Temperature, Heat,

1
Chapter 21

• Temperature, Heat, Expansion

2
Temperature, Heat Expansion

• Atoms and molecules have kinetic energy
• The average kinetic energy of these particles
  causes heat
• To increase the KE of matterwarm the matter

3
Temperature

• Temperaturethe quantity that tells how hot or
  cold something is compared with a standard
• Almost all mater expands when its temperature
  increases and contracts when its temperature
  decreases
• A common thermometer measures temperature by
  showing the expansion and contraction of a liquid
  in a glass tube using a scale

4
Temperature Scales

• Celsiusmost widely used scale
• 0 assigned to temperature in which water freezes
• 100 assigned to temperature in which water boils
• The gap between is divided into 100 equal
  partsdegrees
• Fahrenheitthe US scale
• 32 assigned to temperature in which water freezes
• 212 assigned to temperature in which water boils
• The gap between is divided into 180 equal
  partsdegrees
• F 9/5 C 32 C 5/9 (F 32)
• Kelvinthe SI scale
• 0 assigned to the lowest possible
  temperatureabsolute zero
• Absolute zero (-273ºC 0ºK)
• a substance has no KE (no heat to give)
• K C 273.15
• 1ºC 1ºK

5
Temperature Kinetic Energy

• Do molecules move faster in a cup of hot coffee
  or a cup of cold coffee?
• Hot coffee Greater temperature ? more kinetic
  energy (speed)
• In an ideal gastemperature is proportional to
  the average KE of the translational motion of
  molecules
• Temperature is not a measure of total KE
• In 3 cups of boiling water there is 3 times as
  much KE as 1 cup of boiling waterbut the
  temperatures are the same

6
Heat

• Heat is the energy that flows because of
  temperature differences
• No matter can contain heat
• Heat is energy in transit from a body of higher
  temperature to a body of lower temperature
• Temperature is the measure of internal energy of
  a substance
• When heat is added to a substance, the internal
  energy (temperature) increases
• Temperature is not the same thing as heat

Cup of very high-temperature water contains less
internal energy than large bucket of warm water
Consider boy holding a sparkler 2000oC sparks
dont bother him since they are so small very
little internal energy.
7
Heat

• Objects or substances are in thermal contact when
  heat flows from one object to another
• Heat flows from the higher temperature substance
  to the lower temperature substance (unless
  external work is done)
• Not necessarily from object of high internal
  energy to low internal energy
• If spark from sparkler lands in warm water (water
  has higher internal energy but less temp), heat
  flows from spark to water
• The greater the temperature difference, the
  greater the heat flow
• Greater heat flow if the amount of hotter
  substance is larger.

Here, add the same amount of heat, therefore
increase the internal energy of both the same.
But temperature in the one with less water rises
more.
8
Thermal Equilibrium

• Objects are in thermal equilibrium after they
  reach the same temperature
• Thermometer
• Is in contact with a substance, heat flows
  between them until they have the same temperature
• The temperature of the thermometer is the
  temperature of the substance
• The thermometer has to be small enough that it
  doesnt affect the temperature of the object you
  want to measure
• You can measure your bodys temperature with a
  thermometer but you cant measure the temperature
  of a drop of water with it
• Contact between the thermometer and drop can
  change the drops temperature.
• You heat a half-cup of tea and its temperature
  rises by 4ºC. How much will the temperature rise
  if you add the same amount of heat to a full cup
  of tea?
• 2ºC
• Where does the internal energy go when a cup of
  hot tea cools?
• It goes into warming the surroundings. Soon the
  tea will be cooler and the surroundings warmer.
  They will achieve thermal equilibrium at a common
  intermediate temperature

9
Internal Energy

• There are forms of energy in molecules other than
  translational (linear) KE
• Rotational KE of molecules
• KE due to internal movements of atoms within
  molecules
• PE due to forces between molecules
• All of these energies added together is called
  internal energy
• A substance does not contain heat, it contains
  internal energy
• When heat is absorbed or given off by an object,
  its internal energy changes.
• It may warm up increase translational KE.
• But not always it may change phase
• add heat to ice - melts to water

10
Internal Energy

• A 3.0 x 10-3 kg copper penny drops a distance of
  50.0 m to the ground. If 65 percent of the
  initial potential energy goes into increasing the
  internal energy of the penny, determine the
  magnitude of the internal energy increase.
• The amount of internal energy needed to raise the
  temperature of 0.25 kg of water by 0.20ºC is
  209.3 J. How fast must a 0.25 kg baseball travel
  in order for its kinetic energy to equal this
  internal energy?

