Physics 170 - MechanicsLecture 35

Heat

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Definition and Units of Heat

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Heat is a form of energy, and therefore is measured injoules.

There are other units of heat, the most common one is the kilocalorie:

One kilocalorie (kcal) is defined as the amount of heatneeded to raise the temperature of 1 kg of water by 1Co (from 14.5 Co to 15.5 Co).

One calorie is 0.001 kilocalorie.

Definition and Units of Heat

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Confusingly, the calories listed on nutrition labels in theU.S. are really kilocalories (sometimes called Calories).Some other labels are more accurate (left, Australia;right, Germany).

Definition and Units of Heat

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This figure illustrates the three most common units ofheat.

Definition and Units of Heat

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So, if heat is energy, howdo kilocalories convert tojoules? Careful experimentation using the apparatusgives the answer, as thework done by the fallingweights raises thetemperature of thewater.This relationship is calledthe mechanicalequivalent of heat.

Specific Heat and Calorimetry

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The amount of heat needed to increase the temperature of a solid or liquid depends on the amount of the substance and the properties of the substance itself.

The constant c is called the specific heat of thesubstance.

Specific Heat and Calorimetry

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Specific Heat and Calorimetry

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Calorimetry is the quantitative measurement of heat exchange; it is done using a calorimeter.

A calorimeter is insulated from the environment,minimizing heat exchange.

Therefore, heat lost by one object in the calorimeter must be gained by another.This is one way of measuring specific heats.

Specific Heat and Calorimetry

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Specific heat can be defined for gases as well, but gases do not have constant volume or pressure.We therefore define two specific heats for gases—one at constant volume (cV ), and one at constant pressure (cP ).For a particular gas, cP is always greater than cV .

Phase Changes and Latent Heat

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Three phases of matter: solid, liquid, and gas.Solid has a definite shape and the strongest intermolecular bonds.Liquid flows but is relatively incompressible, so it has adefinite volume.Gas is compressible, and will expand to fill a container.

Phase Changes and Latent Heat

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Phase changes:Solid →liquid: meltingLiquid →gas: evaporating, boilingGas →liquid: condensingLiquid →solid: freezingSolid →gas: sublimating

Latent heat is the amount of heat absorbed or released when a substance undergoes a phase transition.

Phase Changes and Latent Heat

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During a phase transition, the heat energy goes tochanging the intermolecular bonds, and thetemperature does not change.

The heat needed for a phase change is:

Here, L is the latent heat; Lf is the latent heat of fusion(solid ↔liquid) and Lv the latent heat of vaporization.

Phase Changes and Latent Heat

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Latent heat is a property of a particular substance.

Phase Changes and Latent HeatHeat added as ice becomes steam:

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Heat Transfer

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Heat transfer takes place via three mechanisms:

1. Conduction

2. Convection

3. Radiation

Heat Transfer

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Conduction is the transfer of heat through a substance.

If the cup that holds your hot coffee, is a good conductor of heat, you will need a sleeve to keep from burning your hand.

Typically, metals are good conductors of heat—they have electrons that are free to move throughout the material—and nonmetals are not.

Nonconductors of heat are also called insulators.

Heat Transfer

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The heat flow rate through a slab of material isproportional to its surface area and to the temperaturedifference, and inversely proportional to its thickness.

The constant k is called the thermal conductivity.

Heat Transfer

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This diagram illustrates the geometry of heat transfer by conduction

Heat Transfer

The thermal conductivities of substances vary widely.

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Heat Transfer

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When insulating a house, we want materials whosethermal conductivity is as low as possible.

Heat Transfer

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Heat transfer in fluids is mostly by convection, which isthe result of mass transfer; that is, heat is transferredas warmer fluid moves to replace cooler fluid.Convection may be spontaneous (as below) or forced.

Heat Transfer

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Many homes are heated using forced hot air; this is anexample of forced convection.

Heat Transfer

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In the atmosphere, convection includes large- andsmall-scale rising and sinking of air masses and smallerair parcels. These vertical motions distribute heat andmoisture throughout the atmospheric column andcontribute to cloud and storm development (whererising motion occurs) and dissipation (where sinkingmotion occurs)

Heat Transfer

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Radiation is the only type of heat transfer that can take place through a vacuum. You can feel the radiation of heat when you stand near a fireplace. This radiation is in the form of electromagnetic waves, in the infrared part of the spectrum.

Heat Transfer

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The rate of energy radiation is given by Stefan’s law:A is the object’s surface area, T is its temperature, ande is a number between 0 and 1 called the emissivity. σis the Stefan– Boltzmann constant:

A good emitter of radiation is also a good absorber.

Heat Transfer

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Here we see the three types of heat transfer:

Heat Transfer

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Passive solar heating uses the changing angle of theSun to warm buildings in the winter but not in thesummer.

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Example

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