Thermal
Thermal
Thermal
Temperature
4.184 J = 1 cal
KEAv = 3/2 kT
Scale Conversions
180 / 100 = 9 / 5
TK = TC + 273.15
Temperature Practice
1. Normal body temperature is 98.6 0F. What is this on the Centigrade scale?
Conduction
• Wood or tiled floor feels cooler than carpet even though they
are at the same temperature
• Wood or tiled floor feels cooler than carpet even though they
are at the same temperature
Convection
Radiation
• Surfaces that are light in color and smooth (shiny) are poor
radiators (and poor absorbers). The reverse is true for dark and
rough surfaces.
Some substances will absorb a lot of heat for only a small change
in temperature.
Iron with a little heat will shoot up in temperature while water takes
a lot of heat energy to make the temperature higher.
Why is this?
Heat and Specific Heat
Iron’s electrons move rapidly back and forth and the temperature
goes up quickly
c = Q / (m T) units: J / kg K or J / kg 0C
It turns out that water will absorb a whole calorie of heat energy
per gram and only change temperature by 1 degree Celcius
(centigrade)
Q = m c t
How much energy does it take to raise the temperature of 1.5 liters of
water by 200C?
mwater = 1.5 L ( 1kg / 1L) = 1.5 kg cwater = 1000 cal/kg0C or 4184 J/kg0C
T = 20oC
Q = m c t
Some high temperature foods, you can eat comfortably when you take
them out of the oven as they have a low specific heat capacity and
therefore don’t hold much thermal energy but water filled foods like pie
filling you can burn your mouth on as the high temperature food will hold a
lot of energy.
A hot water bottle contains boiling water that cools gradually during the
night releasing a large amount of thermal energy.
Countries surrounded by water (which has a high specific heat) are heated by
the warm winds that have absorbed thermal energy from the ocean (cons of
energy). The ocean cools gradually during the winter so maintains a constant
source of heat energy. The water acts as a temperature moderator, absorbing
energy from the air above in the summer and releasing it in the winter.
Heat and Temperature Change: Specific Heat Capacity
A Hot Jogger
Q mcT
Q 8.0 105
J
T 3.5 C
mc 65 kg 3500 J kg C
Heat and Temperature Change: Specific Heat Capacity
CALORIMETRY
9.00 102 J kg C 0.15 kg 4.0 C 4186 J kg C 0.20 kg 4.0 C
0.040 kg 75.0 C
1300 J kg C
Phase Changes
Why do you feel cold when you get out of a pool and a breeze
is blowing?
Water evaporates off your body and cools you down. How come?
Normal
distribution curves
of molecular
speeds for ideal
gas at 100 K and
900 K respectively
Phase Changes
In hot climates, sacking is put over clay pots and kept wet. Why?
Evaporation of water from the sack cools the sack and thus draws thermal
energy outwards from the water inside the pot. This keeps the drinking
water nice and cool.
They can’t sweat so they create a large surface area (tongue and
bronchial tract) from which liquid can evaporate and therefore cool
them down.
Similarly rubbing your hands under a hand dryer creates a larger
surface for water to evaporate.
Phase Changes
Evaporation depends upon the temperature of the liquid and air
surrounding the liquid, the surface area of the liquid and the moisture
content in the air (humidity). Evaporating molecules form a vapor above
the liquid. Equilibrium is reached when the vapor pressure exerted on the
fluids surface returns liquid molecules to the liquid at the same rate they
leave.
Why are their warning signs on hot tubs about staying in too
long?
In a hot tub you sweat like mad but the water can’t evaporate off the
skin so your body heats up and can be dangerous if you stay in too
long.
Heating the water in a pan is a heating process but the act of boiling is
actually a cooling process. Like evaporation, when boiling, molecules leave
the surface of the liquid taking thermal energy with them.
Heat pan of water, bubbles form but 30km of air pushing down
(atmospheric pressure) squashes these bubbles. With more heating
the temp keeps increasing and the molecules become more energetic
until bubbles are able to withstand the pressure. This temps for water
is 1000C at sea level.
You normally cook eggs for 3 minutes at sea level. Do you cook
them for a longer or shorter time when living at higher altitude?
