Chapter 15 Temperature, Heat & Thermal Expansion

Apr 20, 2023 Physics 1 (Garcia) SJSU

Chapter 15Temperature, Heat & Thermal Expansion

Part III: Heat


Temperature

Temperature of an object indicates average internal energy (due to molecular motion) of the object.

Temperature Scales


Total versus Average

The total amount of money in this room is probably around $1000.

The average amount of money per person is probably around $20.

Temperature of an object depends on the average amount of energy per molecule, not the total.

Bucket of warm water can have more internal energy than cup of hot water.

80 °F

100 °F


Internal EnergyInternal energy of an

object depends on:• Temperature• Mass• Material

1 kg

Temperature Internal

Energy

300 K 120,000 J

200 K 80,000 J

100 K 40,000 J

0 K 0 Joules

Iron


Energy

300 K 120 J

200 K 80 J

100 K 40 J

0 K 0 Joules

Iron

1 gram


Energy

300 K 1,200,000 J

200 K 800,000 J

100 K 400,000 J

0 K 0 Joules

Water

1 kg

1000 grams


Money and Happiness

Some people need a lot of money to make them happy. Some don’t.

Some materials, such as water, need a lot of energy to raise their temperature.

Some materials, such as iron, need little energy to raise their temperature.

Nicole Richie & Paris Hilton

MAHATMA GANDHI


Increasing Internal Energy

Can increase internal energy (and temperature) by tapping energy sources.

Chemical energy released in fire

Electric energy heats burner


Work and Heat

May increase internal energy by exerting a force to do mechanical work.

Rub hands togetherfor warmth

Strike an iron surface with great force and red-hot sparks are created


Demo: Work and Heat

Increase internal energy (and thus temperature) by doing mechanical work on an object.

SHAKE

Bottle of Mercury

Temperature increases by a few degrees


Specific Heat Capacity

Specific heat capacity is the internal energy required to raise one gram of a material by one degree of temperature.

Filling has high specific heat capacityCrust has low specific heat capacity

Filling and crust at same temperature yet mouth burned only by the filling.


Demo: Sparklers

Iron burns red-hot at a temperature of 5000 ºF

Sparks from sparkler don’t burn skin because they have very little energy (small mass and low heat capacity).

Walking on red-hot embers is safe for the same reason.


Demo: Boil Water in Paper Cup

Because of high heat capacity of water, the large amount of heat added by the flame raises the temperature of the water until boiling.

If the cup is filled with sand instead of water then it burns quickly.

Cup with sand

Cup with water


Thermal Expansion

Due to increased molecular motion, most materials expand as temperature increases.

Sidewalk buckles and cracks due to expansion on a hot summer day

Space allows for expansion


Demo: Slowing Air Molecules

Cool balloon using liquid nitrogen

Air molecules slow down and lose kinetic energy

Balloon slowly warms up,

restoring energy

Balloon returns to its original state


Demo: Expansion of a Ring

Metal ball barely fits past the metal ring.

Not surprising that heated ball won’t pass through cold ring.

Will cold ball pass through heated (expanded) ring?


Demo: Bi-metallic Strip

Different materials have different rates of expansion.

STEEL

Brass

Brass expands more than steel when heated

Thermostat

Bi-metallicSpiral strip

HOT COLD


Demo: Heat, Cool, Break

COOL (quickly)

HEAT

Glass expands when heated. If hot glass is cooled quickly, exterior cools before the interior. Exterior contracts faster than the interior, cracking the glass.

GLASS

Cracks form


Chapter 16Heat Transfer


Heat Transfer

Heat always flows from high temperature objects to low temperature objects.

Heat flow stops when temperatures equal.

Various ways by which heat may flow.

98º

32º

75º

Heat flows fromchild and air into the ice cream

Heat flows fromchild into air


Conduction

Conduction is heat flow by direct contact.

Some materials are good thermal conductors, others are insulators.

