chapter 15 temperature, heat, and expansion herriman high physics
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Chapter 15
Temperature, Heat, and Expansion
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Temperature Scales
Temperature is defined as a measure of the average kinetic energy of the molecules. This is often called the internal energy of an object.
Temperature scales were developed using the freezing and boiling points of water at sea level as the standard reference points.
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Temperature Scales
There are three Temperature Scales used in Science Fahrenheit – Used primarily in the
United States Celsius – the standard for the Metric
System Kelvin – Also know as the “Absolute
Zero” Scale.
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Standard Reference Points
Scale Boiling Point
Freezing Point
Fahrenheit 212 °F 32 °F
Centigrade 100 °C 0 °C
Kelvin 373 273
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Conversion Equations
Fahrenheit to CentigradeC = 5/9 (F – 32)Ex: C = 5/9 (212-32) = 100
Centigrade to FahrenheitF = (9/5 C) + 32Ex: F = (9/5)(100) + 32 = 212
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Conversion Equations
Centigrade to KelvinK = C +273Ex: K = 100 + 273 = 373
Kelvin to Centigrade C = K - 273Ex: C = 373 - 273 = 100
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Heat
Heat is a form of Energy Transfer Heat flows from areas of high
energy to areas of lower energy Heat is transferred three way
Conduction – requires contact Convection – mass movement of
molecules Radiation – transfer over a distance
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Conduction
In order for conduction to occur the two objects must begin at different temperatures. Objects at the same temperature will not transfer heat and are said to be at thermal equilibrium.
In order to understand conduction you must understand how heat is tranferred.
Each material has an innate ability to absorb or give off heat – specific heat
The amount of heat an object can transfer depends upon three things:
The mass of the object, m (in Kg) The specific heat of the object, Cp ( in J/g°C) The temperature change of the object, t (in ºC) Heat is transferred according to the equation:
Q = mCpt
The heat transferred is measured in a unit called a calorie.Herriman High Physics
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Sample Problem
How much energy is required to raise the temperature of 5 Kg of water from 0°C to 100°C ? (Cp = 1 calorie/g°C)
Q= m CpΔt =
(5000 g)(1 calorie/g°C)(100°C)= 500,000 calories
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Thermal Expansion
Most objects tend to expand when their temperature rises and to contract when the temperature drops.
Do you know what the one notable exception is?
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Thermal Expansion Solids can expand linearly whereas
liquids expand volumetrically! Each substance has a constant for
which describes its ability to expand.
Linear constants are denoted by the Greek letter alpha, α and the volumetric constant is denoted by the Greek letter beta, β.
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Chapter 16
Heat Transfer
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Convection Unlike conduction which transfers energy
without transferring mass, convection requires a current – or mass movement of molecules in a fluid.
Keep in mind that a fluid in physics is either a liquid or a gas
Convection occurs because when fluids are heated they become less dense and rise past the colder, denser molecules – this creates the current which allows for heat transfer
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Convection
Wind is a form of convection Using the furnace in your house is
a form of convection Boiling water is also a form of
convection
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Radiation
Radiation is the transfer of energy via an electromagnetic wave.
Electromagnetic waves include radio waves, microwaves, and light
Packets of energy called photons are carried on a wave front
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The Greenhouse Effect
A car parked in the sun can get significantly hotter than the outside temperature, this is the greenhouse effect. It occurs because the glass windows let heat in as an electromagnetic wave (sunlight), but then don’t let the resulting heat waves back out.
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Solar Power
Solar Power is the rate at which the Earth receives energy from the sun.
Solar power can be used by either passive solar heating (greenhouse effect) or by using photovoltaic cells, also called solar panels, to convert the solar energy to electricity.
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Change of Phase
Chapter 17
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Phases Changes Phase changes require that a
substance absorb energy or release energy to occur.
There is NO Change in Temperature associated with a phase change!
Different words are used to denote direction when dealing with a phase change.
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Vocabulary of a Phase Change
Freezing – change from liquid to solid. Energy is released!
Melting – change from solid to liquid Energy is absorbed!
Both of these changes happen at the same point; 0° for water!
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Vocabulary of a Phase Change Condensing – change from gas or
vapor to a liquid. Energy is released!
Boiling – change from liquid to gas or vapor Energy is absorbed!
Vapor is the gas phase of any substance which is normally a liquid at room temperature!
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Energy Required for a Phase Change
Just like specific heat is a set amount of energy for each substance – the amount of energy required for a phase change is also substance specific.
Heat of Fusion – ΔHf , is the amount of energy absorbed or released when a substance melts or freezes!
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Energy Required for a Phase Change
Heat of Vaporization – ΔHv , is the amount of energy absorbed or released when a substance boils or condenses!
Mathematically:Q = mΔHf for Fusion
orQ = mΔHv for Vaporization
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Sample Problem
How much heat is required to melt 5 kg of ice at 0° C?
SolutionQ = mΔHf = (5000 g)(80 calories/g)
=400,000 calories or 400 Kilocalories
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Phase Diagram
Ice
Ice - Water
Water
Water-Steam
Steam
-5
0
100
120
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Other Forms of Stored Energy:
Chemical Energy
Stored in the Chemical Bonds that make up a substance Often released by combustion
(burning) Released as
kinetic energy Heat Light Sound
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