Longer as water boils at a lower temp
The more you heat, the more vigorous the boiling but you also have
more liquid escaping therefore more cooling so they offset each other.
(you can also think of it as the heat input going towards breaking
intermolecular bonds and not towards changing the average kinetic
energy of the molecules).
Yes, the lid increases the pressure so temp goes up more before
bubbles stay formed.
Phase Changes
Why are sensitive fruit crops in the South sprayed with water before
a forecasted frost?
In the past, farmers used to put a tub of water in the cellar where
they stored their canned food. Why?
If the temperature in the cellar dropped the water in the tub would
freeze before the liquid in the cans (which had added salt or sugars)
thus releasing heat and warming the room.
Phase Changes
The steam burn is much more serious because each gram of steam
liberates 540 calories when it condenses, whereas water only liberates
1cal for every gram for every degree (or only 100 cal to go from 1000C
to 0 0C).
Note: When 540 cal is used to turn a gram of water into steam it does not
go into increasing the molecules KE, so it must go into increasing their PE.
Intermolecular bonds have to be broken and this takes a lot of energy.
When steam condenses, bonds form and this energy is released.
Phase Changes
Why is an ice rink flooded with hot water to smooth out the ice?
Wouldn’t cold water work better?
Above 800C, hot water freezes faster than warm water. For a large
surface area like an ice skating rink, the rate of cooling by rapid
evaporation is very high because each gram of water that evaporates,
draws at least 540 cals/gram from the water left behind.
This is huge compared with the 1 cal/g/0C that is drawn for each gram
of water that cools by thermal conduction.
Phase Changes
The quantity of heat needed per kg to melt a solid (or solidify a liquid)
at constant temperature and atmospheric pressure.
Q = m Lf Lf units: J/kg
Q = m Lv Lv units: J/kg
Lv,water = 540 cal/g or 2.26 x 106 J/kg
The quantity of heat needed per kg to melt a solid (or solidify a liquid)
at constant temperature and atmospheric pressure.
Q = m Lf Lf units: J/kg
Q = m Lv Lv units: J/kg
Lv,water = 540 cal/g or 2.26 x 106 J/kg
V 1/ P (constant T)
PV = constant
P1V1 = P2V2
Any object that has a mass less than the mass of an equal volume of
the surrounding fluid (air or liquid) will rise.
In other words for air if any object is less dense than the surrounding
air around it, it will rise. Heating air in a hot air balloon has this effect.
Gas Laws
Examples
V2 = 9.90 cm3
Gas Laws
Examples
2. In a jet liner ascending from sea level where the cabin pressure starts off
at 1.01 x 105 Pa to flying altitude where the cabin pressure drops slightly to
1.00 x 105 Pa despite pressurized conditions. A person will feel a strange
sensation in their middle ear, whose volume is 6.0 x 10-7 m3. What is the
new volume of air inside the person’s middle ear and what can they do to
compensate for this change in volume?
P1V / T1 = P2V / T2
= 6.1 x 10-7 m3
3. Mr. Fawcett is diving at a dept of 20m off the coast of Mexico where the
density of water is 1025 kg/m3 and the pressure is 3.06 x 105 Pa. If he
foolishly hold s his breath as he ascends to the surface, how many times
would the volume of his lungs change (assuming the water temp stays
constant)? Would his lungs be crushed or would they expand? What is
the best way to ascend after diving?
4. A 5450 m3 blimp circles Fenway Park during the Word Series, suspended
in earth’s 1.21 kg/m3 atmosphere. The density of the helium in the blimp
is 0.178 kg/m3.
a) What is the buoyant force that suspends the blimp in the air?
b) How does this force compare to the blimp’s weight?
c) How much weight in addition to helium, can the blimp carry and still
continue to maintain a constant altitude?
5. Floating on her back in the beautiful Caribbean during her spring break a
student has a density of 980 kg/m3 and a volume of 0.060 m3. What
buoyant force supports her in the sea, which has a density of 1025
kg/m3?
Note: Buoyant force can’t be more than her weight because she is
floating. She is not submerged as he density is less than that of
sea water.
Gas Laws
Examples