98º

75º

98º

75º

Wood is aninsulator

Tile is aconductor

Tile floor feels colder than wood floor


Demo: Torch the Money

Wrap a dollar bill tightly around a copper pipe. Put it into a flame.


Air is a Poor Conductor

Can safely put your hand in an oven.Metal is good conductor so you need oven mitt to touch it safely (cloth is a poor conductor).

Because air is such a poor conductor, some pizza ovens don’t have a door.


Demo: Boiling Ice Water

Water and glass are relatively poor conductors of heat.

Can boil water at the top of a test tube with ice at the bottom of the tube.

Steel wool prevents ice from floating


Convection

Heat transfer in a fluid often occurs mostly by convection.

Buoyancy causes warm air to rise, which carries thermal energy directly by its motion.


Demo: CandleVery HOT

Warm

Shadows revealrising air currents of hot air.

Rising hot air above a candle carries most of the heat generated by the burning flame.


Convection OvenConvection oven has a fan to enhance the circulation of the air, increasing the transfer of heat.


Fiberglass Insulation

Air is a poor thermal conductor but easily transfers heat by convection.

Fiberglass insulation is mostly air, with the fibers disrupting the convection flow.


Radiation

Light has many different wavelengths, most of which are not visible to the eye.

All light carries energy, thus transfers heat.

Heat Lamp


Emission of Radiant Energy

All objects radiate light; higher the temperature the higher the frequency.

At room temperature the radiated light is at frequencies too low for our eyes to see.

Special cameras are sensitive to this infrared radiation.

Attics in this house were kept warm for growing marijuana.

70

98º

75º


Reflection of Radiant Energy

White objects reflect light, black objects don’t.

Hole in a box with white interior looks black because almost none of the light entering the hole reflects back out.

White tubes look black inside.


Controlling Heat Transfer

Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum.

Silvered interior walls minimize heat transfer by radiation.


Greenhouse EffectGlass is transparent to sunlight (short-wavelength).

Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat.


Greenhouse Carbon Dioxide

Over past 1000 years temperatures nearly constant until CO2 emissions increased starting with the industrial revolution.

Industrial revolution begins


Chapter 17Changes of Phase


Phases of Matter

Four Phases of Matter:

• Solid

• Liquid

• Gas

• Plasma

Change of phase occurs when we pass from one phase to another, such as water (liquid) boiling to change into vapor (gas).

Ice

Water

SteamPlasma


Evaporation

Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid.

GAS

LIQUID

A random molecule at the surface acquires enough energy to escape the attraction force among the molecules (which holds the liquid together).


Evaporative Cooling

Because only the most energetic molecules can escape the surface, evaporation removes internal energy from the liquid, that is, evaporation cools.

WETCLOTH

Wet towel cools head

WETTONGUE

Wet tongue cools dog

WETBODY &TOWEL

Wetness cools person

Brr

HEAT

HEATHEAT


Condensation

Condensation is the reverse of evaporation, a change of phase from gas to liquid that takes place at the surface of a liquid.

GAS

LIQUID

A random molecule from the gas strikes the surface and sticks instead of bouncing back into the gas.

Condensation heats.


Hot and Humid

A 90 degree day in a dry climate, like San Jose, is more comfortable than a 90 degree day in a humid place like New Orleans.

In a dry climate you’re cooled by evaporation, in a wet climate you’re heated by condensation.

Heat index is the apparent temperature a person feels for a given humidity.


Demo: Wet/Dry Bulb Thermometer

Pair of thermometers; one is kept dry while the other’s bulb is wrapped in wet cloth.

Difference of their temperatures gives relative humidity.

Large temperature difference indicates high or low humidity?Low humidity; evaporative cooling is significant.

Dry bulb

Wet bulb


Fog & Clouds

Warm air rises. As it rises, it expands. As it expands, it cools. As it cools, vapor molecules condense into water droplets. This forms a cloud (or fog if warm, moist air cools near the ground).

Warm

Cool

Warm breath feels cool when it expands

Water vapor(gas) is invisible

As vapor expands, it coolsand tiny, visible, water droplets (liquid) condense.


