# Heat Transfer - PowerPoint PPT Presentation

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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

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,

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
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 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

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
• Only ultraviolet, x-rays and gamma rays are
harmful to our cells

14
• 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
• A hot burner on a stove or a fire emits large
amounts of infrared and a smaller amount of
• 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,