chapter 15 temperature, heat, and expansion herriman high physics

Chapter 15

Temperature, Heat, and Expansion

Herriman High Physics

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.


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.


Standard Reference Points

Scale Boiling Point

Freezing Point

Fahrenheit 212 °F 32 °F

Centigrade 100 °C 0 °C

Kelvin 373 273


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


Conversion Equations

Centigrade to KelvinK = C +273Ex: K = 100 + 273 = 373

Kelvin to Centigrade C = K - 273Ex: C = 373 - 273 = 100


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


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

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


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?


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.


Chapter 16

Heat Transfer


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


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


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


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.


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.


Change of Phase

Chapter 17


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.


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!


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!


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!


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


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


Phase Diagram

Ice

Ice - Water

Water

Water-Steam

Steam

-5

0

100

120


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


chapter 15 temperature, heat, and expansion herriman high physics

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