HEAT

TEMPERATURE is a measure of the average kinetic energy per molecule. The infrared radiation coming from the air canal in the ear passes through the optical system of the thermometer and is converted to an electrical signal that gives a digital reading of body temperature.

TemperatureTemperature is related to the kinetic activity of the molecules, whereas expansion and phase changes of substances are more related to potential energy.

2½mvT

N

Although not true in all cases, a good beginning is to define temperature as the average kinetic energy per molecule.

Temperature vs. Internal Energy

The large pitcher and the small one have the same temperature, but they do not have the same thermal energy. A larger quantity of hot water melts more of the ice.

Temperature EquilibriumHeat is defined as the transfer of thermal energy that is due to a difference in temperature.

Hot Coals

Cool Water Same Temperature

Thermal Equilibrium

Insulated Container

Two objects are in thermal equilibrium if and only if they have the same temperature.

Thermometer

A thermometer is any device which, through marked scales, can give an indication of its own temperature.

T = kXT = kX

X is thermometric property: Expansion, electric resistance, light wavelength, etc.

Limitations of Relative Scales

The most serious problem with the Celsius and Fahrenheit scales is the existence of negative temperatures.

Clearly, the average kinetic energy per molecule is NOT zero at either 00C or 00F!

-250C ?

T = kX = 0 ?T = kX = 0 ?

Comparison of Four Scales

1 C0 = 1 K1 C0 = 1 K

5 C0 = 9 F5 C0 = 9 F

095 32F Ct t 09

5 32F Ct t

059 32C Ft t 05

9 32C Ft t

TK = tC + 2730TK = tC + 2730

ice

steam

Absolute zero

1000C

00C

-2730C

Celsius

CFahrenheit

320F

-4600F

2120F

F

273 K

373 K

Kelvin

0 K

KRankine

0 R

460 R

672 R

R

Volume Expansion

Expansion is the same in all directions (L,

W, and H), thus:

The constant is the coefficient of volume expansion. 0

V

V t

0

V

V t

FOUNDRY: It requires about 289 Joules of heat to melt one gram of steel. In this PowerPoint, we will define the quantity of heat to raise the temperature and to change the phase of a substance.

Photo © Vol. 05 Photodisk/Getty

Heat Defined as Energy

Heat is not something an object has, but rather energy that it absorbs or gives up. The heat lost by the hot coals is equal to that gained by the water.

Heat is not something an object has, but rather energy that it absorbs or gives up. The heat lost by the hot coals is equal to that gained by the water.

Hot coals

Cool water

Thermal Equilibrium

Units of Heat

One calorie (1 cal) is the quantity of heat required to raise the temperature of 1 g of water by 1 C0.

10 calories of heat will raise the temperature of 10 g of water by 10 C0.

Example

Units of Heat (Cont.)

10 Btu of heat will raise the temperature of 10 lb of water by 10 F0.

Example

One British Thermal Unit (1 Btu) is the quantity of heat required to raise the temperature of 1 lb of water by 1 F0.

The SI Unit of HeatSince heat is energy, the joule is the preferred

unit. Then, mechanical energy and heat are measured in the same fundamental unit.

Since heat is energy, the joule is the preferred unit. Then, mechanical energy and heat are

measured in the same fundamental unit.

1 cal = 4.186 J1 cal = 4.186 J

Comparisons of Heat Units:

1 kcal = 4186 J1 kcal = 4186 J

1 Btu = 778 ft lb1 Btu = 778 ft lb

1 Btu = 252 cal1 Btu = 252 cal

1 Btu = 1055 J1 Btu = 1055 J

Temperature and Quantity of Heat

200 g

600 g

200C

200C

220C

300C

The effect of heat on temp- erature depends on the quantity of matter heated.

The same quantity of heat is applied to each mass of water in the figure.

The larger mass experiences a smaller increase in temperature.

Two objects are made of the same material, but have

different masses and temperatures. If the objects are

brought into thermal contact, which one will have the

greater temperature change?

(A) the one with the higher initial temperature

(B) the one with the lower initial temperature

(C) the one with the greater mass

(D) the one with the smaller mass

(E) the one with the higher specific heat

Quiz 1

Pre-Lecture Quiz 14

Heat CapacityThe heat capacity of a substance is the heat required to raise the temperature a unit degree.

Lead Glass Al Copper Iron

Heat capacities based on time to heat from zero to 1000C. Which has the greatest heat capacity?

37 s 52 s 60 s 83 s 90 s

1000C 1000C 1000C 1000C 1000C

Conservation of EnergyWhenever there is a transfer of heat within a system, the heat lost by the warmer bodies must equal the heat gained by the cooler bodies:

Hot iron

Cool water

Thermal Equilibrium

(Heat Losses) = (Heat Gained) (Heat Losses) = (Heat Gained)

Change of Phase

Solid LiquidGas

Q = mLf Q = mLv

fusion

Vaporization

When a change of phase occurs, there is only a change in potential energy of the molecules. The temperature is constant during the change.

