Energy in Thermal Processes

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Chapter 11

• Energy in Thermal Processes

Quick quizzes 1,3,5c Conceptual questions
3,9,11 Problems 22,59
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Internal Energy and Heat

• Internal Energy, U, is the energy associated with
  the microscopic components of the system
• It includes kinetic and potential energy of
  translational, rotational and vibrational motion
  of the atoms or molecules
• Heat is a mechanism by which energy is
  transferred between a system and its environment
  because of a temperature difference between them
• Q is used to represent the amount of energy
  transferred by heat between a system and its
  environment

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Units of Heat

• Calorie
• A historical unit
• A calorie is the amount of energy necessary to
  raise the temperature of 1 g of water from 14.5
  C to 15.5 C .
• A Calorie (food calorie) is 1000 cal
• 1 cal 4.186 J
• This is called the Mechanical Equivalent of Heat

• BTU stands for British Thermal Unit
• A BTU is the amount of energy necessary to raise
  the temperature of 1 lb of water from 63 F to
  64 F

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Specific Heat

• Every substance requires a unique amount of
  energy per unit mass to change the temperature of
  that substance by 1 C
• The specific heat, c, of a substance is a measure
  of this amount
• Units
• J / kg C
• cal / g C (historical units)

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Table,specific heat
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Heat and Specific Heat

• Q m c ?T
• ?T T(final) - T(initial)
• When the temperature increases, ?T and ?Q are
  positive and energy flows into the system
• When the temperature decreases, ?T and ?Q are
  negative and energy flows out of the system

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Quick Quiz 11.1

• Imagine you have 1 kg each of iron, glass, and
  water, and that all three samples are at 10C.
• (a) Rank the samples from lowest to highest
  temperature after 100J of energy is added to each
  by heat.
• (b) Rank them from least to greatest amount of
  energy transferred by heat if enough energy is
  transferred so that each increases in temperature
  by 20C.

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Consequences of Different Specific Heats

• Water has a high specific heat compared to land
• On a hot day, the air above the land warms faster
• The warmer air flows upward and cooler air moves
  toward the beach

9
Phase Changes

• A phase change occurs when the physical
  characteristics of the substance change from one
  form to another
• Common phases changes are
• Solid to liquid melting
• Liquid to gas boiling
• Phases changes involve a change in the internal
  energy, but no change in temperature

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Latent Heat

• During a phase change, the amount of heat is
  given as
• Q m L
• L is the latent heat of the substance
• Latent means hidden or concealed
• Latent heat of fusion is used for melting or
  freezing
• Latent heat of vaporization is used for boiling
  or condensing

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Table, latent heat
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Graph of Ice to Steam
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Problem Solving Strategies

• Transfers in energy are given as Qmc?T for
  processes with no phase changes
• Use Q m Lf or Q m Lv if there is a phase
  change
• In Qcold - Qhot be careful of sign
• ?T is Tf - Ti

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Problem 11-22

• How much energy is required to change a 40-g ice
  cube from ice at 10oC to steam at 110oC?
• Solution is outlined in section 11.4, pages 338
  and 339

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Methods of Heat Transfer

• Conduction
• Less energetic particles gain energy during
  collisions with more energetic particles
• Conduction occurs when there is a difference in
  temperature between two parts of the medium
• Convection
• Radiation

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Conduction example

• The molecules vibrate about their equilibrium
  positions
• Particles near the flame vibrate with larger
  amplitudes
• These collide with adjacent molecules and
  transfer some energy
• Eventually, the energy travels entirely through
  the rod

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Conduction, equation

• The slab allows energy to transfer from the
  region of higher temperature to the region of
  lower temperature

In the figure Dx is L
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Conduction

• A is the cross-sectional area
• L ?x is the thickness of the slab or the length
  of a rod
• P is in Watts when Q is in Joules and t is in
  seconds
• k is the thermal conductivity of the material

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Problem 11-59

• A bar of gold is in thermal contact with a bar
  of silver of the same length and area (Fig.
  P11.59). One end of the compound bar is
  maintained at 80.0C, and the opposite end is at
  30.0C. When the energy flow reaches steady
  state, find the temperature at the junction.

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Home Insulation

• Substances are rated by their R values
• R L / k
• More multiple layers, the total R value is the
  sum of the R values of each layer
• Wind increases the energy loss by conduction in a
  home

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TableR values
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Quick Quiz 11.3

• Will an ice cube wrapped in a wool blanket
  remain frozen for
• (a) less time,
• (b) the same length of time, or
• (c) a longer time than an identical ice cube
  exposed to air at room temperature?

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Convection

• Energy transferred by the movement of a substance
• When the movement results from differences in
  density, it is called natural conduction
• When the movement is forced by a fan or a pump,
  it is called forced convection

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Convection example

• Air directly above the flame is warmed and
  expands
• The density of the air decreases, and it rises
• The mass of air warms the hand as it moves by

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Radiation

• All objects radiate energy continuously in the
  form of electromagnetic waves due to thermal
  vibrations of the molecules
• P sAeT4 (Stefans Law)
• P is the rate of energy transfer, in Watts
• s 5.6696 x 10-8 W/m2 K4
• A is the surface area of the object
• e is a constant called the emissivity
• e varies from 0 to 1
• T is the temperature in Kelvins

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Radiation example

• The electromagnetic waves carry the energy from
  the fire to the hands
• No physical contact is necessary

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Ideal Absorber and Reflector

• An ideal absorber is defined as an object that
  absorbs all of the energy incident on it
• e 1
• This type of object is called a black body
• An ideal absorber is also an ideal radiator of
  energy
• An ideal reflector absorbs none of the energy
  incident on it
• e 0

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Applications of Radiation

• Clothing
• Black fabric acts as a good absorber
• White fabric is a better reflector
• Thermography
• The amount of energy radiated by an object can be
  measured with a thermograph
• Body temperature
• Radiation thermometer measures the intensity of
  the infrared radiation from the eardrum

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Quick Quiz 11.5c

• Star A has twice the radius and twice the
  absolute temperature of star B. What is the
  ratio of the power output of star A to that of
  star B due to electromagnetic radiation? The
  emissivity of both stars can assumed to be 1.
• (a) 4 (b) 8 (c) 16 (d) 32 (e) 64

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Conceptual questions

• Dogs pant when they are hot. How does it help
  them to cool off?
• 3. In usually warm climates that experience
  occasional hard freeze, fruit growers spray the
  fruit trees with water, hoping that a layer of
  ice will form on the fruit. Why is this
  advantageous?

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Conceptual questions

• 9. A tile floor may feel cold to your bare feet,
  but a carpeted floor at the same temperature
  feels warm. Explain.
• 11. Concrete has a higher specific heat than does
  soil. Explain why a city has a higher
  temperature than a countryside. Would you expect
  evening breeze to blow from city to countryor
  from country to city?

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Thermos

• Minimizes energy transfer to surroundings
• Space between walls is evacuated to minimize
  conduction and convection
• Silvered surface minimizes radiation
• Neck size is reduced

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Global Warming

• Greenhouse example
• Visible light is absorbed and re-emitted as
  infrared radiation
• Convection currents are inhibited by the glass
• Earths atmosphere is also a good transmitter of
  visible light and a good absorber of infrared
  radiation