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