Efficiency
Physics 2A
Chapter 11 : Using Energy
and &12:Thermal Properties
Of Matter
Efficiency
Thermometers
Heat
Convection
Radiation
Efficiency
Thermometers
⇒ For example, most gases, liquids, and solids
expand when heated so their volume changes
common thermometer ⇒ temperature measured
by looking at changes in the volume of mercury
or alcohol
⇒ thermometers are calibrated by placing them in
thermal contact with objects at two calibration
temperatures, usually the ice point (0 oC) and the
steam point (100 oC)
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Heat
⇒ When hot and cold water are mixed together,
they will eventually reach the same temperature.
Energy is transferred from the hot object to
the cold object.
heat ⇒ energy transferred between objects
because of a temperature difference
Heat
⇒ Two objects are said to be in thermal contact if
they can exchange heat.
⇒ Two objects in thermal contact will eventually
reach thermal equilibrium.
The flow of heat ceases because there is no
temperature difference (they have reached the
same temperature).
Convection
⇒ When part of a fluid (liquid or gas) is warmed, such
as the air above a fire, the volume of the fluid expands
and therefore the density of the fluid decreases.
⇒ The surrounding fluid is cooler and more dense.
⇒ Because of this, the surrounding cooler fluid exerts a
buoyant force on the warmer fluid and pushes it upward.
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Convection
⇒ As the warmer fluid rises, the surrounding fluid
replaces it. The cooler fluid, in turn, is warmed and
pushed upward.
Convection
⇒ Because of convection, hot air rises and creates
updrafts or thermals.
⇒ This process continues, and a continuous flow called
a convection current is established, which transfers heat.
convection ⇒ the process in which heat is carried
from place to place by the bulk movement of a
fluid.
Examples of Convection Currents
An Electromagnetic Wave
⇒ An electromagnetic wave is a transverse
wave consisting of oscillating electric and
magnetic fields.
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The Electromagnetic Spectrum
⇒ The electromagnetic spectrum consists of
electromagnetic waves of all frequencies.
Radiation
⇒ Any object above absolute zero emits
electromagnetic radiation:
⇒ The higher the temperature (in Kelvin), the
more energy is radiated in the form of EM waves.
The Electromagnetic Spectrum
⇒ Visible light makes up a very small portion (less
than one millionth of 1%) of the measured
electromagnetic spectrum.
Radiation
⇒ At lower temperatures
(3000 K), most of the EM
radiation is in the red part of
the visible spectrum.
⇒ At higher temperatures
(10,000 K), EM radiation is
emitted at higher frequencies
and the objects glow white.
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Radiation
⇒ The average rate at which energy is radiated
away depends upon the surface area A, the
temperature (T), and the emissivity ε
P=
Q
= σε AT 4
Δt
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