Heat, Temperature, and Expansion
Chapter 21
All matter is composed of atoms which are constantly in motion. Previously
we learned that kinetic energy or “energy of motion” can be expressed with
the equation: http://phet.colorado.edu/en/simulation/states-of-matterbasics
KE = ½ mv2
Therefore, it makes sense that the faster atoms or molecules move, the more
kinetic energy they contain. We can increase this motion many different
ways. Here are some examples:
• stirring a liquid
• passing an electric current through a wire
• increasing the pressure of a gas
• hitting a penny with a hammer
• friction between two surfaces
The temperature of the atoms and molecules in all of these examples is
represented as their average kinetic energy.
Temperature Scales:
Fahrenheit scale - named after German physicist Daniel Gabriel
Fahrenheit, this scale was proposed in 1724 and is still used in the United
states but has
been replaced by the Celsius scale in most countries. On
the Fahrenheit scale, water freezes at 32 degrees and boils at 212
degrees.
Celsius scale – named after the Swedish astronomer Anders Celsius, this
scaled is used by most countries. Water freezes at 0 degrees on this scale
and boils at 100 degrees.
Kelvin Scale – absolute temperature scale named after Lord Kelvin who
introduced this scale in 1848. Water freezes at 273.15 degrees Kelvin and
boils at 373.15 degrees. Zero degrees Kelvin on this scale corresponds to a
state in which atoms of a substance have zero kinetic energy. This state is
called absolute zero.
Temperature Scales:
It is incorrect to say that matter contains heat. All atoms and molecules are in
constant motion, and contain energy by virtue of their motion (kinetic energy).
calorie = amount of energy needed to raise the temperature of 1 gram of
water by 1 degree Celsius
1 Calorie = 1000 calories = 1 kilocalorie
4.184 joules = 1 calorie
Specific heat = amount of energy needed to raise the temperature of 1 gram
of a substance by 1 degree Celsius
Heat capacity = amount of energy needed to raise the temperature of a
substance by 1 degree Celsius. ***NOTE HEAT CAPACITY DEPENDS ON THE
MASS OF THE SUBSTANCE*** 1 gallon of water has more heat capacity
than 1 cup of water.
Specific Heat vs. Heat Capacity
Compare the cup of water to the ocean on the right:
Which has the greater specific heat?
Answer:
Cup of Water
Neither. The specific heat
in both places is 4.18 J/g ̊C
Which has the greater heat capacity?
Answer:
The lake. There is more water in the lake
which can absorb more total heat energy.
Lake
Equation for specific heat:
Q = mC∆T
Q = heat in joules
m = mass of substance
C = specific heat of substance
∆T = change in temperature (Tf – Ti)
(Tfinal – Tinitial)
Thermal Expansion
Different materials expand at different rates. In a bimetallic
strip, two sides of different materials, say one of brass and the
other of iron are welded together. When the strip is heated,
the difference in the amounts of expansion of brass and iron
shows up easily.
A thermostat is a practical application of a bimetallic strip.
Bimetallic strip
used in a
thermostat:
Above picture: Expansion Joint on a bridge
Expansion of Water
• Solids, liquids, and gases expand while heated and
contract when cooled.
• Water is an exception to this rule.
• Water is less dense as solid than it is as a liquid.
*** Note water molecules are closer together in liquid form ***