11
Measurements of Heat

• Heat is flow of energy
• measured in energy units (Joules)
• calorie (or kilocalorie 1000 cal)
• 1 calorie amount of heat required to change
  temp of 1 gram of water by 1oC.
• 1 kilocalorie amount of heat required to change
  temp of 1 kilogram of water by 1oC.
• 1 calorie 4.186 joules.
• Sometimes kilocalorie is called Calorie, with
  capital C
• Energy rating of foods/fuel is determined by
  burning them and measuring energy released.
• 1 L of 30ºC water is mixed with 1 L of 20ºC
  water. What is the final temperature of the
  mixture?
• 25ºC the internal energy lost by the warmer
  water is gained by the cooler water
• You heat a cup of water on the campfire, raising
  its temp by 5oC. How long would it take to heat
  two cups of water 5oC on this campfire?
• 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.

12
Specific Heat Capacity

• Different objects have different abilities to
  retain heat.
• Heated apple pie the crust cools off quicker
  than the inside filling.
• 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 amounts of internal
  vibration/rotation or potential (doesnt raise
  temp), and jiggling (does raise temp).
• Water takes much longer to bring from room
  temperature to boiling, than it takes same amount
  of oil to reach same temperature
• We say water has a higher specific heat
  capacity (or just specific heat) than oil.

13
Specific Heat Capacity

• Specific heat capacity ? thermal inertia
• Resistance of substance to change in its
  temperature
• The common unit for specific heat (c) is the
  J/kgºC (joule per kilogram degree celsius)
• The transfer of heat from an object depends upon
  the objects mass, the specific heat, and the
  difference in temperature between the object and
  its surroundings
• Q heat (joules)
• m mass (kg)
• c specific heat (J/kgºC)
• T temperature (ºC)

14
Specific Heat Capacity

• Hypothermia can occur if the body temperature
  drops to 35.0ºC,although people have been known
  to survive much lower temperatures. On January
  19, 1985, 2 year old Michael Trode was found in
  the snow near his Milwaukee home with a body
  temperature of 16.0ºC. If Michaels mass was 10.0
  kg, how much heat did his body lose, assuming
  that his normal body temperature is 37.0ºC and
  the specific heat of the human body is 3470
  J/kgºC?
• Gwyns bowl is filled with 0.175 kg of 60.0ºC
  soup (mostly water) that she stirs with a 20.0ºC
  silver spoon with a mass of 0.0400 kg. The spoon
  slips out of her hand and slides into the soup.
  What equilibrium temperature will be reached if
  the spoon is allowed to remain in the soup and no
  heat is lost to the outside air? (cspoon 240.
  J/kgºC, cwater 4187 J/kgºC) Assume that the
  temperature of the bowl does not change.

15
Specific Heat

• Water has exceptionally high specific heat
• A small amount of water can absorb a lot of heat
  while only changing temperature a little
• 1 gram of water requires 1 calorie of energy to
  raise temp by 1oC (i.e. specific heat 1
  cal/(K.g) 4186 J/kgºC))
• 1 gram of oil requires 0.5 calorie of energy to
  raise temp by 1oC (i.e. specific heat 0.5
  cal/(K.g) 2093 J/kgºC)
• 1 gram of iron requires 0.125 calorie of energy
  to raise temp by 1oC (i.e. specific heat 0.125
  J/(K.g) 523.25 J/kgºC)
• Water is a good cooling agent (in cars, engines)
• Once heated, water stays warm for long time
  (hot-water bottles on cold nights)
• Why will a watermelon stay cool for a longer time
  than sandwiches when both are removed from a
  cooler on a hot day?
• Water has a high specific heat capacity, so takes
  a long time to heat up or cool down. The water in
  the watermelon resists changes in temp, so once
  cooled will stay cooler longer than sandwiches or
  other non-watery substances.

16
The Specific Heat of Water Climate

• Water moderates the climate more energy needed
  to warm water than to warm land
• islands/peninsulas dont have extreme temps like
  interior lands do
• Europe is at about the same latitude as parts of
  northeastern Canada but is not 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 energy into
  the air goes into westerly winds to warm
  Europe.
• If water didnt have such a high specific heat,
  Europe would be as cold as northeastern Canada!
• Ocean doesnt vary its temperature much from
  summer to winter, because of high specific heat
• In the winter, it warms the air (air does change
  temp more, small specific heat),
• In the summer, it cools the air
• Westerly winds keep San Francisco warmer in
  winter, and cooler in summer than Washington DC
  even though they are at the same latitude.
• The air moves from San Francisco to DC across
  land. Land has a lower specific heat capacity
  than water.

17
Thermal Expansion

• Matter expands when heated and contracts when
  cooled can understand in terms of increased
  (heated) or decreased (cooling) jiggling motion
  of molecules.
• Telephone wires become longer and sag on hot day.
  
