Unit 6a - Temperature and Heat Transfer

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27 Οκτ 2013 (πριν από 4 χρόνια και 17 μέρες)

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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)


Read the temperature

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,
and Radiation

Conduction


Heat transfer when two objects in
contact

Convection


Heat transferred by fluid motion


Warm air rises


Lava Lamp


Radiation


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



Watch your +/
-

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

Add more heat


temp. increases to boiling point


-

Liquid becoming vapor


-

During vaporization, no temp. increase

Melting

Vaporizing

Freezing

Condensing

Temperature

Heat Added


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