12/7/2014
Warm Up: 12/02/14
Heat Equation & Thermal Expansion
CH21 Continued
Why does water expand when it freezes?
Physics Fall, 2014
Mrs. Kummer
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Are Heat and Temperature The Same?
• ABSOLUTELY NOT!
• Heat is an Energy, the ability to do thermal
work, uses symbol Q
• Heat is measured in Joules (J)
• Temperature is measuring how fast/slow the
molecules are moving in a substance, uses
symbol T
• Temperature is measured in °C
• JUST LIKE: mass ≠ weight
• TEMP ≠ HEAT
Thermal Expansion
First, let’s describe what happens when T is
increased for a body
What are the molecules doing?
– How fast/slow are they moving?
– Do they move closer/further apart when moving as T
is increased?
Now, let’s relate it to most solids (s), liquids (l),
gases (g)
– Most s,l, & g EXPAND WHEN HEATED, AND CONTRACT
WHEN COOLED
– Thermal Expansion: material property where the
material expands when heated, and contracts when
cooled
How about an example? Your gasoline car
engine…check out these cool videos!
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Thermal Expansion
Thermal Expansion
How Stuff Works: Internal Combustion Engine
In your gasoline engine the following occurred:
– The fuel is injected into the air; this mixture
dumped into combustion chamber
– Piston comes up to compress the mixture
– Spark ignites the high P, high T mixture
– This explosion drives the piston down, as the
“power stroke”
– Piston comes back up one more time, with
exhaust valve open, to remove the spent mix
4stroke and injection explain
Massive engines show
http://videos.howstuffworks.com/discovery/3
1671-massive-engines-combustion-enginevideo.htm
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Thermal Expansion
Thermal Expansion
At 4(°C) and greater, water (l) begins to EXPAND
with increase (↑) T, like all other liquids do
But what happens between 0(°C) and 4(°C)?
You can also see this on your way home
tonight:
– Sidewalks
– Driveways
– Expressways
Thermal expansion SUPER obvious with H2O
– Water turns to ICE
– ICE turns to Water
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Thermal Expansion
Water (Liquid)
Thermal Expansion
– The opposite!
– USE CAUTION IN THIS RANGE WHETHER YOU ARE
TALKING ABOUT WATER (l) OR ICE (s)
– Water (l) actually CONTRACTS between 0(°C) and 4(°C)
– In this range, water has its smallest volume, or greatest
density at 4(°C)….remember, Density Equation:
ρ=m
(Eq. 1)
V
– This makes water (l) at 4 (°C) SINK to the bottom of
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ponds, lakes, etc.
Why, then, does cold solid ice float on cold
water?
– Due to water’s funky crystal structure
• Most materials have molecules very close together in solid
state
• Water, due to its angular six-sided crystal structure, has a lot
of air gaps between molecules in solid state (e.g. the
snowflake)
• Therefore, ice has a greater volume than cold water
– At 0°C, ice is < dense than liquid H2O
So, Ice floats on liquid H2O
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How Do We Measure Heat?
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How Do We Measure Heat?
When using Q = m∙c∙ΔT
(Eq. 25)
YOU NEVER SOLVE FOR c, YOU ALWAYS HAVE
THIS
UNITS MUST BE AS STATED IN THE
EQUATION!
If they are not, your first step is to convert
them to proper units!
The Heat Equation!
Q = m∙c∙ΔT
(Eq. 25)
Q = Heat Energy (J)
m = mass (kg)
c = Specific Heat ( J )
kg°C
(°C)
ΔT= Tfinal – Tinitial
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Heat Equation Example #1
Heat Equation Example #2
How much heat is needed to heat my aluminum
stick, with a mass of 10 (kg), from room temp. to
150 (°C)?
How much material is needed to heat my
aluminum rod from room temp. to 150 (°C),
producing 560,000 (J) of heat?
GIVEN:
GIVEN:
MATERIAL, Al: c=900 (J/kg°C)
mass, m = 10 (kg)
Tfinal = 150 (°C)
Tinitial = _______ (°C)
and, ΔT = Tfinal – Tinitial
so, ΔT=__________ (°C)
UNKNOWN:
FORMULA:
PLUG-IN:
SOLVE: Answer: 1125000 (J) or 1125 (kJ)
MATERIAL, Al: c=900 (J/kg°C)
Q = 560,000 (J)
Tfinal = 150 (°C)
Tinitial = _______ (°C)
and, ΔT = Tfinal – Tinitial
so, ΔT=__________ (°C)
UNKNOWN:
FORMULA:
PLUG-IN:
SOLVE: Answer: 4.98 kg
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LET’S PRACTICE!
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