Ch 12 Heat
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Updated 5/26/03
PA
STANDARDS: a] Analyze energy sources and transfers of heat.
b] Use conservation & entropy to solve energy & heat problems.
c] Apply and analyze energy sources and conversions and their
relationship to heat and
temperature.
AIR
1.10 ETHANOL
2.45 LITHIUM
3.41 H2SO4
1.54
ALUMINUM 0.903 FREON 11
0.856 MERCURY
0.138 TIN
0.220
AMMONIA 4.60 HELIUM
3.12 METHANOL
2.37 WATER 4.18
BRASS 0.376 GLASS
0.664 NITROGEN
1.039
ICE
2.06
CARBON 0.716 GOLD
0.129 OXYGEN
0.909
STEAM 2.02
CO2
0.775 IRON
0.450 SILVER
0.235 ZINC
0.386
COPPER 0.385 LEAD
0.130 STEEL
0.450
1
a] Describe and interpret the Food Color Demo.
1
One drop of food color is put into a beaker of cold water and a beaker of hot
water. The drop in cold water falls
to the bottom. The drop in hot
water diffuses within 5 min. The
diffusion is caused by the more rapid motion of hot water molecules.
Thus, temp is a measure of how fast the molecules move.
b] Temperature is the measure of the average internal kinetic energy of the
particles (atoms & molecules) in a material. It does NOT include the external K.E. of the whole object if
it is in motion.
c] Heat is the transfer of internal K.E. from one object to another.
This transfer occurs through the collisions of moving surface atoms
(vibrations for solids, translations for fluids).
Heat is energy in motion.
d] Thermal equilibrium between two objects in contact (A & B) occurs when
the transfer from "A" to "B" equals the transfer from
"B" to "A".
2
a] THERMOMETERS reach thermal equilibrium with an object and then measure
some temperature dependent property.
b] LIST 6 EXAMPLES OF HOW THERMOMETERS WORK.
Thermometer
| Temperature Dependent Property
Liquid in glass |
Volume
Dial (meat)
| Length
Digital
| Electrical Resistance
Thermocouple (oven) | Voltage
Color Strip
| Crystal Angle
Pyrometer (fire) |
Color
Liquid in glass thermometers are limited by the freezing point and boiling point
of the liquid. For example, a
mercury thermometer cannot measure temperatures below -39 oC or above 357 oC
c] Calibrate: to adjust an
instrument so that its numbers match those of a standard.
d] To calibrate a thermometer
1] Put it in ice water and
mark 0o on the thermometer
2] Put it in boiling water
and mark 100o on the thermometer
3] Divide the distance
between the marks into 50 equal spaces
each worth
2o.
3
a] HEAT GAIN = MASSxSPECIFIC HEAT x /\T
Q Jules
Q =
m x C
X /\T. b] C = ----- = -----
m /\T g*oC
SPECIFIC HEAT CAPACITY [C] is the
heat needed to raise 1 g 1 oC
It is a measure of how much heat an object can store.
d] It is a property of matter, and can be used to identify unknowns.
e] CH2O/CStone = 4.18/0.664 = 6.30 ---> CH2O = 6.30xCStone
f] Implications: H2O stabalizes
atmospheric temperature. Ex.:
Night vs day temp changes are more extreme in a desert than in an island.
4
a] 50 g OF SILVER & 50 g OF GOLD ARE HEATED TO 100 oC WHICH METAL
WILL CAUSE MORE ICE TO MELT? EXPLAIN.
Specific Heat is a measure of how much heat an object can store.
The object that stores more heat will melt more ice.
Since silver stores 0.235 J per g (=CAg) while gold stores only
0.129 J per g (= CAu), silver will melt more ice. CAg/CAu = 0.235/0.129 = 1.82 ==>silver holds 82% more heat
& melts 82% more ice
b] WILL AN ICE CUBE LOWER THE TEMP OF A GLASS OF WATER OR A GLASS OF WINE
[ETHANOL] FASTER? EXPLAIN.
A gram of ice can absorb a fixed amount of heat, (80 cal).
For equal masses of melted ice the
heat absorbed by water = heat absorbed by ethanol:
Cw/\Tw = Ce/\Te
4x10 =
2x20
Thus, the liquid that stores less heat (small C) must have a larger /\T to
release the same amount of heat. Cethanol
= 2.45 cools faster.