• 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.
• Golden Gate bridge in San Francisco contracts
  more than a meter in cold weather.
• Generally liquids expand more than solids
• glass thermometer filled with mercury liquid
  mercury expands more than the glass. If not, it
  wouldnt increase height with increasing
  temperature
• Important to account for expansion in building
  and construction.
• Filling material for tooth cavities has same rate
  of expansion as teeth.
• Concrete roads or bridges are intersected by gaps
  (often with tar) so concrete can freely expand in
  summer, contract in winter.

18
Thermal Expansion

• Railroad tracks
• No gaps in tracks in picture here, so tracks
  buckled on a very hot day.
• Generally, they have gaps that make a
  clickety-clack sound.
• Now instead they are welded together to
  eliminate the sound and are laid down on
  hottest days to avoid buckling due to heat.
• On cooler days, the tracks contract, but that
  just stretches the tracks, not distorting them

• Different materials expand at different rates
• Generally, something that expands more when
  heated, also contracts more when cooled.
• Bimetallic strip (two strips of different
  metals) brass expands and contracts more than
  iron.

19
Thermal Expansion

• A thermostat is an application of a bimetallic
  strip
• The back and forth bending of the strip opens and
  closes an electric circuit
• When a room becomes too coldthe coil bends
  toward the brass side, closing the circuit,
  turning on the heat
• When a room becomes too warm, the coil bends
  toward the iron side, opening the thermostat,
  turning off the heat
• The amount of expansion of a substance depends on
  its change in temperature
• Liquids expand appreciably with increases in
  temperature

20
Thermal Expansion

• When a metal ball is heated in a Bunsen flame,
  which undergoes a change volume, mass, or
  density?
• Volume and density. Mass remains the same.
• (2) Why is it advisable not to completely fill
  the gas tank in a car that may sit in sunlight in
  a hot day after being filled?
• As it warms, it expands, overflowing and causing
  a hazard.
• (3) Place a dented ping-pong ball in boiling
  water, the dent is removed. Why?
• Because the ball expands as its temperature
  rises, so it pops back out into a sphere.

21
Thermal Expansion

• Linear Expansion
• Ernesto is knitting his wife a sweater in his
  18C air-conditioned living room with 0.30 m long
  aluminum knitting needles, when he decides to
  knit outside in the 27C air. How much will the
  knitting needles expand when Ernesto takes them
  outside? ( 24 x 10-6 C-1)
• Area Expansion
• Volume Expansion

22
Expansion of Water

• Almost all liquids expand when they are heated.
• Ice-cold water contracts when the temperature is
  increased
• Water contracts from 0 4 degrees Celsius
• At 4 degrees, it begins to expand
• At what temp does water have its greatest
  density?
• At 4oC, smallest volume

23
Expansion of Water

• Will a sample of 4ºC water expand, contract, or
  remain unchanged in volume when heated?
• Expand
• Will a sample of 4ºC water expand, contract, or
  remain unchanged when it is cooled?
• Expand
• Why would water, instead of mercury or alcohol,
  be a poor liquid for a thermometer when
  near-freezing temperatures are to be measured?
• The column height would be ambiguous for 0 8ºC.
  You couldnt distinguish between temperatures
  below and above 4ºC.

24
Expansion of Water

• Ice has crystalline structure open-structured
  crystals due to angular shape of water molecules.
  
• In ice-cold water, most molecules are in liquid
  phase (water) but also a few ice-crystals here
  and there.
• Water molecules in this open structure occupy a
  greater volume than they do in the liquid state ?
  ice is less dense than water (it floats)

25
Expansion of Water

• Understanding the dip in the volume curve
• Two competing effects as you add heat to ice-cold
  water
• (i) (spaced-out) ice-crystals collapsing (ii)
  faster molecular motion

26
Why can fish exist in ponds in the winter?

• The dip in the curve!

Consider if there was no dip so water would
continue getting denser through to freezing point
(like most liquids). Then coldest water would be
at bottom of pond, since it is densest. Organisms
would be killed in winter ?
Fortunately, the densest water is at 4oC, so this
is the temp at bottom of a pond in winter.
Instead, ice (water at freezing point, 0oC) is
less dense so floats to the top, leading to happy
fish at 4oC ?
Ponds freeze from surface downward. As it cools,
water sinks until all pond is 4oC. After that,
lower temperatures can be reached and this floats
on top and freezes right at top of water is ice
at 0oC.
This, together with waters high specific heat,
means that very deep bodies of water are not
ice-covered even in the coldest winter need to
get whole body to 4oC first, not just the upper
part.