Heat and TemperatureLight

The Nature of Matter• All matter consists of tiny particles called molecules.• These molecules are in a constant state of motion.

– Molecules in liquids and gases move freely.– Molecules in solids simply vibrate.

• This means that all molecules possess their own kinetic energy (KE), the energy of motion.

Solid Liquid/Gas

Temperature• Look at the jar below. What do you notice about the

speed of the molecules?– They move at different speeds.

• Now let’s add some heat.– Now they are all moving faster!

• Temperature relates to the average kinetic energy of the molecules in a substance.

The higher the temperature, the faster the molecules move.

Temperature Scales• There are three major temperature scales that we will

deal with.– The Fahrenheit Scale (°F)– The Celsius Scale (°C)– The Kelvin Scale (K)

• Conversion formulas are shown below. 325

9CF TT

3295

FC TT

273.15K CT T

273.15C KT T

95 32F CT T 9

5 21 32FT 69.8FT F

273.15K CT T 21 273.15KT 294.15KT K

Sample Temperature Conversion• A beaker of water at room temperature is

measured to be at 21°C.

• What is the Fahrenheit and Kelvin temperature?

Special Temperatures• There are several key temperatures that you will need to

know in relation to the temperature scales.– The freezing point of water:

– The boiling point of water:

• There are some other *special temperatures* that you may care to remember.– Body temperature:

0CT C 32FT F 273.15KT K

100CT C 212FT F 373.15KT K

37CT C 98.6FT F 310.15KT K

Absolute Zero• The final temperature to note is absolute zero, 0K.• This is the lowest possible temperature.• Here molecules are in a complete state of rest, which

means that there is no kinetic energy.• Absolute zero has never been reached.• However, scientists have come within 0.1K

0K

Thermal Energy• The sum of all the kinetic energies within a material is

known as thermal energy.• Both full cups of coffee are at the same temperature.

– Which cup contains greater thermal energy?– Which cup contains a higher average kinetic energy within the

molecules?

AB

B

Both are the same!

What is Heat?• Heat is energy in transfer from an object of

higher temperature to one of lower temperature.• In order for heat to be transferred, the objects

must be in thermal contact.• Eventually, they will reach the same

temperature. This situation is called thermal equilibrium.

Cold Hot

Heat Flow

Warm Warm

Heat Transfer• Heat, being energy, is measured in Joules (J)• The variable we will use for heat is Q.

– If heat is gained, Q > 0– If heat is lost, Q < 0

• Heat transfer causes temperature change (T).

• Every material has its own specific heat (c). This is the amount of heat required to raise 1kg of the substance by 1°C or 1K.

F IT T T

Common Specific Heat Capacities• Recall, units are measured in

Aluminum 903 Gold 129 Silver 233

Brass 377 Ice 2060 Steam 2020

Carbon 710 Iron 450 Water 4184

Copper 385 Lead 130 Zinc 388

Glass 664 Methanol 2450

or J Jkg C kg K

Specific Heat Video Clip

Heat Equation• The amount of heat required is proportional to mass (m),

specific heat (c), and change in temperature (T).• The equation is shown below.

• Celsius and Kelvin are interchangeable here.

Q mc T

Heat (J) Mass (kg)

Temp Change (K or °C)

Specific Heat

or J J

kgK kg C

Sample Heat Problem• How much heat is required to raise the temperature of a

2.4kg gold ingot (c = 129J/kgK) from 23°C to 45°C?

2.4kgQ mc T

2.4 129 22JkgKQ kg C

6811.2Q J

F IT T T 45 23T C C 22T C

Light

Sample Heat Problem 2• The ingot (m = 2.4kg) is then plunged into a bucket of

cold water causing it to lose 9216J of heat.• What is the temperature change (T) of the ingot?

• If the ingot was initially at 45°C, then what is the final temperature?

2.4kgQ

Tmc

9216

2.4 129 JkgK

JT

kg

30T C

F IT T T F IT T T

30 45FT C C 15FT C

Q mc T

Calorimetry• Recall, all heat transfers require at least two substances,

so simply calculating for one is not always enough.• Recall, a heat transfer requires the objects to be at

different temperatures. • One always receives heat, while the other loses.

