Unit 6a - Temperature and Heat Transfer

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27 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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Kinetic Molecular Theory of Heat

Atoms held by electromagnetic forces

Atoms vibrate

Thermal energy is a measure of this
motion.

Add heat = increase motion and thermal
energy.

3D Lattice Model

Temperature

A measure of average particle kinetic
energy

A measure of thermal energy.

Not dependent on mass.

Thermometer

Measures temperature

Place in contact with object

Wait for thermal equilibrium
(temperatures are equal)

Thermal equilibrium

The energy transfers back and forth
between two objects are equal

OR net zero energy transfer

Temperature Scales

Celsius:

freezing point = 0

boiling points = 100.

Kelvin:

absolute zero = 0

atoms are motionless.

Kelvin = SI unit of temperature.

Compare Scales

Kelvin

Celsius

Fahrenheit

Boiling

373

100

212

Freezing

273

0

32

Absolute

Zero

0

-
273

-
459.67

Conversions

K = ºC + 273

Heat:

energy flows between objects

Due to temp. difference

Always from hot to cold

Heat Transfer:

Conduction, Convection,

Conduction

Heat transfer when two objects in
contact

Convection

Heat transferred by fluid motion

Warm air rises

Lava Lamp

Energy transfer by electromagnetic
waves.

Sunlight, toaster, campfire

Solar collector

Specific Heat

C: material property, the amount of
energy required to raise 1 kg by 1 K.

Unit = J/kg∙K

Examples:

Water C = 4180 J/kg∙K

Iron

C = 450

Copper C = 385

Heat Transferred

Amount of heat gained or lost by an
object

Q = mC∆T

Q = heat transferred (unit = joules)

m = mass

C = specific heat

∆T = change in temp.

Example

1 kg. of Iron is heated from 37 ºC to

57 ºC. The specific heat of iron is 450

J/kgK. What was the heat transferred

to the iron?

Q

= mC∆T

= (1 kg)(450 J/kgC)(57 ºC

37 ºC)

= 1 x 450 x 20

= 9,000 J

Heat Transfer and Temperature

Heat lost by one must equal heat gained by
other

Both end up at same final temperature

hot

object

cold

object

Heat

insulation

Q
lost

= Q
gained

mC∆T
lost

= mC∆T
gained

mC(T
f
-
T
i
)
hot object

= mC(T
f
-
T
i
)
cold material

-

signs

Example

Drop 0.020 kg Iron at 180 ºC into 0.40

kg water at 10 ºC. What’s the final temp?

mC(T
f
-
T
i
)
iron

+ mC(T
f
-
T
i
)
water

= 0

(0.02)(450)(Tf

180) + (0.4)(4180)(Tf

10) = 0

9(Tf
-
180) + 1672(Tf
-
10) = 0

9Tf

1620 + 1672Tf

16720 = 0

1681Tf

18340 = 0

1681Tf = 18340

Tf = 18340/1681

Tf = 10.9 ºC

Change of State

3 Phases: Solid

Liquid
-

Gas

Kinetic Molecular model: atoms joined
by bonds

Add heat, increase temp, bonds break

-

Solid becomes liquid at Melting Point

-

During melting, no temp. increase

temp. increases to boiling point

-

Liquid becoming vapor

-

During vaporization, no temp. increase

Melting

Vaporizing

Freezing

Condensing

Temperature

Heat of Fusion: energy needed to melt
1 kg of solid.

Q=mH
f

H
f

= heat of fusion

m = mass

Heat of Vaporization: energy needed to
vaporize 1 kg of liquid.

Q=mH
v

H
v

= heat of vaporization

m = mass

Thermal Expansion

As temp increases, matter expands

Increase length

Increase volume

Water exception

expands as it
freezes