When a change of phase occurs, there is only a change in potential energy of the molecules. The temperature is constant during the change.

Terms: Fusion, vaporization, condensation, latent heats, evaporation, freezing point, melting point.

Terms: Fusion, vaporization, condensation, latent heats, evaporation, freezing point, melting point.

Change of Phase

The The latent heat of fusionlatent heat of fusion ( (LLff) of a substance is ) of a substance is

the heat per unit mass required to change the the heat per unit mass required to change the substance from the solid to the liquid phase of substance from the solid to the liquid phase of its melting temperature.its melting temperature.

The The latent heat of vaporizationlatent heat of vaporization ( (LLvv)) of a of a

substance is the heat per unit mass required substance is the heat per unit mass required to change the substance from a liquid to a to change the substance from a liquid to a vapor at its boiling temperature.vapor at its boiling temperature.

For Water: Lf = 80 cal/g = 333,000 J/kgFor Water: Lf = 80 cal/g = 333,000 J/kg

For Water: Lv = 540 cal/g = 2,256,000 J/kgFor Water: Lv = 540 cal/g = 2,256,000 J/kg

f

QL

mf

QL

m

v

QL

mv

QL

m

Example 3: How much heat is needed to convert 10 g of ice at -200C to steam at

1000C?

First, let’s review the process graphically as shown:temperature

t

Qice

steam only

-200C

00C

1000

C

steam and

water

540 cal/g

ice and water

80 cal/gwater only

1 cal/gC0

ice steam

cice= 0.5 cal/gC0

Example 3 (Cont.): Step one is Q1 to convert 10 g of ice at -200C to ice at 00C

(no water yet).

t

Qice-200C

00C

1000

C

cice= 0.5 cal/gC0

Q1 = (10 g)(0.5 cal/gC0)[0 - (-200C)]

Q1 = (10 g)(0.5 cal/gC0)(20 C0)

Q1 = 100 calQ1 = 100 cal

-200C

00C

Q1 to raise ice to 00C: Q1 = mct

t

Q-200C

00C

1000

C

Example 3 (Cont.): Step two is Q2 to convert 10 g of ice at 00C to water at 00C.

MeltingQ2 to melt 10 g of ice at 00C: Q2 = mLf

80 cal/g

ice and water

Q2 = (10 g)(80 cal/g) = 800 cal

Q2 = 800 calQ2 = 800 cal

Add this to Q1 = 100 cal: 900 cal used to this point.

t

Q-200C

00C

1000

C

water only

1 cal/gC0

Step three is Q3 to change 10 g of water at 00C to water at 1000C.

00C to 1000C

Q3 to raise water at 00C to 1000C.Q3 = mct ; cw= 1

cal/gC0 Q3 = (10 g)(1 cal/gC0)(1000C - 00C)

Q3 = 1000 calQ3 = 1000 cal

Total = Q1 + Q2 + Q3

= 100 +900 + 1000 = 1900 cal

Step four is Q4 to convert 10 g of water to steam at 1000C? (Q4 = mLv)

Q-200C

00C

1000

C

vaporization

Q4 to convert all water at 1000C to steam at 1000C. (Q = mLv)

Q4 = (10 g)(540 cal/g) = 5400 cal

100 cal

icewater only

ice and water

800 cal1000 cal steam

and water

5400 cal Total Heat:

7300 cal7300 cal

1 kg of water at 100 oC is poured into a bucket that

contains 4 kg of water at 0 oC. Find the equilibrium

temperature (neglect the influence of the bucket).

(A) 0 oC

(B) 20 oC

(C) 50 oC

(D) 80 oC

(E) 100 oC

Quiz 2

Pre-Lecture Quiz 14

TRANSFER OF HEAT is minimized by multiple layers of beta cloth. These and other insulating materials protect spacecraft from hostile environmental conditions. (NASA)

Heat Transfer by Conduction

Conduction is the process by which heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move.

Conduction Direction

From hot to cold.

Given your experience of what feels colder when you

walk on it, which of the surfaces would have the highest

thermal conductivity?

(A) a rug

(B) a steel surface

(C) a concrete floor

(D) has nothing to do with thermal conductivity

Quiz 3

Pre-Lecture Quiz 14

Heat Transfer by Convection

Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid.

ConvectionHeated fluid rises and is then replaced by cooler fluid, producing convection currents.

Convection is significantly affected by geometry of heated surfaces. (wall, ceiling, floor)

Heat Transfer by Radiation

Radiation

Sun

Radiation is the process by which heat energy is transferred by electromagnetic waves.

Atomic

No medium is required !

Summary: Heat Transfer

Convection is the process by which heat energy is transferred by the actual mass motion of a heated fluid.