Tiny bubbles grow due to evaporation at their surface

BoilingWhen the temperature of a liquid is high enough

that evaporation occurs everywhere, not just the surface, then the liquid boils.

The temperature required depends on the pressure; lower the pressure, the lower the boiling temperature (boiling point).


Liquid NitrogenLiquid nitrogen boils at

atmospheric pressure and room temperature.

Boiling point is -320 ºF and freezes at -346 ºF.


Demo: Slowing Air Molecules

Cool balloon using liquid nitrogen

Air molecules slow down and lose kinetic energy

Balloon slowly warms up,

restoring energy

Balloon returns to its original state


Demo: Low Pressure Boiling

Water boils at room temperature if the pressure is low.

Cooking at high altitudes is difficult due to this effect; coffee brewed in the mountains always tastes lukewarm.


MeltingMelting is the change

of phase from solid to liquid.

Melting is a cooling process; the solid must absorb heat to melt.


Sublimation

Sublimation is change of phase from solid to gas without passing through liquid phase.

Solid carbon dioxide (dry ice) sublimates at a chilly -109 °F.

Put dry ice into warm water to create dense fog of tiny water droplets.


Demo: Carbon Dioxide

Carbon dioxide, released when dry ice sublimates, is heavier than air.

Bubbles float on layer of dry ice.

(a) Burning candle(b) Extinguished under CO2 layer

(c) Scoop out some CO2 in a cup(d) Pour it on candle to extinguish


Freezing

Freezing is the opposite of melting, that is, the change of phase from liquid to solid.

Heat must be removed from a liquid in order to freeze it into a solid.

Lava (liquid) freezes into rock (solid), heating the seawater.

Seawater (liquid) boils into vapor (gas), cooling the lava.


Demo: Freeze Solid

Materials become brittle when frozen solid.

Organic materials appear solid but cells contain large amounts of liquid water.


Energy & Changes of Phase


Heats of Fusion & Vaporization

Heating a gram of water

80 cal 100 cal 540 cal 720 calHeat of Fusion

Heat Capacity Heat of Vaporization

Total Energy


Check Yourself

Is boiling a cooling or a warming process?Boiling is a cooling process.So can you cool your hand by putting it in

boiling water?NO! Ouch!So why is boiling a cooling process?Because when a liquid boils it cools by itself

releasing its most energetic molecules, just as with cooling by evaporation.

18 Thermodynamics

Adiabatic Processes• Compressing or expanding a gas while no heat enters or

leaves the system is said to be an adiabatic process (from the Greek for “impassable”).

• Adiabatic conditions can be achieved by thermally insulating a system from its surroundings (with Styrofoam, for example) or by performing the process so rapidly that heat has no time to enter or leave.

• In an adiabatic process, therefore, because no heat enters or leaves the system, the “heat added” part of the first law of thermodynamics must be zero. Then, under adiabatic conditions, changes in internal energy are equal to the work done on or by the system.

• For example, if we do work on a system by compressing it, its internal energy increases: We raise its temperature. We notice this by the warmth of a bicycle pump when air is compressed. If work is done by the system, its internal energy decreases: It cools. When a gas adiabatically expands, it does work on its surroundings, releasing internal energy as it becomes cooler. Expanding air cools.

Meteorology and the First Law

Air temperature rises as heat is added or as pressure is increased.

Heat is added by solar radiation, by long-wave Earth radiation, by moisture condensation, or by contact with the warm ground resulting in an increase in air temperature

The atmosphere may lose heat by radiation to space, by evaporation of rain falling through dry air, or by contact with cold surfaces. The result is a drop in air temperature.

Some changes are —small enough that the process is nearly adiabatic. Then we have the adiabatic form of the first law: Air temperature rises (or falls) as pressure increases (or decreases).

The temperature of a parcel of dry air that expands adiabatically decreases by about 10°C for each kilometer of elevation.

