Zeroth Law of Thermodynamics
Physics 2A
Chapter 12 - 14: Heat
Transfer & the Ideal Gas
Zeroeth Law of Thermodynamics
Thermal Expansion
Unusual Behavior of Water
Heat
Convection
Zeroth Law of Thermodynamics
⇒ If the zeroth law of thermodynamics were not
true, than on object could have many different
temperatures depending on what objects were in
thermal equilibrium with it.
How can we use object T as a thermometer?
Measure some change in its physical properties.
thermometric property ⇒ a physical property that
changes with temperature
T
A
T
B
A
B
zeroth law of thermodynamics ⇒ if bodies A and
B are each in thermal equilibrium with a third
body T, then they are in equilibrium with each
other
This allows us to define temperature as a property
of an object and use T to measure temperature.
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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Thermal Expansion
Thermal Expansion
⇒ Extreme heat caused these railroad tracks to buckle
⇒ To compensate for thermal expansion, bridges have
expansion joints and one end of the bridge rides on
rockers (the other end is fixed).
because of thermal expansion.
The Unusual Behavior of Water
The Unusual Behavior of Water
⇒ Although most materials expand when heated, a few
do not.
⇒ For example, if water at 0oC is heated, its volume
actually decreases until its temperature reaches 4oC.
⇒ Above 4oC water behaves normally, and its volume
increases as the temperature increases.
⇒ The unusual behavior of water is very important to
⇒ As the temperature of water decreases below 4oC, the
aquatic life on this planet. Why?
volume of water increases so the density of water
decreases.
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The Unusual Behavior of Water
⇒ Because of this, ice is less dense than water and
therefore floats. Because of this, ponds and lakes freeze
from the top down.
⇒ If water behaved like most liquids, its volume would
decrease as its temperature decreased. Ice would then be
more dense than water and would sink.
⇒ Bodies of water would then freeze from the bottom
up and many bodies of water would remain frozen all
year round – aquatic life as we know it could not exist!
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).
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Convection
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.
⇒ As the warmer fluid rises, the surrounding fluid
replaces it. The cooler fluid, in turn, is warmed and
pushed upward.
⇒ The surrounding fluid is cooler and more dense.
⇒ This process continues, and a continuous flow called
a convection current is established, which transfers heat.
⇒ Because of this, the surrounding cooler fluid exerts a
buoyant force on the warmer fluid and pushes it upward.
convection ⇒ the process in which heat is carried
from place to place by the bulk movement of a
fluid.
Convection
Examples of Convection Currents
⇒ Because of convection, hot air rises and creates
updrafts or thermals.
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