16.1 Thermal Energy and Matter
• In the 1700’s, heat was believed to be a fluid,
caloric, which flowed between objects.
• Ben Thompson, Count Rumford, disproved
this in 1798. He made cannons and noticed
that brass heated while drilling (boiled water)
but cooled when drilling stopped.
• The conclusion was heat was not a separate
piece of matter, but related to the drilling due
to friction.
Work and Heat
• The transfer of thermal energy from one
object to another because of a temperature
difference is heat.
• Heat flows spontaneously from hot objects to
cold objects (“stealing” heat).
Temperature
• A measure of how hot or cold an object is
compared to a reference point is temperature.
• Absolute zero is 0 Kelvin.
• Temperature is related to the average kinetic
energy of the particles in an object due to their
random motions through space.
• Hot objects move faster, and heat is transferred
by collisions.
• High energy loses in collisions, and low energy
gains in collisions (parked car that is hit moves).
Thermal Energy
• Thermal energy depends on the mass,
temperature, and phase (solid, liquid, or gas)
of an object.
• Thermal energy is the total potential and
kinetic energy of all the particles in an object.
• Thermal energy depends on mass (longer to
heat bucket of water than a droplet).
Phases of Matter: Solid
• Definite shape
• Definite volume
• slow-moving
Phases of Matter: Liquid
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•
•
•
•
Definite volume
Indefinite shape
Faster moving than solid
Capillary action- can go against gravity
Surface tension- the surface can hold (Red
Rover)
Phases of Matter: Gas
•
•
•
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Indefinite shape
Indefinite volume
Faster than liquid
Moves erratically
Thermal Contraction and Expansion
• A balloon on a cold day will shrink, and tires deflate.
This is due to the gas moving more slowly and exerting
less force, or thermal contraction.
• A balloon on a hot day could pop due to thermal
expansion, or an increase in volume due to increase in
temperature.
• Thermal expansion occurs when particles of matter
move farther apart as temperature increases.
• Gases expand the most.
• Thermometers depend on the expansion on alcohol
(red fluid rises) or coil unwinding for a stove.
Specific Heat
• You do not get pizza burn from the crust, but the crust
is the same temperature as the cheese that can burn
the roof of your mouth.
• This is because crust and cheese have different specific
heats.
• Specific heat is the amount of heat needed to raise the
temperature of one gram of a material by one degree
Celsius.
• The lower a material’s specific heat, the more its
temperature rises when a given amount of energy is
absorbed by a given mass.
• Water has a high specific heat of 4.18 J/gdegreeCelsius.
Specific Heat
•
•
•
•
•
Q = mcT
Q = heat (J)
M = mass (g)
C = specific heat (J/gdegree Celsius)
T = change in temperature (degrees Celsius)
Specific Heat problems
• An iron skillet has a mass of 500 g. The specific
heat of iron is 0.5 J/gdegreesCelsius. How
much heat must be absorbed to raise the
temperature 90 degrees Celsius?
• Q = mcT
• Q = (500)(0.5)(90)
• Q = (250)(90)
• Q = 22500 J, or 22.5 kJ
Specific Heat Problems
• How much heat would raise 100 g of water
100 degrees Celsius?
• Q = mcT
• Q = (100)(4.18)(100)
• Q = (418)(100)
• Q = 41800 J or 41.8 kJ
Specific Heat Problems
• How much heat is absorbed by 70 g of iron
that goes from 25 degrees Celsius to 125
degrees Celsius?
• Q = mcT
• Q = (70)(0.5)(125-25)
• Q = (70)(0.5)(100)
• Q = (35)(100)
• Q = 3500 J or 3.5 kJ
Specific Heat Problems
• An aquarium transfers 1200 kJ heat to 75,000
g of water. What is the increase in
temperature?
• Q = mcT
• 1200000 = (75000)(4.18)T
• 1200000 = 313500T
• T = 4 degrees Celsius
Specific Heat Problems
• To release a diamond, a jeweler must heat a
10 g ring with 24 J of heat. What was the
temperature change?
• Q = mcT
• 24 = (10)(0.2)T
• 24 = 2T
• T = 12 degrees Celsius
Specific Heat Problems
• What mass of water will change 3 degrees
Celsius when 525 J of heat is added to it?
• Q = mcT
• 525 = (m)(4.18)(3)
• 525 = (m)(12.54)
• M = 42 g
Measuring Heat Changes
• A calorimeter is an instrument used to
measure changes in thermal energy.
• A calorimeter uses the principle that heat
flows from a hotter object to a cooler object
until both reach the same temperature.
• Thermal energy of the burning food is
absorbed by the surrounding water.
• Water is stirred for even distribution of heat.
• The calorimeter is made of an insulator.