Transcript Document

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.
Temperature
Temperature is related to the kinetic activity of
the molecules, whereas expansion and phase
changes of substances are more related to
potential energy.
Although not true in all cases, a good beginning
is to define temperature as the average kinetic
energy per molecule.
T
2
½mv

N
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 Equilibrium
Thermal Equilibrium
Hot Coals
Insulated
Container
Cool Water Same Temperature
Heat is defined as the
transfer of thermal energy
that is due to a difference
in temperature.
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 = 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!
T = kX = 0 ?
-250C ?
Comparison of Four
Scales
1000C
2120F
373 K
672 R
1 C0 = 1 K
460 R
5 C0 = 9 F
steam
00C
Celsius
C
-2730C
273 K
ice
K
Kelvin
320F
Fahrenheit
F
Absolute
zero
0 K -4600F
R
Rankine
0R
t F  t  32
9
5 C
tC 
5
9

0
t F  320
TK = tC + 2730

Volume Expansion
Expansion is the same
in all directions (L,
W, and H), thus:
The constant b is the coefficient
of volume expansion.
V
b
V0 t
Photo © Vol. 05
Photodisk/Getty
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.
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.
Cool
water
Hot coals
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.
Example
10 calories of heat will
raise the temperature of
10 g of water by 10 C0.
Units of Heat (Cont.)
One British Thermal Unit (1 Btu) is the
quantity of heat required to raise the
temperature of 1 lb of water by 1 F0.
Example
10 Btu of heat will raise
the temperature of 10 lb of
water by 10 F0.
The SI Unit of Heat
Since heat is energy, the joule is the preferred
unit. Then, mechanical energy and heat are
measured in the same fundamental unit.
Comparisons of Heat Units:
1 cal = 4.186 J
1 Btu = 778 ft lb
1 kcal = 4186 J
1 Btu = 252 cal
1 Btu = 1055 J
Temperature and Quantity of
Heat
The effect of heat on temperature depends on the
quantity of matter heated.
The same quantity of heat
is applied to each mass of
water in the figure.
200C
220C
600 g
200C
The larger mass
experiences a smaller
increase in temperature.
200 g
300C
Quiz 1
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
Pre-Lecture Quiz 14
Heat Capacity
The heat capacity of a substance is the heat
required to raise the temperature a unit degree.
Lead
Glass
Al
Copper
Iron
1000C
1000C
1000C
1000C
1000C
37 s
52 s
60 s
83 s
90 s
Heat capacities based on time to heat from zero
to 1000C. Which has the greatest heat capacity?
Conservation of Energy
Whenever 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:
 (Heat Losses) =  (Heat Gained)
Cool
water
Hot
iron
Thermal Equilibrium
Change of Phase
When a change of phase occurs, there is only a
change in potential energy of the molecules. The
temperature is constant during the change.
Liquid Vaporization
Solid
Gas
fusion
Q = mLf
Q = mLv
Terms: Fusion, vaporization, condensation, latent
heats, evaporation, freezing point, melting point.
Change of Phase
The latent heat of fusion (Lf) of a substance is
the heat per unit mass required to change the
substance from the solid to the liquid phase of
its melting temperature.
Q
Lf 
m
For Water: Lf = 80 cal/g = 333,000 J/kg
The latent heat of vaporization (Lv) of a
substance is the heat per unit mass required
to change the substance from a liquid to a
vapor at its boiling temperature.
Q
Lv 
m
For Water: Lv = 540 cal/g = 2,256,000 J/kg
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
ice
steam 540 cal/g
0
100 C
1 cal/gC0
80 cal/g
water
0
ice
ciceand
= 0.5 cal/gC
only
-200C ice water
00C
steam
and
water
steam
only
Q
Example 3 (Cont.): Step one is Q1 to
convert 10 g of ice at -200C to ice at 00C