5
HOW MUCH HEAT IS ABSORBED BY 250 g OF WATER WHEN IT IS HEATED FROM
10 oC TO 85 oC? Q = mC/\T--->Q =
250 g*4.18 J/goC*(85-10)oC = 78375 J
6
A 100 g OF GLASS AT 90oC IS MIXED WITH 100 g OF WATER AT 20 oC
a] DERIVE THE EQUATION FOR THE FINAL TEMPERATURE OF A MIXTURE,
HEAT LOST BY HOT = HEAT
GAINED BY COLD
Qh =
Qc
Mh Ch (Th - Tf) = Mc Cc (Tf
- Tc)&nbbsp; Mh & Mc = the hot
& cold mass
Mh Ch Th + Mc Cc Tc Ch
& Cc = the hot & cold SpHeat
Algebra--->Tf = -------------------
Th & Tc = the hot & cold temp
Mh Ch + Mc Cc
Tf = Final Temp
Weighted Avg
Note: If both masses are equal:
Mh = Mc
Ch Th + Cc Tc
Tf =
------------- a weighted
average
Ch + Cc
Note: If both materials are the
same: Ch = Cc
Mh Th + Mc Tc
Tf =
------------- a weighted
average
Mh + Mc
Note: If both masses are equal
& both materials are the same.
Th + Tc
Tf =
-------& an average
2
b] RANGE OF POSSIBLE ANSWERS: Th
> Tf > Tc
c] FINAL TEMPERATURE OF THE MIXTURE. To
avoid mistakes.
1] Identify the given
[2] Use the most general equation
Ex: PRACT 1: 600 g CARBON AT 70oC IS PLACED IN 500 g OF H2O AT 30oC
Hot: Mh = 600 g, Th = 70oC, Ch =
0.716 (Carbon)
Cold: Mc = 500 g, Tc = 30oC, Cc = 4.18 (water)
Mh Ch Th + Mc Cc
Tc 600*0.716*70 + 500*4.18*20
71,872
Tf = ------------------- = -------------------------- = ------ = 28.53
Mh
Ch + Mc Cc
600*0.716 + 500*4.18
2,519
Note: to avoid errors divide the
calculation into three steps
(top,
bottom, divide).
d] ASSUMPTION: No heat lost to or
gained from the surroundings.
7
a] DERIVE THE EQUATION FOR THE SPECIFIC HEAT OF A MATERIAL.
Heat Lost by Hot = Heat Gained by Cold
Qh =
Qc
Mh Ch (Th - Tf)
= Mc Cc (Tf - Tc) [Divide by Mh (Th - Tf)
Mc
Cc (Tf - Tc)
Ch = ---------------
Mh (Th - Tf)
b] A 50 g MASS AT 100oC IS PLACED
IN 100 g OF WATER AT 10oC, Tf = 13.87oC. FIND
THE SPECIFIC HEAT & THE MATERIAL.
Mc Cc (Tf - Tc)
100*4.18*(13.87 - 10) 1617.66
Ch = --------------- = --------------------- = ------- = 0.376
Mh (Th -
Tf) 50*(100
- 13.87)> 4306.50
Note: to avoid errors divide the
calculation into three steps
(top,
bottom, divide).
ANS: Ch = 0.376-->brass (see
table)
8
a] THE 5 TYPES OF HEAT TRANSFER.
RADIATION--Heat transfer by infrared light.
Ex: Sun warming the earth, toaster.
Note: Microwave Ovens transfer
energy not heat, they warm by vibrating water and don't heat things without
water, i.e. styrofoam.
NATURAL CONVECTION--Heat transfer by the flow of a fluid caused by a hot or cold
spot. Ex:
Heat flow around a flame or radiator.
FORCED CONVECTION--Heat transfer by the flow of a fluid caused by a pump or a
fan. Ex: hot water flowing through a radiator.
CONDUCTION--heat transfer by direct contact.
Ex: Heat flow through a metal spoon
left in a cup of hot coffee.
EVAPORATION: heat transfer by a
liquid to gas phase change.
Liquids absorb heat to evaporate: Ex:
sweating, each gram of sweat absorbs 2,260 J from your body to evaporate.
b] See board.
Heat
Theory
Updated 5/26/03
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CLASSIFY
8 TYPES OF HEAT ENGINES.
RECIPROCATING ROTARY
EXTERNAL COMBUSTION STEAM
[1] STEAM TURBANS [5]
INTERNAL COMBUSTION GASOLINE
ROTARY [6]
4 CYCLE [2] GAS
TURBAN [7]
2 CYCLE [3]
DIESEL [4] ROCKET
[8]
KINETIC
THEORY: A] ALL MATTER CONSISTS OF
MINUTE PARTICLES WHICH ARE IN CONSTANT MOTION.