– This occurs until thermal equilibrium is reached.Lost GainedQ Q

Calorimetry Equation• The goal here is to find the final temperature when

combining two substances of differing temperature.

Lost GainedQ Q

a a a b b bm c T m c T

a a F aI b b F bIm c T T m c T T

a a F a a aI b b F b b bIm c T m c T m c T m c T

a a F b b F b b bI a a aIm c T m c T m c T m c T

F a a b b b b bI a a aIT m c m c m c T m c T

a a aI b b bIF

a a b b

m c T m c TT

m c m c

Calorimetry Equation• The equation applies to any number of substances by

following the existing pattern.

a a aI b b bIF

a a b b

m c T m c TT

m c m c

Sample Problem (Calorimetry)• A hot iron rod (TaI = 340°C) is dropped into a bucket of

cold water (TbI = 10°C).

• If the mass of the rod is 0.5kg and the water 1.2kg, what is the final temperature of the mixture?

a a aI b b bIF

a a b b

m c T m c TT

m c m c

0.5 450 340 1.2 4184 10

0.5 450 1.2 4184

J JkgK kgK

F J JkgK kgK

kg C kg CT

kg kg

24.15FT C

Heat Transfer Types• Heat is transferred in three major ways:

– Conduction– Convection– Radiation

• Again, heat only flows when there is a difference in temperature.

Conduction• Conduction is the transfer of heat through molecular

collisions.• This form of heat transfer best occurs in solids where

molecules are closely packed.• Materials that conduct heat well are called conductors.

(Eg. metals such as copper and iron)• Materials that conduct heat poorly are called insulators.

(Eg. foam, air, and asbestos)Magnification

Convection• Convection is the transfer of heat through moving fluids.• A fluid is any substance that flows, which includes all

liquids and gases.• Examples include convection ovens and cloud formation.

Convection ?

• Have you ever watched a pot of water when it is being heated, especially while boiling? What do you notice?

• You will notice that there are currents within the pot.

The Heating Water Pot

The process of convection transfers heated water from the bottom of the pot to the top, where it is exchanged for cooler water.

Radiation (not radioactivity)

• Radiation is the transfer of heat via electromagnetic waves.

• These waves include visible light, but are mostly infrared.

• No matter is required for this type of heat transfer.

• Examples include the sun’s heat and warmth felt from a flame.

Open Space

?Radiation

More on Radiation• All objects emit heat in the form of radiation (radiant heat).• Hotter objects emit more energetic waves.• Some extremely hot objects can emit visible light, a form

of electromagnetic radiation.• In fact, all forms of radiation travel at the speed of light.

82.998 10 ms

Objects in thermal equilibrium will emit the same amount of radiation that they receive from other

objects.

Absorption and Emission• Certain colored objects will absorb radiant

heat better than others.

• Generally, objects of darker colors are these better absorbers.

Heat Transfer Question• Consider a camp fire burning vigorously.

– How is heat normally transferred while warming its viewers?• Radiation

– How is heat transferred when you put a hand in the smoke?• Convection

– How is heat transferred to a stick when it is placed in the hot coals?

• Conduction

• Some situations involve

multiple heat transfer

types like this.

The Thermos Bottle• The Thermos® bottle is designed to effectively prevent

heat loss/entry from affecting its contents.• Conduction and convection are prevented by the

vacuum between the two layers of glass.• Radiation is minimized by the shiny metal that is on the

unexposed side of each of the layers.

Reflective Metal

Vacuum

Glass

•Some coffee pots contain this bottle.

Latent Heat (Enthalpy)• As you know, an object requires a heat transfer (in/out)

in order to have a temperature change (T).• A heat transfer is also required to change phase. • Heat must be added to go to a more energetic phase.• Heat must be removed to go to a less energetic phase.

Solid Vapor

Vapor

Liquid

+Q +Q

-Q -Q

Heats of Fusion and Vaporization• The heat of fusion (Hf) is the amount of heat energy

required change 1kg of substance in the solid state into the liquid state.

• The equation is as follows:

• The heat of vaporization (Hv) is the amount of heat energy required to change 1kg of substance in the liquid state into the vapor state.

• The equation is as follows:

f fQ mH

v vQ mH

Sample Heating Across Phase• Consider a block of ice (m = 1.2kg) at -10°C being

heated to water at 80°C.• How much heat is used?

