed to raise the temperature of
  1 kg of water by 1 degree.
• 1 Calorie 4186 Joules
• Calories are what you are used to when discussing
  energy content of food
• BTU or British Thermal Unit is the amount of heat
  required to raise the temperature of 1 pound of
  water by 1 degree
• Example 11 and 13 (CJ6 Chapter 12)

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Thermal Physics

• Heat and Phase Changes Latent Heat
• Changing phase latent heat
• Funny things happen when a substance changes
  phase. Heat can be transferred in or out without
  any change in temperature, because of the energy
  required to change phase. What is happening is
  that the internal energy of the substance is
  changing, because the relationship between
  neighboring atoms and molecules changes. Going
  from solid to liquid, for example, the solid
  phase of the material might have a particular
  crystal structure, and the internal energy
  depends on the structure. In the liquid phase,
  there is no crystal structure, so the internal
  energy is quite different (higher, generally)
  from what it is in the solid phase.
• The change in internal energy associated with a
  change in phase is known as the latent heat. For
  a liquid-solid phase change, it's called the
  latent heat of fusion. For the gas-liquid phase
  change, it's the latent heat of vaporization,
  which is generally larger than the latent heat of
  fusion. Latent heats are relatively large
  compared to the heat required to change the
  temperature of a substance by 1 C.
• Table 12.3 on page 354 has latent heats of Fusion
  and Vaporization for some common substances

Phase Change Solid-liquid-gas
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Thermal Physics

• Heat and Phase Changes Latent Heat
• Changing phase latent heat of water
• Temperature vs. Heat for water

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Thermal Physics

• Heat Transfer
• There are three basic ways in which heat is
  transferred.
• In fluids, heat is often transferred by
  convection, in which the motion of the fluid
  itself carries heat from one place to another.
  The motion is caused by warmer parts of the
  liquid which are less dense, rising to the
  surface due to buoyant forces (remember
  Archimedes Principle???)
• Another way to transfer heat is by conduction,
  which does not involve any motion of a substance,
  but rather is a transfer of energy within a
  substance (or between substances in contact).
• The third way to transfer energy is by radiation,
  which involves absorbing or giving off
  electromagnetic waves.

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Thermal Physics

• Heat Transfer
• Convection
• Heat transfer in fluids generally takes place via
  convection. Convection currents are set up in the
  fluid because the hotter part of the fluid is not
  as dense as the cooler part, so there is an
  upward buoyant force on the hotter fluid, making
  it rise while the cooler, denser, fluid sinks.
  Birds and gliders make use of upward convection
  currents to rise, and we also rely on convection
  to remove ground-level pollution.
• Forced convection, where the fluid does not flow
  of its own accord but is pushed, is often used
  for heating (e.g., forced-air furnaces) or
  cooling (e.g., fans, automobile cooling systems).

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Thermal Physics

• Heat Transfer
• Conduction
• When heat is transferred via conduction, the
  substance itself does not flow rather, heat is
  transferred internally, by vibrations of atoms
  and molecules. Electrons can also carry heat,
  which is the reason metals are generally very
  good conductors of heat. Metals have many free
  electrons, which move around randomly these can
  transfer heat from one part of the metal to
  another.
• The equation governing heat conduction along
  something of length (or thickness) L and
  cross-sectional area A, in a time t is
• k is the thermal conductivity, a constant
  depending only on the material, and having units
  of J / (s m C).
• Copper, a good thermal conductor, which is why
  some pots and pans have copper bases, has a
  thermal conductivity of 390 J / (s m C).
  Styrofoam, on the other hand, a good insulator,
  has a thermal conductivity of 0.01 J / (s m C).

You will not be tested on use of this equation,
but understand the concept
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Thermal Physics

• Heat Transfer
• Radiation
• The third way to transfer heat, in addition to
  convection and conduction, is by radiation, in
  which energy is transferred in the form of
  electromagnetic waves.
• An electromagnetic wave is basically an
  oscillating electric and magnetic field traveling
  through space at the speed of light. You're
  already familiar with many kinds of
  electromagnetic waves, such as radio waves,
  microwaves, the light we see, X-rays, and
  ultraviolet rays. The only difference between the
  different kinds is the frequency and wavelength
  of the wave.
• Note that the radiation we're talking about here,
  in regard to heat transfer, is not the same thing
  as the dangerous radiation associated with
  nuclear bombs, etc. That radiation comes in the
  form of very high energy electromagnetic waves,
  as well as nuclear particles. The radiation
  associated with heat transfer is entirely
  electromagnetic waves, with a relatively low (and
  therefore relatively safe) energy.
• Everything around us takes in energy from
  radiation, and gives it off in the form of
  radiation. When everything is at the same
  temperature, the amount of energy received is
  equal to the amount given off. Because there is
  no net change in energy, no temperature changes
  occur. When things are at different temperatures,
  however, the hotter objects give off more energy
  in the form of radiation than they take in the
  reverse is true for the colder objects.
• Examples of radiant energy
• Heat from the sun warming just about everything
  on planet Earth!
• Heat from a light bulb (when not touching the
  light bulb)
• Heat from hot coils in an electric stove or oven
• Others?