DUE TO THEIR MOTION THEY HAVE KINETIC ENERGY. B] THESE PARTICLES MUST ATTRACT ONE ANOTHER.
OTHERWISE, NO SOLID OR LIQUID EXIST
IT
IS IMPOSSIBLE AND IMPRACTICAL TO TRY TO FIND THE TOTAL INTERNAL ENERGY OF ANY
GIVEN OBJECT. HOWEVER, STUDYING THE
CHANGE IN INTERNAL ENERGY WHEN ENERGY IS ADDED OR REMOVED FROM AN OBJECT WILL
YIELD GREAT SUCCESS IN SOLVING PROBLEMS. THIS
TRANSFER OF ENERGY IS CALLED HEAT ENERGY.
SYSTEM--ANY
WELL-DEFINED COLLECTION OF OBJECTS: A
SINGLE SPRING, OR A MAGNET, OR AN IDEAL GAS IN A CONTAINER, OR SOME COMPLICATED
GADGET WITH MANY INTERNAL PARTS.
COLD--A
TERM THAT DESCRIBES THE HEAT ENERGY OF ONE OBJECT RELATIVE TO THE HEAT ENERGY OF
ANOTHER. COLD DOES NOT FLOW, HEAT
FLOWS.
1
DEFINE THERMAL ENERGY, HEAT, TEMPERATURE, AND ABSOLUTE ZERO
THERMAL ENERGY--THE TOTAL INTERNAL K.E. & P.E. OF A SYSTEM
HEAT--A MEASURE OF THE INTERNAL K.E. TRANSFERRED FROM ONE OBJECT TO ANOTHER
TEMPERATURE--THE MEASURE OF AN OBJECT'S AVG INTERNAL KINETIC ENERGY.
ABSOLUTE ZERO [-273 oC, -460 oF]--ALL TRANSLATIONAL ENERGY DISAPPEARS BUT A
GREAT RESERVOIR OF P.E. AND A SMALL AMOUNT OF VIBRATIONAL ENERGY REMAIN.
[AVERAGE K.E. = 0, GAS VOLUME = 0].
2
NAME THE 4 TEMPERATURE SCALES AND THEIR STANDARDS.
WATER. C = 5/9[F - 32], F = 9/5 C +
32; AN IDEAL GAS.
K = oC + 273, OR oC = K - 273
R = oF + 460, OR oF = R - 460
4a
HOW DO THERMOMETERS WORK? THEY
REACH EQUILIBRIUM WITH THE OBJECT THEY CONTACT, AND THEN MEASURE SOME
TEMPERATURE DEPENDENT PROPERTY.
b LIST 5 TYPES OF
THERMOMETERS & THE PROPERTY USED TO MEASURE TEMP.
LIQUID IN GLASS--MERCURY OR ALCOHOL -- VOLUME.
THERMOCOUPLES [OVEN THERMOSTATS] -- VOLTAGE;
RESISTANCE [DIGITAL] -- ELECTRICAL RESISTANCE [-259 TO 962 oC]
BIMETALIC [HOUSE THERMOSTATES] -- DISTANCE;
COLOR STRIP [FISH TANK] -- CRYSTAL ORIENTATION
HELIUM -- VAPOR PRESSURE [VERY LOW TEMPERATURES]
OPTICAL PYROMTER-- COLOR OF GLOWING SUBSTANCES [VERY HIGH TEMPS]
c WHAT LIMITS THE
USE OF Hg THERMOMETER? BP & FP OF Hg, -39oC TO 357 oC
11
DEFINE AND GIVE ONE EXAMPLE OF THE 4 TYPES OF HEAT TRANSFER.
RADIATION--HEAT TRANSFER BY INFRARED LIGHT.
NATURAL CONVECTION--HEAT TRANSFER BY THE FLOW OF A FLUID CAUSED BY A HOT OR COLD
SPOT.
FORCED CONVECTION--HEAT TRANSFER BY THE FLOW OF A FLUID CAUSED BY A PUMP OR A
FAN.
CONDUCTION--HEAT TRANSFER BY DIRECT CONTACT.
12
WRITE THE EQUATION FOR THE RATE OF HEAT TRANSFER & DEFINE THE
VARIABLES.
2
USE THE EQUATION FOR HEAT GAIN [Q = mC/\T] TO EXPLAIN YOUR ANSWERS TO:
a] FIFTY GRAMS OF METAL X AND FIFTY GRAMS OF METAL Y ARE HEATED TO THE
SAME TEMPERATURE.
