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Key points from this lecture are: Temperature, Heat, Expansion, Van De Graaff Generator, Electrical Energy Storage, Thermometers, Themal Equilibrium, Internal Energy, Measuring Heat, Specific Heat Capacity, Thermal Expansion, Thermal Inertia Topics covered in Basic concepts of Physics course are: Newton’s Laws of Motion, Linear Motion, Momentum, Energy, Rotation, Gravity, Liquids, Gase, Plasmas, Heat, Waves, Sound, Electrostatics, Electric current, Magnetism, Electromagnetic Induction, Color, Li
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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 = 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.
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.
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 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)
-^ 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.
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.
-^ 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.
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
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.