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Thermal Physics

• Thermodynamics
• Systems and Surroundings
• Collection of objects on which attention is being
  focused is a system, everything else is called
  the surroundings
• A system and its surroundings are separated by a
  wall of some kind
• Diathermal wall permits heat to flow through it
• Perfectly insulating walls (no heat flow) are
  called adiabatic wall
• The state of a system is usually given by the
  pressure, volume, temperature, and mass
• The Zeroth Law of Thermodynamics
• Thermal equilibrium is an important concept in
  thermodynamics. When two systems are in thermal
  equilibrium, there is no net heat transfer
  between them. This occurs when the systems are at
  the same temperature. In other words, systems at
  the same temperature will be in thermal
  equilibrium with each other.

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Thermal Physics

• Thermodynamics
• Thermodynamics is the study of systems involving
  energy in the form of heat and work. A good
  example of a thermodynamic system is gas confined
  by a piston in a cylinder. If the gas is heated,
  it will expand, doing work on the piston this is
  one example of how a thermodynamic system can do
  work.
• The first law of thermodynamics relates changes
  in internal energy to heat added to a system and
  the work done by a system. The first law of
  Thermodynamics is simply a conservation of energy
  equation
• The internal energy has the symbol U. Q is
  positive if heat is added to the system, and
  negative if heat is removed W is positive if
  work is done by the system, and negative if work
  is done on the system.
• Example 1 page 419

Uf - Ui
W
Q
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Thermal Physics

• Thermodynamics
• We've discussed heat transfer, and what Q (heat)
  means. What does it mean for the system to do
  work? Work is simply a force multiplied by the
  distance moved in the direction of the force. A
  good example of a thermodynamic system that can
  do work is the gas confined by a piston in a
  cylinder, as shown in the diagram.
• If the gas is heated, it will expand and push the
  piston to the right, doing work on the piston. If
  the piston is pushed to the left, the piston does
  work on the gas and the gas does negative work on
  the piston. This is an example of how work is
  done by a thermodynamic system.
• The pressure-volume graph
• As has been discussed, a gas enclosed by a piston
  in a cylinder can do work on the piston, the work
  being the pressure multiplied by the change in
  volume. If the volume doesn't change, no work is
  done. If the pressure stays constant while the
  volume changes, the work done is easy to
  calculate. On the other hand, if pressure and
  volume are both changing it's somewhat harder to
  calculate the work done.
• As an aid in calculating the work done, it's a
  good idea to draw a pressure-volume graph (with
  pressure on the y axis and volume on the x-axis).
  If a system moves from one point on the graph to
  another and a line is drawn to connect the
  points, the work done is the area underneath this
  line. We'll go through some different
  thermodynamic processes and see how this works.

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Thermal Physics

• Thermodynamics
• Types of thermodynamic processes
• There are a number of different thermodynamic
  processes that can change the pressure and/or the
  volume and/or the temperature of a system. To
  simplify matters, consider what happens when
  something is kept constant. The different
  processes are then categorized as follows
• Isobaric
• the pressure is kept constant. An example of an
  isobaric system is a gas, being slowly heated or
  cooled, confined by a piston in a cylinder. The
  work done by the system in an isobaric process is
  simply the pressure multiplied by the change in
  volume, and the P-V graph looks like

The work done is the area (shaded area)
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Thermal Physics

• Thermodynamics
• Types of thermodynamic processes
• Isochoric (isometric) - the volume is kept
  constant. An example of this system is a gas in a
  box with fixed walls. The work done is zero in an
  isochoric process, and the P-V graph looks like

The work done is ZERO
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Thermal Physics

• Thermodynamics
• Types of thermodynamic processes
• Isothermal - the temperature is kept constant. A
  gas confined by a piston in a cylinder is again
  an example of this, only this time the gas is not
  heated or cooled, but the piston is slowly moved
  so that the gas expands or is compressed. The
  temperature is maintained at a constant value by
  putting the system in contact with a
  constant-temperature reservoir (the thermodynamic
  definition of a reservoir is something large
  enough that it can transfer heat into or out of a
  system without changing temperature).
• If the volume increases while the temperature is
  constant, the pressure must decrease, and if the
  volume decreases the pressure must increase.

