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Chap 15 on “Temperature, Heat, andExpansion”

Temperature

•^

How hot something feels is a measure of the kinetic energy of theconstituent atoms/molecules – these are continually randomlyjiggling. We’ll study concepts and relationships betweentemperature, heat, energy, expansion. Temperature Tells us how warm or cold an object is with respect to some standard.Proportional to the average “translational” kinetic energy of molecules

i.e. motion carrying molecule from one place to

another, as opposed to rotational or vibrational motion – the latter twodon’t directly affect temperature. Eg. Microwave oven: microwaves cause water molecules in food to oscillate

with considerable

rotational

KE. But to get the food to cook (i.e. temp to

rise), these molecules bounce into neighboring molecules imparting theirKE.

Themal EquilibriumThe expansion of the liquid in a thermometer depends on the

liquid’s

temperature. So how come we say it’s reading is the temp of the objectsurrounding it ??Because of “

thermal equilibrium

”:^

Energy flows between two

objects in contact with each other until they reach the sametemperature. The thermometer must be small enough that it doesn’t affect the tempof the object you want to measure.E.g. Can measure your body’s temp with thermometer but can’tmeasure temp of drop of water with it, since contact between thethermometer and drop can change the drop’s temp.

Heat

•^

Heat = energy transferred from one object to another due to atemperature difference between them

-^

Not

a property of the material – i.e. don’t say an object “contains heat”, rather heat is

energy in transit

c.f.

idea of work)

•^

Rather, an object contains “

internal energy

” – sum of translation

kinetic (giving rise to temp), rotational kinetic, vibrational, andpotential (from intermolecular forces).

-^

Note, temperature is

not

the same thing as heat!

Eg. Consider boy holding a sparkler – 2000

o C sparks don’t bother him since

they are so small – very little internalenergy although very high temperature

Eg. Cup of very high-tempwater contains

less

internal

energy than large bucket ofwarm water.

Clicker Question

When you touch a giant iceberg,A) heat flows from your finger to the iceberg because your finger has

higher internal energy B) heat flows from your finger to the iceberg because your finger has

higher temperature C) heat flows from the iceberg to your finger because the iceberg has

higher internal energy due to its size D) no heat is transferredAnswer: BHeat always flows from the hotter object to the cooler one, no matterwhat the internal energies of the objects are.

Clicker Question

You heat a cup of water, half-filled, on the campfire, raising its

temp by 5

oC. How long would it take to heat this cup with

twice the amount of water to the same temperature 5

oC on

this campfire?

A) The same amount of timeB) Twice as longC) More than twice as longD) None of the aboveAnswer: Twice as long.There are twice as many molecules, so each molecule would,

in the same time, gain an average of half as much energy.

Specific Heat Capacity

•^

Different objects have different abilities to retain heat. Eg. Heated apple pie – the crust cools off quicker than the inside

filling. Eg. Toast cools off much quicker than a bowl of soup.•^

Similarly,

same

amounts of

different

objects require different

amounts of heat to be raised to the same temperature. Why? Because the applied energy gets apportioned into different

proportions of internal vibration/rotation or potential (doesn’traise temp), and jiggling (does raise temp). Eg. Water takes much longer to bring from room temperature to

boiling, than it takes same amount of oil to reach same temp. We say water has a higher “specific heat capacity” (or just

“specific heat”) than oil.^ The specific heat capacity of any substance is defined as thequantity of heat required to change the temperature of a unitmass of the substance by 1 degree.

Specific Heat cont.

•^

Specific heat is like

thermal inertia –

resistance to change temp

when heat is added.

-^

Water has exceptionally high specific heat

i.e. small amount of

water can absorb a lot of heat while only changing temp. a little. 1 gram of water requires 1 calorie of energy to raise temp by 1

o C (i.e.

specific heat = 1 cal/(K.g)) 1 gram of oil requires 0.5 calorie of energy to raise temp by 1

o C (i.e.

specific heat =0.5 cal/(K.g)) 1 gram of iron requires 0.125 calorie of energy to raise temp by 1

o C

(i.e. specific heat = 0.125 cal/(K.g))

-^

So, water is a good cooling agent (eg in cars, engines…)

-^

Equally, once heated, it keeps warm for long time (eg. hot-waterbottles on cold nights)

Specific Heat of water and climate

-^ Water moderates the climate: more energy needed to warmwater than to warm lande.g. islands/peninsulas don’t have extremetemps like interior lands do• Europe is at about the same latitude as parts of northeastern Canada but isnot so cold. Why?The Gulf Stream carries warm water northeast from the Caribbean,remaining warm, even up to coast of Europe. Here it cools, releasing energyinto the air – goes into westerly winds (i.e. winds from the west) to warmEurope. If water didn’t have such a high specific heat, Europe would be ascold as northeastern Canada!• Ocean doesn’t vary its temp much from summer to winter, because of highspecific heat – so, in winter, it warms the air (air changes temp more, smallspecific heat), whereas in summer, it cools the air. Hence, westerly windskeep San Francisco warmer in winter, and cooler in summer than inWashington DC even though same latitude.

Thermal Expansion

•^

Generally, matter expands when heated, contracts when cooled – canunderstand in terms of increased (heated) or decreased (cooling)jiggling motion of molecules.

-^

E.g.

Telephone wires become longer and sag on hot day.

•^

E.g.

Opening a stiff metal lid on glass jar – easier to do if hold under hot water for a while since metal expands more than the glass.

-^

E.g.

NY’s Verrazano bridge’s roadway is 12 feet lower in the summer than in winter, because of thermal expansion/contraction of the steelcables; Golden Gate bridge (San Francisco) contracts more than ameter in cold weather.

-^

Generally liquids expand more than solids.This is important for a glass thermometer filled with mercury liquid –mercury expands more than the glass. If not, it wouldn’t increaseheight with increasing temp.

-^

Important to account for expansion in building and construction. E.g

Filling material for tooth cavities has same rate of expansion as teeth.

Thermal expansion cont.

-^ Different materials expand at different rates: e.g. brass more than ironGenerally, something that expands more when heated, also contractsmore when cooled.

eg. Bimetallic strip: brass expands and contracts more than iron – explainscurves in strip shown (outer curve longer than inner)Useful in devices, eg in thermostats, bending in response to temp

change

can open/close circuit in a heating or cooling unit.

Clicker Question

When a metal ball is heated in a Bunsen flame, which

undergoes a change: volume, mass, or density?

A) Volume aloneB) Density aloneC) Volume and DensityD) Mass aloneE) Mass, Volume and DensityAnswer: CVolume increases and density decreases. Mass remains the same.

Anomolous expansion of water

•^

All common liquids expand whenheated – but

not

water at temps

near the freezing point!

Ice-cold water at melting temp, 0

oC = 32

oF,

contracts

when temp is increased – until

o 4 C, after which it does expand like normal

materials: At what temp does water have its greatest density?At 4

o C, smallest volume.

When water is solid ice (just below 0

o C), its volume is larger, and density

smaller (hence ice floats on water). But if further cooled, then it willcontract. Ice has crystalline structure – open-structured crystals due to angularshape of water molecules.In ice-cold water, most molecules are in liquid phase (water) but also afew ice-crystals here and there.