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Heat Transfer

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Heat transfer is one way of transferring energy to a body (work is the other) ... Heat energy is transferred when there is a difference in temperature ... – PowerPoint PPT presentation

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Title: Heat Transfer


1
Heat Transfer
  • Conduction, Convention and Radiation

2
Principles of Heat Transfer
  • Heat transfer is one way of transferring energy
    to a body (work is the other)
  • Occurs only when there is a temperature
    difference between the two bodies (heat flows
    from hot to cold)
  • Occurs through three processes conduction,
    convection, and radiation

3
From Hot to Cold
  • Heat energy is transferred when there is a
    difference in temperature
  • In an isolated system heat flows from hot to cold
    until both bodies are at the same temperature

4
The Three Types of Heat Transfer
  • Conduction Heat is transferred through a
    material (e.g. insulation or glass)
  • Convection Heat is transferred by air or water
    currents (e.g. ocean currents)
  • Radiation Heat is transferred when a hot body
    emits radiation (e.g. infrared radiation given
    off by a fire)

5
Conduction
  • Conduction depends on the following
  • Type of Material thermal conductivity (e.g.
    metal spoons transfer heat better than plastic)
  • Temperature Difference
  • Area (e.g. a thin stirring stick transfers less
    heat than a thick spoon)
  • Thickness (the distance heat has to travel)

6
Heat Conduction Equation
  • QC/t heat transferred per unit of time
  • c thermal conductivity
  • A area
  • T2 - T1 temperature difference
  • ? thickness

7
Examples of Conduction
  • Why does crushed ice melt faster than ice cubes?
  • Answer Because the exposed area is larger
  • Why do you save money by turning down the
    thermostat in cold weather?
  • Answer Because the temperature difference
    (between inside and outside) is smaller

8
Convection
  • Warm air (water) rises and cool air (water) sinks
  • Why? Because warm air (water) is less dense and
    floats on cooler air (water)
  • The rising of warm air (water) creates
    circulating convection currents
  • Convection can occur in any gas or fluid

9
Examples of Convection
  • The sea breeze is caused by differences in
    temperature between the ocean and the shore
  • In fact, all weather and ocean currents are
    caused by convection
  • A draft in a cold room is caused by convection
    currents from air leaking through a window or
    door
  • A rolling boil in a pot is the result of
    convection

10
Radiation
  • Radiation results in heat being emitted at the
    speed of light
  • Radiated heat requires no medium (e.g. air) and
    can propagate through empty space
  • Heat is emitted as electromagnetic radiation
    (e.g. light)
  • Here, radiation does not refer to the emissions
    of radioactive substances

11
The Wave Nature of Light
  • Light is a form of electromagnetic radiation
  • Electromagnetic radiation consists of waves of a
    definite wavelength
  • Wavelength is the distance from one crest to the
    next

12
The Speed of Light
  • All electromagnetic waves travel at the speed of
    light
  • The speed of light is 186,000 miles/second or 3 x
    108 m/s
  • Light can travel around the earth about 7 times
    in one second

13
The Electromagnetic Spectrum
  • Radio, Microwave, Infrared, Visible Light,
    Ultraviolet, X-Rays and Gamma Rays are all
    electromagnetic radiation
  • Only ultraviolet, x-rays and gamma rays are
    harmful to our cells

14
Radiation from Hot Objects
  • Hot objects emit radiation over a wide range of
    wavelengths
  • Room temperature objects emit radiation that is
    mostly infrared
  • Object hotter than 1000 C begin to emit visible
    light

Light from the Sun
15
Examples of Radiation
  • A hot burner on a stove or a fire emits large
    amounts of infrared and a smaller amount of
    visible radiation
  • Mammals (40 C) emit mostly infrared radiation
  • Our sun (6000 C) emits a large amount of
    visible light
  • Incandescent lights (regular light bulbs) have
    heated filaments that emit visible light

16
An Example of Heat Transfer
  • A radiator works by circulating steam through a
    series of pipes, where it condenses and releases
    heat
  • Heat is transferred by conduction, convection,
    and radiation
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