(no water yet).
-200C
00C
Q1 to raise ice to 00C: Q1 = mct
t
1000C
Q1 = (10 g)(0.5 cal/gC0)[0 - (-200C)]
Q1 = (10 g)(0.5 cal/gC0)(20 C0)
Q1 = 100 cal
00C
-200C ice
cice= 0.5 cal/gC0
Q
Example 3 (Cont.): Step two is Q2 to convert
10 g of ice at 00C to water at 00C.
Melting
t
1000C
Q2 to melt 10 g of ice at 00C: Q2 = mLf
Q2 = (10 g)(80 cal/g) = 800 cal
Q2 = 800 cal
00C
-200C
80 cal/g
ice and
water
Add this to Q1 = 100 cal:
900 cal used to this point.
Q
Step three is Q3 to change 10 g of water at
00C to water at 1000C.
Q3 to raise water at 00C to 1000C.
Q3 = mct ; cw= 1 cal/gC0
00C to 1000C
t
1000C
Q3 = (10 g)(1 cal/gC0)(1000C - 00C)
1
00C
-200C
cal/gC0
Q3 = 1000 cal
Total = Q1 + Q2 + Q3
= 100 +900 + 1000
water
= 1900 cal
only
Q
Step four is Q4 to convert 10 g of water to
steam at 1000C? (Q4 = mLv)
Q4 to convert all water at 1000C
vaporization to steam at 1000C. (Q = mLv)
1000C
Q4 = (10 g)(540 cal/g) = 5400 cal
800 cal
100 cal
00C
ice and
-200C ice water
1000
cal
water
only
5400 cal
Total Heat:
steam
and
water
7300 cal
Q
Quiz 2
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
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.
Quiz 3
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
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.
Heated fluid rises and is then
replaced by cooler fluid, producing
convection currents.
Convection is significantly affected
by geometry of heated surfaces. (wall,
ceiling, floor)
Convection
Heat Transfer by Radiation
Radiation is the process by
which heat energy is transferred
by electromagnetic waves.
Radiation
Atomic
No medium is required !
Sun
Summary: Heat Transfer
Conduction: Heat energy is
transferred by adjacent molecular
collisions inside a material. The
medium itself does not move.
Convection is the process by
which heat energy is
transferred by the actual
mass motion of a heated
fluid.
Radiation is the process by which
heat energy is transferred by
electromagnetic waves.
Examples of Thermal Conductivity
Comparison of Heat Currents for Similar Conditions:
L = 1 cm (0.39 in.); A = 1 m2 (10.8 ft2); t = 100 C0
2050 kJ/s
4980 Btu/h
3850 kJ/s
9360 Btu/h
Concrete or
Glass:
8.00 kJ/s
19.4 Btu/h
Corkboard:
0.400 kJ/s
9.72 Btu/h
Aluminum:
Copper:
THERMODYNAMICS
Thermodynamics 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.
Object C
Thermal Equilibrium
A
A
Object C
B
B
Same Temperature
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.
Quiz 4
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°
c water  4186
J
kgCo
(B) 0.12 C°
(C) 0.37 C°
(D) 0.42 C°
P.E.=mgh
Joule = newton/meter
Heat 14 (13 of 42)
TWO WAYS TO INCREASE THE
INTERNAL ENERGY, U.
+U
WORK DONE
ON A GAS
(Positive)
HEAT PUT INTO A
SYSTEM
(Positive)
TWO WAYS TO DECREASE
THE INTERNAL ENERGY, U.
Wout
Qout
-U
Decrease
hot
WORK DONE BY
EXPANDING GAS:
W is positive
hot
HEAT LEAVES A
SYSTEM
Q is negative
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.
HEAT ENGINES
Hot Res. TH
Qhot
Engine
Qcold
Cold Res. TC
Wout
A heat engine is any
device which through
a cyclic process:
• Absorbs heat Qhot
• Performs work Wout
• Rejects heat Qcold
THE SECOND LAW OF
THERMODYNAMICS
Hot Res. TH
Qhot
Engine
Wout
Qcold
Cold Res. TC
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)!
The Second Law of Thermodynamics
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:
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
Hot Res. TH
QH
W
Engine
QC
The efficiency of a heat engine
is the ratio of the net work
done W to the heat input QH.
e=
W
QH
=
Cold Res. TC
e=1-
QH- QC
QH
QC
QH
REFRIGERATORS
Hot Res. TH
Qhot
Win
Engine
Qcold
Cold Res. TC
A 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
Entropy
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.