The work done is the area (shaded area)
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Thermal Physics

• Thermodynamics
• Types of thermodynamic processes
• Adiabatic - in an adiabatic process, no heat is
  added or removed from the system. The first law
  of thermodynamics is thus reduced to saying that
  the change in the internal energy of a system
  undergoing an adiabatic change is equal to -W.
  Since the internal energy is directly
  proportional to temperature, the work becomes

The work done is the area (shaded area)
35
Thermal Physics

• Thermodynamics
• Thermal Processes
• Isobaric (constant Pressure)
• Isochoric (constant volume)
• Isothermal (constant temperature)
• Adiabatic (no heat flow)

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A gas turbine  has a compressor to draw in and
compress air -a combustor (or burner) to add fuel
to heat the compressed air -and a turbine to
extract power from the hot air flow
37

• Thermodynamics
• Heat Engines-Otto Cycle (e.g. gasoline engine)
• 1st Law of Thermodynamics cannot be denied!
• Intake stroke (modeled as isobaric process 1 to
  2)
• piston moves down, air and fuel are drawn in
  (usually with a fuel injector controlled by the
  engine computer or ECU)
• Compression stroke (modeled as adiabatic process
  2-3)
• Intake valve is closed, and piston moves
  up-compressing the air-fuel mixture
• Combustion (modeled as isochoric process 3-4)
• Air-fuel mixture is ignited (usually a spark
  plug, with proper timing provided by a computer
  ECU which monitors exhaust gases, intake air
  temperature, engine temperature, coolant
  temperature and numerous other engine parameters)
• Power stroke (modeled as adiabatic process 4-5)
• The exploding air-fuel mixture is allowed to
  expand by pushing down on the piston and doing
  work
• Heat Rejection and Exhaust stroke (modeled as
  isochoric 5-6 followed by isobaric 2 to 1)
• Exhaust valve opens and some gas escapes followed
  by a pushing of all gases out of the cylinder by
  the upward motion of the piston
• The cycle then repeats.

38
Thermal Physics

• Thermodynamics Heat Engines

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Thermal Physics

• Thermodynamics
• The 2nd Law of Thermodynamics
• Heat flows from an object of higher temperature
  to an object of lower temperature, the reverse is
  not possible

Hot To Cold
41
Thermal Physics

• Thermodynamics
• Heat Engines
• Any device that uses heat to do work
• Heat is supplied to the engine at a relatively
  high temperature (with respect to the cold
  reservoir) from the hot reservoir
• Part of the input heat is used to perform work
  by the working substance of the engine (for
  instance the gasoline/air mixture in a car
  engine)
• The input heat not converted to work output is
  sent to the cold reservoir, which has a
  temperature colder than the input temperature
  (e.g. the exhaust of a car engine)
• An important measure of a heat engine is its
  efficiency how much of the input energy ends up
  doing useful work? The efficiency is calculated
  as a fraction (although it is often stated as a
  percentage)
• If the input heat is converted entirely to work
• the engine would have an efficiency of 1.00, or
  100

42
Thermal Physics

• Thermodynamics
• Heat Engines
• Conservation of Energy
• A heat engine like any device must obey the
  principle of conservation of energy
• Only some of an engines heat is converted into
  work
• Neglecting any other losses the conservation of
  energy for a heat engine looks like this

Example 6 on page 427
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44

• Thermodynamics
• 1st Law of Thermodynamics cannot be denied!

Physics comics by Irene
45
Thermal Physics

• Thermodynamics
• Heat Engines
• Carnot's principle
• How can an engine achieve its maximum efficiency?
  
• It must operate using reversible processes a
  reversible process is one in which the system and
  the surroundings can be returned to state they
  were in before the process began
• If energy is lost to friction during a process,
  the process is irreversible if energy is lost as
  heat flows from a hot region to a cooler region,
  the process is irreversible.
• The efficiency of an engine using irreversible
  processes can not be greater than the efficiency
  of an engine using reversible processes that is
  working between the same temperatures. This is
  known as Carnot's principle, named after Sadi
  Carnot, a French engineer.
• For any reversible engine (known as a Carnot
  engine) operating between two temperatures, TH
  and TC, the efficiency is given by
• The efficiency is maximized when the cold
  reservoir is as cold as possible, and the hot
  temperature is as hot as possible.
• Example 7, page 429

46

• Thermodynamics
• Heat Energy
• 1st Law of Thermodynamics cannot be denied!

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• Thermodynamics
• Heat Energy in different fuels

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• Thermodynamics
• Heat Energy-Heat Engines-future energy sources
• EROEI estimates (approximate value only)