202-Temperature and Heat

7/23/2019 202-Temperature and Heat

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Temperature and Heat

Welcome to Thermodynamics

(or welcome to heat transfer)

Up to now: mass, length, time, current

The fourth quantity in physics: Temperature and related another form

of energy, HEAT



What is temperature?

Let’s define it is a variable to measure “hot” and “cold”. (We will see that it

is a measure of an internal kinetic energy)

What is heat?

Energy transferred between objects because of a temperature difference

Transferring an amount of heat to an object may raise its temperature.

Are heat and temperature the same or maybe the change

in Temperature?

NO! Heat neither can be the same thing as temperature

nor the increase in temperature. The amount of heat to

increase temperature depends how much mass and what

material is being heated



Thermal equilibrium

No heat transfer between two objects (when they are in thermal contact)(Note: a good thermal insulator, doesn’t totally prevent hat transfer, it

just slows it down)

Let’s summarize this in the

Zeroth law of thermodynamics

Two objects are in thermal equilibrium when

their temperature is the same

or

If object A is in thermal equilibrium with B and

B is in thermal equilibrium with C, then A and

C are in thermal equilibrium.

(Is it obvious?)



Let’s begin how we measure temperature……….

Temperature Scales

The Celsius Scale (Anders Celsius 1701-1744)

Reference points:

1. Water freezes at zero degrees Celsius: 0˚C

2. Water boils at hundred degrees Celsius: 100˚C

A change in temperature is described as “Celsius degrees” C˚

Lowest possible temperature -273.15˚C

The Fahrenheit Scale (Gabriel Fahrenheit 1686-1736)

Reference Points:

1. Coldest temperature he was able to achieve in his lab.

2. Body temperature 96 degrees (today 98.6 ˚F). Nobody knows why 96 ˚F

Therefore water freezes at 32˚F and boils at 212˚F

A change in temperature is described as “Fahrenheit degrees” F˚



C T C

F T C F

°+°

°= 325

9

C T F

C T

C F °−°

°= 32

9

5

Be careful!

1 degree is not necessarily not to the same as 1 degree!

Celsius Fahrenheit

Fahrenheit Celsius



The Kelvin Scale (William Thomson, Lord Kelvin 1824-1907)

Kelvin K = SI Unit

Constant-Volume Gas Thermometer

Pgas = Pat + ρmercuryhg

Extrapolation of linear relationship between pressure and temperature leads to absolute zero (-273.15˚C)

Absolute zero is exactly T = 0K

The difference between Celsius and Kelvin is just a shift of the zero level

T = TC + 273.15



Degrees ˚ is not used in the Kelvin scale !



Thermal expansion (a key issue to build a thermometer)

Most substances expand with temperature (one exception

is water. The fish in the pond are happy about it!)

Thermometer: Fluid in a tube. The height of the fluid is a

measure for the temperature.

Change of length (1 D), area (2 D) and volume (3 D) is

related to temperature: Thermal Expansion



]:

.)(0

1-[Kexpansionlinear of tCoefficienα

T LT const L Δ=Δ=Δ α

1D – Linear Expansion



T A A Δ≈Δ α 2

2 D Area Expansion

Calculation gives us for area of any shape



α β β

β β α

3

]:3

=

Δ=Δ≈Δ

useonesubstancecertainafor listednoisthereIf

[KExpansionVolumeof tCoefficien 1-T V T V V

3 D Volume Expansion

Calculation gives us for area of any shape



Coefficient of linear Expansion,

α (K –1)

Substance

0.50 x 10 –6Quartz

3.3 x 10 –6Pyrex glass

11 x 10 –6Window glass

12 x 10 –6Concrete

12 x 10 –6Iron (Steel)

17 x 10 –6Copper

19 x 10 –6Brass

24 x 10 –6 Aluminum

29 x 10 –6Lead

Coefficient of volume

expansion, β (K –1)

Substance

0.18 x 10 –3Mercury

0.21 x 10 –3Water

0.68 x 10 –3

Olive oil

0.95 x 10 –3Gasoline

1.01 x 10 –3 Alcohol

1.18 x 10 –3Carbon tetrachloride

1.51 x 10 –3Ether



Special properties of Water (Why the fish are so lucky that the lakeis freezing from the top to the bottom)



Application of thermal expansion: Bimetallic strip



Connection between heat and mechanical work

Rub over a piece of wood (or watch the film Castaway how to make fire) and

you can feel that the temperature of the wood increase. Remember that

energy cannot be created or destroyed. But energy can be transferred. Forexample, transfer mechanical work into heat.

The first unit of heat was calorie (cal).

1 kcal is defined as the heat to increase the temperature of 1kg water from

14.5°C to 15.5°C.

James Prescott Joule showed with his machine that

1 kcal = 4187 Joule

or 1 cal = 4.187 J 1000 cal = 1 C (food calorie)

(also called the mechanical equivalent of heat)



Specific Heat

The heat required to increase the temperature of anarbitrary substance is given by the specific heat, c

)]/()/([ °⋅=⋅Δ= C kg J K kg J T m

Q

calways positive

If Q is positive then change in temperature

positiveIf Q is negative then change in temperature

negative

Large specific heat means take large quantities

of heat with little change in temperature

Example:

Specific heat of 1kg water is cwater = 4186 J/(kg/K)



W W bb

W W W bbb

W W W bbb

W B

cmcm

T cmT cmT

T T cmT T cm

QQ

+

+=

=−+−

=+

0)()(

0

Calorimetry

(Dropping an object into a lightweight, insulating flask containing water)



][:mK

W tyconductivithermal k

t LT kAQ Δ=

Thermodynamics describes heat transfers. How can we exchange heat?

Three options: Conduction, Convection, Radiation

1.Conduction

Heat flows directly through a material

Heat flow fast good thermal conductor

Heat flow very slow thermal insulator We find that the amount of heat flowing through an object is ~A, ~ΔT, ~t, ~1/L



0.0234 Air

0.040Wool

0.10Wood

0.25 Asbestos

0.60Water

0.84Glass

1.3Concrete

1.6Ice

16.3Stainless steel—alloy 302

34.3Lead

66.9Steel, low carbon

217 Aluminum

291Gold

395Copper

417Silver

Thermal Conductivity, k[W/(mK)]

Substance





Convection

Heat transfer by movement of matter in a fluid.

Forced convection (Fan, Pump)

Natural convection: (Heater,

e.g. circulation - warm air raises up, cold air sink down)









Radiation (there is more or less nothing between earth an sun!)

All objects give off energy in form of radiation and absorb it in same way.

There is no need of transportation of matter – electromagnetic waves

Radiated Power (Energy per time)

Stefan’s Law

)(44

4

S net T T Ae P

AT e P

−=

=

σ

σ

T)gsurroundin:(sradiationNet

e: emissivity (between 0 and 1)0 ideal reflector, 1 black body

σ: Stefan-Boltzmann constant = 5.67 10-8 W/(m2K4)

Use Kelvin!

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202-Temperature and Heat