– Consider the following process:

• These problems have 1-5 steps.• Cooling uses negative numbers.

Ice -10°C Ice 0°C Water 0°C

Water 80°C

3 Steps

Heating Melting Heating

Q mc T fQ mH Q mc T

fQ mc T mH mc T 51.2 2060 10 1.2 3.34 10 1.2 4184 80J J J

kg C kg kg CQ kg C kg kg C 826800Q J

The Heating Curve (H2O)• As H2O is heated from ice below freezing to steam above boiling,

the temperature can be plotted with respect to time.• What do you notice about the curve?• Where are the flat spots located? What does this tell you about

heating during a phase change?

Temperature remains constant

during phase changes.

Once each phase change is complete, temperature can rise

again.

Heating Curve of Water

-20

0

20

40

60

80

100

120

140

0 5 10 15 20

Time (min)

Tem

per

atu

re (

C)

HeatHeatingMeltingBoiling Heat

The Cooling Curve (H2O)• The cooling curve appears very similar to the heating curve, except

for one very small difference. What is it?• What can you conclude about temperatures when cooling between

phases?

Cooling Curve of Water

-20

0

20

40

60

80

100

120

140

0 5 10 15 20

Time (min)

Tem

per

atu

re (

C)

SolidifyingCoolingCondensingCooling also requires the temperature to be held constant while in the midst of a phase

change.

The temperature can continue to drop after all of the substance is converted into the next

phase.

Cool

Thermal Expansion• Most substances expand when heated and

contract when cooled. This property is known as thermal expansion.

• Thermal expansion applies to all phases (solid, liquid, and vapor.)

Linear Expansion of Solids• As mentioned, solids expand upon heating.

• The change in length for a solid (L) can be calculated, due to a linear relationship with temperature change (T).

• The thermal expansion equation is shown below.

• The term , the coefficient of linear expansion is a value that depends on the material.

• Other helpful equations include:

iL L T

Change in Length (m)Initial Length (m) Coefficient of Linear Expansion (°C-1)

Temperature Change (°C)

f iL L L

Sample Linear Expansion Problem• A train rail is initially 20m long in the morning when the

temperature outside is 10°C. By how much will the rail expand in the heat of the day when the temperature reaches 35°C?

• The coefficient of linear expansion for steel is:

• Now find L.

• What is the final length?

6 112.0 10 C

iL L T 620 12.0 10 35 10L m C C 0.006L m

f iL L L f iL L L

20 0.006 20.006fL m m m

The Bimetallic Strip• A bimetallic strip consists of two metal strips

pressed together into a single strip.• A common strip consists of steel and brass. • Since different materials expand at different

rates, the strip will bend.

Room TempBrass

Steel

High Temp

Brass

Steel

Low TempBrass

Steel

The Bimetallic Strip (Cont.)• Bimetallic strips are most often used in temperature

sensitive instruments as thermostats.

• The strip is wound into a coil.

• As temperature changes, the coil expands or contracts to activate a switch controlling a heating/cooling system.

Heat

AC

I’m Hot!!!I’m

Cold!!

Unique Properties of Water• Like other substances, water expands and contracts

when heated and cooled, except within a special range.• However, water behaves strangely around its freezing

point. Consider the graph shown below.

•Where does water appear most dense?

•What happens to water as it cools from 4°C to 0°C?

•What happens to water as it freezes into ice?

•What happens to ice as it cools below 0°C?

4 C

Unique Properties of Water (Cont.)• In liquid water, the molecules move freely in no particular

order or array. This is what allows them to flow.

• When water freezes, the molecules form a hexagonal pattern.

• Why may ice possess less density?

O

HH

OHH

OHH

O

H

H

O HH

OH

H

O

H

H

O HH

OHH

O

HH

OH

H

O HH

OHH

OH H

O HH

OHH

OH

H

OHH

Liquid Water

OH

H

OHH

OHH

OHH

OHH

OH

H

Solid Ice

OH

H

OHH

OHH

OHH

OHH

OH

HO

HH

OHH

OHH

OHH

OHH

OH

H

Open Space

Applications of Linear Expansion• Linear expansion must be taken into consideration for

engineers designing something that will experience a range of temperatures.

• Here are just a few select examples:

• Can you think of any more?

Railroads Bridges Metal Roofs