chapter 14 heat - siena collegerfinn/courses/phys110/heat.pdf · chapter 14 heat 14-1 heat as...

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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?