THE PIECES OF METAL ARE PLACED ON A BLOCK OF ICE
WHICH METAL WILL CAUSE MORE
ICE TO MELT? SINCE m & /\T ARE
EQUAL THE METAL THAT HOLDS MORE HEAT WILL BE THE ONE WITH THE GREATER SPECIFIC
HEAT CONTENT [C].
COMPARE SILVER & GOLD.
Csilver/Cgold = 0.235/0.129 = 1.822; thus silver holds 82.2% more heat.
b] WILL AN ICE CUBE LOWER THE TEMPERATURE OF A GLASS OF WATER OR A
GLASS CONTAINING AN
ALCOHOLIC BEVERAGE FASTER? WHY?
It takes 4.18 J to lower a gram of water, and 2.45 J.
A gram of ice can absorb a fixed amount of heat, Ex 100 J.
Thus it will lower a g of H2O /\T = Q/mC = 100 J/(1g 4.18) = 19.31oC
& a g of ETHANOL /\T = Q/mC = 100 J/(1g 2.45) = 28.99 oC
Heat
Engines & the 2nd Law
Updated 5/26/03
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ADDITIONAL
QUESTIONS
STATE THE FIRST LAW OF THERMODYNAMICS.
Q = /\U + W: Q = HEAT
SUPPLIED TO A SYSTEM, /\U = CHANGE IN INTERNAL E, W = WORK DONE BY SYS
STATE & GIVE AN EXAMPLE OF THE SECOND LAW OF THERMODYNAMICS.
HEAT ENERGY IS ___ WHEN CHANGED TO ANOTHER FORM.
DOES TEMPERATURE FLOW FROM A HOT OBJECT TO A COLD ONE?
EXPLAIN.
SKETCH & LABEL THE PARTICLE FRACTION vs K.E. FOR 2 TEMPS.
[CHEM 324]
___ NEVER DECREASES IN NATURAL PROCESSES.
IS ENERGY LOST WHEN A FUEL IS BURNED?
EXPLAIN
___ NEVER DECREASES IN NATURAL PROCESSES.
A ___ ENGINE CONTINUOUSLY CONVERTS THERMAL ENERGY TO MECHANICAL ENERGY
A ___ USES MECHANICAL ENERGY TO TRANSFER HEAT FROM AN AREA OF LOWER TEMP
TO AN AREA OF HIGHER TEMP.
IF YOU PLACE IDENTICAL UNCOVERED PAILS OF HOT AND COLD WATER OUTDOORS ON
A DRY SIDEWALK ON A VERY COLD DAY, THE HOT WATER USUALLY STARTS TO FREEZE FIRST.
EXPLAIN. WHAT WOULD HAPPEN
IF THE PAILS WERE COVERED?
EXPLAIN WHY FRUIT GROWERS SPRAY THEIR TREES WITH WATER, WHEN FROST IS
EXPECTED, TO PROTECT THE FRUIT FROM FREEZING.
WHY DOES WATER IN A CANTEEN STAY COOLER IF IT HAS A CANVAS COVER THAT IS
KEPT WET?
14
a DEFINE
HEAT ENGINE & HEAT PUMP.
HEAT ENGINES: ANY DEVICE OPERATING
THROUGH A CYCLE, CONVERTS [SOME] HEAT TO WORK & DISCRADS THE REMAINDER INTO
A COLD RESERVOIR.
HEAT PUMP: A HEAT ENGINE RUN
BACKWARDS--CONVERTS WORK INTO HEAT
THE
2nd LAW OF THERMODYNAMICS--NATURAL PROCESSES GO IN THE DIRECTION THAT INCREASES
RANDOMNESS--ENTROPY. EX:
HEAT FLOWS SPONTANEOUSLY FROM A HOTTER TO A COLDER OBJECT BUT NOT THE
REVERSE. EVEN THE MOST IDEAL ENGINE
WILL GENERATE SOME WASTE HEAT.
TO
USE THE ENERGY STORED IN CHEMICAL OR NUCLEAR FUELS, SUCH AS COAL, OIL, NATURAL
GAS, OR URANIUM, ONE TYPICALLY FIRST CONVERTS THE FUEL'S POTENTIAL ENERGY INTO
THERMAL ENERGY AND THEN CONVERTS SOME OF THE THERMAL ENERGY TO WORK.
