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Chapter 14
Heat14-1 Heat As Energy Transfer
We often speak of heat as though it were amaterial that flows from one object to another; itis not. Rather, it is a form of energy.
Unit of heat: calorie (cal)
1 cal is the amount of heat necessary to raise thetemperature of 1 g of water by 1 Celsius degree.
The calories on food labels are kilocalories.
Ch 14: Problem 1
• How much heat (in joules) is required toraise the temperature of 30.0 kg of waterfrom 150C to 950C?
14-1 Heat As Energy TransferIf heat is a form of energy, it ought to be possibleto equate it to other forms. The experiment belowfound the mechanical equivalent of heat by usingthe falling weight to heat the water:
14-1 Heat As Energy TransferDefinition of heat:
Heat is energy transferred from one object toanother because of a difference in temperature.
• Remember that the temperature of a gas isa measure of the kinetic energy of itsmolecules.
14-2 Internal Energy
The sum total of all the energy of all themolecules in a substance is its internal (orthermal) energy.
Temperature: measures molecules’ averagekinetic energy
Internal energy: total energy of all molecules
Heat: transfer of energy due to difference intemperature
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14-2 Internal Energy
Internal energy of an ideal (atomic) gas:
But since we know the average kinetic energyin terms of the temperature, we can write:
(14-1)
14-2 Internal Energy
If the gas is molecularrather than atomic,rotational andvibrational kineticenergy needs to betaken into account aswell.
14-3 Specific Heat
The amount of heat requiredto change the temperatureof a material is proportionalto the mass and to thetemperature change:
(14-2)
The specific heat, c, ischaracteristic of thematerial.
14-3 Specific Heat
Specific heats of gases are more complicated,and are generally measured at constantpressure (cP) or constant volume (cV).
Some samplevalues:
14-4 Calorimetry – Solving Problems
Closed system: no mass enters or leaves, butenergy may be exchanged
Open system: mass may transfer as well
Isolated system: closed system where noenergy in any form is transferred
For an isolated system,
Energy out of one part = energy into another part
Or: heat lost = heat gained
14-4 Calorimetry – Solving Problems
Calorimeter - makesquantitative measurementsof heat exchange.
A sample is heated to awell-measured hightemperature, plunged intothe water, and theequilibrium temperaturemeasured. This gives thespecific heat of thesample.
Qlost = Qgained
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14-4 Calorimetry – Solving Problems
Another type of calorimeter is called a bomb
calorimeter; it measures the thermal energy
released when a substance burns.
This is the way the Caloric content of foods
is measured.
Ch 14: Problem 8
• An automobile cooling system holds 16 Lof water. How much heat does it absorb ifits temperature rises from 200C to 900C?
Ch 14: Problem 13
• A hot iron horseshoe (mass =0.40 kg), justforged, is dropped into 1.35 L of water in a0.30-kg iron pot initially at 25.00C. If thefinal equilibrium temperature if 25.00C,estimate the initial temperature of the hothorseshoe.
14-5 Latent Heat
Energy is required for a material to changephase, even though its temperature is notchanging.
14-5 Latent HeatHeat of fusion, LF: heat required to change 1.0 kgof material from solid to liquid
Heat of vaporization, LV: heat required to change1.0 kg of material from liquid to vapor
14-5 Latent HeatThe total heat required for a phase changedepends on the total mass and the latent heat:
(14-3)
The latent heat of vaporization applies toevaporation as well as boiling.
On a molecular level, the heat added breaks theclose bonds between them so the next phase canoccur.
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Ch 14: Problem 21
• How much heat is needed to melt 16.50kg of silver that is initially at 200C?
Ch 14: Problem 25
• A cube of ice is taken from the freezer at-8.50C and placed in a 95-g aluminumcalorimeter filled with 310 g of water atroom temperature of 20.00C. The finalsituation is observed to be all water at17.00C. What was the mass of the icecube?