Conduction: Heat energy is transferred by adjacent molecular collisions inside a material. The medium itself does not move.

Radiation is the process by which heat energy is transferred by electromagnetic waves.

Examples of Thermal Conductivity

Aluminum:Aluminum:

Comparison of Heat Currents for Similar Conditions: L = 1 cm (0.39 in.); A = 1 m2 (10.8 ft2); t = 100

C0

Copper:Copper:

Concrete or Concrete or Glass:Glass:

Corkboard:Corkboard:

2050 kJ/s2050 kJ/s 4980 Btu/h4980 Btu/h

3850 kJ/s3850 kJ/s 9360 Btu/h9360 Btu/h

8.00 kJ/s8.00 kJ/s 19.4 Btu/h19.4 Btu/h

0.400 kJ/s0.400 kJ/s 9.72 Btu/h9.72 Btu/h

THERMODYNAMICSTHERMODYNAMICSThermodynamics is the study of energy relationships that involve heat, mechanical work, and other aspects of energy and heat transfer.

Central Heating

Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics: If two objects A and B are separately in equilibrium with a third object C, then objects A and B are in thermal equilibrium with each other.

AObject C

A B

Thermal Equilibrium

Same TemperatureBObject C

A THERMODYNAMIC SYSTEM

• A system is a closed environment in which heat transfer can take place. (For example, the gas, walls, and cylinder of an automobile engine.)

Work done on gas or work done by gas

INTERNAL ENERGY OF SYSTEM

• The internal energy U of a system is the total of all kinds of energy possessed by the particles that make up the system.

Usually the internal energy consists of the sum of the potential and kinetic energies of the working gas molecules.

The water flowing over Niagara Falls drops a distance of 50 m. Assuming that all the gravitational energy is converted to thermal energy, by what temperature does the water rise?

(A) 0.10 C°

(B) 0.12 C°

(C) 0.37 C°

(D) 0.42 C°

Quiz 4

Heat 14 (13 of 42)

oCkgJ

water 4186c

P.E.=mgh

Joule = newton/meter

TWO WAYS TO INCREASE THE INTERNAL ENERGY, U.

HEAT PUT INTO A SYSTEM

(Positive)

++UU

WORK DONE WORK DONE ONON A GAS A GAS (Positive)(Positive)

WORK DONE BY EXPANDING GAS: W is

positive

WORK DONE BY EXPANDING GAS: W is

positive

--UUDecreasDecreas

ee

--UUDecreasDecreas

ee

TWO WAYS TO DECREASE THE INTERNAL ENERGY, U.

HEAT LEAVES A SYSTEM Q is

negative

QQoutout

hot

WWoutoutWWoutout

hot

THE FIRST LAW OF THERMODYAMICS:

• The net heat put into a system is equal to the change in internal energy of the system plus the work done BY the system.

Q = U + W final - initial)

• Conversely, the work done ON a system is equal to the change in internal energy plus the heat lost in the process.

• Absorbs heat Qhot

• Performs work Wout

• Rejects heat Qcold

A heat engine is any device which through a cyclic process:

Cold Res. TC

Engine

Hot Res. THQhot Wout

Qcold

HEAT ENGINES

THE SECOND LAW OF THERMODYNAMICS

It is impossible to construct an engine that, operating in a cycle, produces no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work.

Not only can you not win (1st law); you can’t even break even (2nd law)!

Wout

Cold Res. TC

Engine

Hot Res. TH

Qhot

Qcold

The second law of thermodynamics is a statement about which processes occur and which do not. There are many ways to state the second law; here is one:

The Second Law of Thermodynamics

Heat will flow spontaneously from a hot object to a cold object.

It will not flow spontaneously from a cold object to a hot object.

EFFICIENCY OF AN ENGINE

Cold Res. Cold Res. TTCC

Engine

Hot Res. THot Res. THH

QH W

QC

The efficiency of a heat engine is the ratio of the net work done W to the heat input QH.

e = 1 - QC

QH

e = = W

QH

QH- QC

QH

REFRIGERATORSA refrigerator is an engine operating in reverse: Work is done on gas extracting heat from cold reservoir and depositing heat into hot reservoir.Win + Qcold = Qhot

WIN = Qhot - Qcold

Cold Res. Cold Res. TTCC

Engine

Hot Res. Hot Res. TTHH

Qhot

Qcold

Win

Entropy

Entropy is a measure of the disorder of a system. This gives us yet another statement of the second law:Natural processes tend to move toward

a state of greater disorder.Example: If you put milk and sugar in your coffee and stir it, you wind up with coffee that is uniformly milky and sweet.

No amount of stirring will get the milk and sugar to come back out of solution.

Entropy

Another example: when a tornado hits a building, there is major damage.

You never see a tornado approach a pile of rubble and leave a building behind when it passes.

Another consequence of the second law: In any natural process, some energy becomes unavailable to do useful work.

Entropy