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27 Οκτ 2013 (πριν από 4 χρόνια και 2 μήνες)

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a.k.a. Heating and Cooling

Thermal Comfort

What is heat?

Heat is a form of energy


All matter has heat energy.


Heat is the excited motion
of the atoms of a substance.


http://
www.youtube.com/watch?v
=f
1eAOygDP5s&feature=related


When a substance gets hot


it’s
molecules disperse


Object when hot


expand


Objects when cold


contract


http://
www.youtube.com/watch?v
=
tPJLFDekxZA


Many implications in construction, e.g. concrete sidewalks

Sources of Heat


The sun


Earth interior


Heat produced when atoms are broken down (nuclear
energy)


Friction



Conservation of Heat

1
st

Law of Thermodynamics


energy is neither created nor destroyed in the conversion of
heat to or from other forms of energy.

How Heat Travels

2
nd

Law of Thermodynamics


When an object is at a
different temperature from
another body or its
surroundings, heat flows so
that the body and its
surroundings reach the
same temperature so that
they are in thermal
equilibrium.

History of Heat

Daniel Bernoulli

1700
-
1782 Swiss physician and mathematician


first proposed that gases consist of many
molecules moving in all directions and that
their impact on a surface causes the gas
pressure.


Also proposed that the internal energy of a
substance is the sum of the kinetic energy
associated with each molecule


And proposed that heat transfer occurs from
regions with energetic molecules, and so high
internal energy, to those with less energetic
molecules, and so lower internal energy.

James Joule

1818
-
1889 English physicist and brewer


Developed the first law of thermodynamics
(conservation of energy)


Developed Joule’s law (closely related to Ohm’s law)
expressing the relationship between the heat
generated by the current flowing through a
conductor.


Q = I
2

* R *
t



Q = heat


I = current


R = resistance



t

= time


The unit Joule is named after him. 1 Joule = the
energy expended (or work done) in applying a force
of one
newton

through a distance of one meter.

William Thomson, 1st Baron Kelvin

1824
-
1907 mathematical physicist and engineer


Discovered that there was a lower limit to
temperature which he called “absolute
zero.”


Based on laws of thermodynamics.


Absolute zero = temperature in which
molecular movement stops.


Kelvin scale


measurement of
thermodynamic temperature.


0 degrees Kelvin = −273.15

°
C


How Heat Travels

Methods of Heat Transfer


Conduction



when a warm object touches a cool
object


it’s heat will go to it


Convection



if a warm object sits in cool air


its heat
will go off into the cool and rise as warm air.


Radiation



warm objects give off heat


http://
www.youtube.com/watch?v
=SJKCO2XK3wM


Conduction

Transferring heat from molecule to molecule.


Metals are better conductors
than heat than wood, glass or
plastic.


Plastics are bad conductors of
heat and are called insulators.


View video demo:
http://
www.youtube.com/watch?v
=9Ux
U0ELgYOA&feature=related

Conduction Experiment

Why does a koozie work?


How does this concept
relate to your home?


Convection

Process of a liquid or gas transmitting heat by means of
the movement of heated matter.


Molecules when heated get
excited and move farther
away from one another, i.e.
the substance expands and
becomes less dense.

Convection

Molecules of warm air moves faster and disperse more
quickly than cold air.


A gallon of cold air weighs
more than a gallon of
warm air


This is why hot air rises.


What is a convection current?


A convection
current is a flow
of a fluid or air
due to heating
and cooling of the
fluid or air. Hot
air rises. Cold air
falls. If there is
local heating of
the air at some
point, a
convection
current will be
set up.

Convection experiments


http://www.youtube.com/watch?v=m232DHncGjk


http://
www.youtube.com/watch?v
=07sNPZBgGv8&featur
e=related

Radiation


Radiation is a method of heat
transfer that does not rely upon any
contact between the heat source
and the heated object.


We feel heat from the sun even
though we are not touching it.


Objects emit radiation when high
energy electrons in a higher atomic
level fall down to lower energy
levels.


The energy lost is emitted as light
or electromagnetic radiation.


Energy that is absorbed by an atom
causes its electrons to "jump" up to
higher energy levels.

Radiation

How do you control the amount of heat an object absorbs?


Materials with a dark or rough surface will absorb more
infrared radiation than materials with a white or shiny
surface.


Black cars get hotter than white cars.

http://www.tom
-
morrow
-
land.com/tests/cartemp/index.htm

Quantifying Heat


How do we measure heat?


Heat is measured via temperature


Can be measured in Celsius, Fahrenheit or
Kelvins
.



How does a thermometer measure
temperature?


A thermometer works because
of the mercury. When
mercury gets hot it expands
and moves up the
thermometer.


At what rate will an object warm up?

Specific heat is the amount of heat required to cause a
unit of mass to change its temperature by 1
°
C.


The specific heat capacity of solid aluminum (0.904 J/
g
/
°
C) is
different than the specific heat capacity of solid iron (0.449
J/
g
/
°
C). This means that it would require more heat to increase the
temperature of a given mass of aluminum by 1
°
C compared to the
amount of heat required to increase the temperature of the same mass
of iron by 1
°
C. In fact, it would take about twice as much heat to
increase the temperature of a sample of aluminum a given amount
compared to the same temperature change of the same amount of iron.
This is because the specific heat capacity of aluminum is nearly twice
the value of iron.

How do we measure heat energy it will
take to warm something up?

Heat energy is measured in Joules: Q =
m•C•ΔT


Q = Joules


M = mass


C = specific heat


Delta T = temp difference


What quantity of heat is
required to raise the
temperature of 450 grams
of water from 15
°
C to
85
°
C? The specific heat
capacity of water is 4.18
J/
g
/
°
C.


Q =
m•C•
Δ
T

= (450
g)•(4.18 J/g/
°
C)•(70.
°
C)


Q = 1.3x10
5

J = 130 kJ

Figure out the specific heat of an
unknown metal


A 12.9 gram sample of an unknown metal at 26.5
°
C is
placed in a Styrofoam cup containing 50.0 grams of water at
88.6
°
C. The water cools down and the metal warms up until
thermal equilibrium is achieved at 87.1
°
C. Assuming all the
heat lost by the water is gained by the metal and that the cup
is perfectly insulated, determine the specific heat capacity of
the unknown metal. The specific heat capacity of water is
4.18 J/
g
/
°
C.


See:
http://www.physicsclassroom.com/class/thermalP/u18l2b.cfm

Creating Heating and Cooling in the
Home

Low Tech Options

Fire


Not very efficient.


Heat is lost through the
chimney.


Heats through radiation.


Loses heat through
convection


Low Tech Options

Fan


Simulates the “
windchill

effect”


Moving air increases
convective heat loss


making it easier for sweat
to evaporate from the skin.


Result: you feel cooler.

Heating Shelters

Electric Heaters


Electric current is forced
through wires that have a
lot of resistance.


Q = I
2

* R *
t



Q


heat


I = current


R = resistance


t

= time

Heating Shelters

Gas Heater


Takes in cold air


cleans it with an air filter


heats it up with a gas burner


distributes the warm air with
a blower motor through your
home's ductwork


Heated air then cools down
in your home's various rooms
and returns to the furnace
through return air grills and
duct work.

Cooling Shelters

How air conditioners work


Works by drawing warm air out of
the room.


Coils inside air conditioner contain
freon



a material that absorbs heat
when converting from a liquid to a
gaseous state.


A compressor converts the
freon

back to liquid and expels the heat
generated to the outside.


Repeated in a cycle to continue
cooling.


http://
www.youtube.com/watch?v
=1MiQCBIx1mM