Figure 18.6

. As a parcel flows up the side of a mountain, its pressure lessens, allowing it to expand and cool. The reduced pressure results in reduced temperature. 5 Measurements show that the temperature of a parcel of dry air will decrease by 10°C for a decrease in pressure that corresponds to an increase in altitude of 1 kilometer. So dry air cools 10°C for each kilometer it rises

On the other hand, if air at a typical temperature of −20°C at an altitude of 6 kilometers descends to the ground, its temperature would be a whopping 40°C. A dramatic example of this adiabatic warming is the chinook—a wind that blows down from the Rocky Mountains across the Great Plains. Cold air moving down the slopes of the mountains is compressed into a smaller volume and is appreciably warmed

Chinooks, which are warm, dry winds, occur when high-altitude air descends and is adiabatically warmed

Figure 18.8

• When the upper regions of the atmosphere are warmer than the lower regions, we have a temperature inversion. If any rising warm air is denser than this upper layer of warm air, it will rise no farther. It is common to see evidence of this over a cold lake where visible gases and particles, such as smoke, spread out in a flat layer above the lake rather than rise and dissipate higher in the atmosphere. Temperature inversions trap smog and other thermal pollutants

temperature inversion.

Figure 18.9

Smog in Los Angeles is trapped by the mountains and a temperature inversion caused by warm air from the Mojave Desert overlying cool air from the Pacific Ocean.

• Do rising parcels of molten material cool faster or slower than the surrounding material?

• Do sinking parcels heat to temperatures above or below those of the surroundings? The answers to these questions are not known at this writing.

Do convection currents in the Earth’s mantle drive the continents as they drift across the global

surface?

Figure 18.12 • These stages can be put differently:

• (a) suck, • (b) squeeze, • (c) bang, • (d) push, and • (e) blow

• A four-cycle internal-combustion engine.

.

A simplified steam turbine. • The turbine turns because

pressure exerted by high-temperature steam on the front side of the turbine blades is greater than that exerted by low-temperature steam on the back side of the blades.

• Without a pressure difference, the turbine would not rotate and deliver energy to an external load (an electric generator, for example).

• The presence of steam pressure on the back side of the blades, even in the absence of friction, prevents the turbine from being a perfectly efficient engine.

The Laws of Thermodynamics

• The first law of thermodynamics states that energy can neither be created nor destroyed. It speaks of the quantity of energy.

• The second law qualifies this by adding that the form energy takes in transformations “deteriorates” to less useful forms. It speaks of the quality of energy, as energy becomes more diffuse and ultimately degenerates into waste.

• Heat, diffused into the environment as thermal energy, is the graveyard of useful energy.

In natural processes, high-quality energy tends to transform into lower-quality energy—order tends

toward disorder.

• Processes in which disorder returns to order without any external help don’t occur in nature. Interestingly, time is given a direction via this thermodynamic rule. Time’s arrow always points from order to disorder. 12

Disordered energy can be changed to ordered energy only with organizational effort or work input

• there is always an increase of disorder somewhere else to more than offset the increase of order

Entropy

• a measure of the amount of disorder in a system.

• More entropy means more degradation of energy

• The net entropy in the universe is continually increasing (continually running “downhill”).

• Energy must be transformed into the living system to support life. When it isn’t, the organism soon dies and tends toward disorder.

• The first law of thermodynamics is a universal law of nature to which no exceptions have been observed. The second law, however, is a probabilistic statement. Given enough time, even the most improbable states may occur; entropy may sometimes decrease. For example, the haphazard motions of air molecules could momentarily become harmonious in a corner of the room, just as a barrelful of pennies spilled on the floor could all come up heads. These situations are possible, but they are not probable. The second law tells us the most probable course of events, not the only possible one.

• Why is the motto of this contractor—“Increasing entropy is our business”—so appropriate?

The laws of thermodynamics are often stated this way:

• You can’t win (because you can’t get any more energy out of a system than you put into it),

• you can’t break even (because you can’t get as much useful energy out as you put in),

• you can’t get out of the game (entropy in the universe is always increasing).

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Chapter 15 Temperature, Heat & Thermal Expansion