CHEMICAL OR NUCLEAR POTENTIAL ENERGY CAN BE CONVERTED TO THERMAL ENERGY
WITH 100% EFFICIENCY. HOWEVER, A PROCESS IN WHICH HEAT IS CONVERTED TO WORK THROUGH
THE USE OF A HEAT ENGINE OPERATING OVER A CYCLE CAN NEVER HAVE 100% EFFICIENCY.
A
HEAT ENGINE IS DEFINED AS ANY DEVICE THAT IN OPERATING THROUGH A CYCLE, CONVERTS
SOME HEAT TO WORK AND DISCARDS THE REMAINDER INTO A COLD RESERVOIR.
A HEAT ENGINE IS ALWAYS RETURNED TO ITS INITIAL STATE IN A CYCLE.
THE HEAT ENGINES WE CONSIDER ARE IDEALIZED:
THEY HAVE NO FRICTION. BUT
EVEN IF NO ENERGY IS LOST TO FRICTION, AN ENGINE CAN NEVER BE PERFECTLY
EFFICIENT. THE REASONS ARE FAR MORE
FUNDAMENTAL.
EFFICIENCY
= WORK OUT/HEAT IN =
W/Q
W = WORK OUT PER CYCLE AND Q = THE HEAT IN PER CYCLE. WHEN WE APPLY THE FIRST LAW OF THERMODYNAMICS TO ONE CYCLE,
WE GET
Q = /\U + W
Qh - Qc = 0 + W; Qh = HEAT IN AT THE HIGH TEMP, Qc = HEAT OUT AT COLD TEMP.
THIS SAYS THAT THE WORK OUT EQUALS THE NET HEAT IN.
IN COMPLETING ONE CYCLE, THE SYSTEM RETURNS TO ITS INITIAL STATE. THEREFORE, /\U = 0. USING
THIS RESULT WE HAVE: EFFICIENCY = (Qh
- Qc)/Qh = 1 - Qc/Qh.
THIS
SHOWS THAT AN ENGINE CAN HAVE 100% EFFICIENCY ONLY IF Qc =0.
AN ENGINE CAN BE PERFECTLY EFFICIENT ONLY IF NO THERMAL ENERGY IS
EXHAUSTED TO THE COLD RESERVOIR. THIS IS IMPOSSIBLE.
THE
REASON? LIKE ANY OTHER FUNDAMENTAL
LAW IN PHYSICS, IT IS CONFIRMED BY THE CIRCUMSTANCES THAT NO EXCEPTION TO IT
HAVE EVER BEEN FOUND.
THE
SECOND LAW OF THERMODYNAMICS CAN BE STATED AS:
NO HEAT ENGINE, REVERSIBLE OR IRREVERSIBLE, OPERATING IN A CYCLE, CAN TAKE IN
THERMAL ENERGY FROM ITS SURROUNDINGS AND CONVERT IT ALL TO WORK.
LORD KELVIN 1851.
A
HEAT ENGINE RUN IN REVERSE IS A HEAT PUMP.
WORK W IS DONE ON A SYSTEM, HEAT Qc IS EXTRACTED FROM THE LOW-TEMP
RESERVOIR, AND HEAT Qh IS EXHAUSTED TO THE HIGH-TEMP RESERVOIR. THE NET EFFECT IS THAT HEAT IS PUMPED FROM THE LOW TO THE
HIGH RESERVOIR. A HEAT PUMP IS
EFFECTIVELY A REFRIGERATOR.
.C.*PHASE
CHANGE
1
CAN YOU ADD THERMAL ENERGY TO AN OBJECT WITHOUT INCREASING ITS
TEMPERATURE? EXPLAIN
2
IF YOU PLACE IDENTICAL UNCOVERED PAILS OF HOT AND COLD WATER OUTDOORS ON
A DRY SIDEWALK ON A VERY COLD DAY, THE HOT WATER USUALLY STARTS TO FREEZE FIRST.
a] EXPLAIN; b] WHAT WOULD HAVE HAPPENED HAD THE PAILS BEEN COVERED?
3
ARE THE COILS OF AN AIR CONDITIONER THAT ARE INSIDE THE HOUSE THE
LOCATION OF VAPORIZATION OR CONDENSATION OF THE FREON?
EXPLAIN.
4
EXPLAIN WHY FRUIT GROWERS SPRAY THEIR TREES WITH WATER, WHEN FROST IS
EXPECTED, TO PROTECT THE FRUIT FROM FREEZING.
5
WHY DOES WATER IN A CANTEEN STAY COOLER IF IT HAS A CANVAS COVER THAT IS
KEPT WET?
6
WHY DO SOME FUSION REACTORS LIQUID LITHIUM AS A COOLANT
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