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Ch 14 Heat
What is heat?
• Heat is energy that moves from an object at a
higher temperature to an object of lower
temp.
• Heat was originally described as movement of
a fluid substance called ‘caloric’.
Mechanical Equivalent of Heat
• James Prescott Joule performed experiments
to show how heat, like work, transfers energy.
• Joule found that a given amount of work done
was always equal to the same amount of heat
input.
4.186 J = 1 cal
4.186 x 103 J = 1 kcal
Units of Heat Energy
• Units include
– calorie (cal)- amount of heat necessary to raise
the temp of 1g of water 1°C
– Kilocalorie (kcal)- 1000 calories or same definition
as above for 1 kg of water by 1°C (also called the
Calorie (C) as in energy content of food.
– British Thermal Unit (Btu)- heat needed to raise 1
lb of water 1°F
– 1 Btu = 0.252 kcal = 1055 Joules
Heat transfer as Energy
• Heat transfer came to be understood as a
transfer of energy from a hot object to a
cooler one.
• SI unit for heat is the Joule. (Energy)
• KE is an indication of heat content of a
material.
• If work is done on an object, its KE and temp
increase. Vice versa holds true for cooling.
Example 14-2
• When a 3.0g bullet, traveling at 400m/s passes through a
tree its speed is reduced to 200m/s. How much heat Q is
produced and shared by the bullet and tree?
• Solution: Energy conservation says Kei = Kef + Q so
1
Q  m(v i2  v 2f )
2
1
 (3.0x103 kg) 400m /s2  200m /s2 
2
180J
 180J 
 43cal
4.186J /cal


This is distributed between the bullet and the tree according to each of the
materials’ abilities to transmit and store heat energy.
Temp, Heat, & Internal Energy
• Thermal energy or internal energy is the sum total
energy of all molecules in an object.
• Heat is the amount of energy transferred from one
object to another at a different temperature.
• Temperature (in Kelvin) is a measure of the average
kinetic energy of individual molecules.
• Two chunks of iron may have the same temperature
but if one has twice the mass, it has twice the
thermal energy as the other.
Internal Energy of a gas
• Internal Energy, U, of an ideal gas depends only
on the temperature and number of moles of gas.
1
2
U  (mv )
2
3
U  NkT N is the total number of molecules
2
3
U  nRT n is the number of moles
2
Specific Heat
Q=mcΔT
• If heat is put into an object, its temperature increases…
how much depends on the mass, type of material, (its
ability to hold heat content), and the temperature
difference.
• Specific Heat, c, of a material is the energy required to
raise 1kg of a material’s temp 1°C.
• Units for c are kcal/kg°C or J/kg°C
• For water at 15°C, c = 1.00 kcal/kgC° or
4.19x103 J/kgC°
• Air heats up ‘faster’ than water on a sunny day due to a
lower specific heat, c, than water’s. See table 14-2.
Latent Heat
• During a phase change, heat is added (or
removed) and a certain amount of energy is
required…
• Heat of Fusion is the energy required for a phase
change from solid to liquid, LF. For water LF is
79.7 kcal/kg or 333kJ/kg.
• Heat of Vaporization is the energy required for a
phase change from liquid to gas, LV. For water LV
is 539 kcal/kg or 2260kJ/kg.
Latent Heat Diagram
Energy moves
both directions.
The same
amount of energy
is required or
released as phase
changes occur.
Latent heat is the value for the heat of fusion or vaporization, L.
The total heat involved during a phase change depends on the latent
heat, L and on the total mass of the substance: Q = mL. (Q is the heat
required or given off during the phase change.
Heat Transfer: Conduction
• Conduction is transfer of thermal energy by
collisions between molecules at different
temperatures. (Objects in contact with ΔT).
• Rate of heat flow by conduction depends on
temperature difference and size and shape of
objects.
•
Q
T2  T1
 kA
T
l where A is cross-sectional area, l
is distance between two ends, and k is
proportionality constant, thermal conductivity. If k
is large, substances conduct heat rapidly and are
good conductors. See table 14-4.
Ex 14-11: Heat loss through windows
• Calculate the rate of heat flow through a glass
window 2.0 m x 1.5 m in area and 3.2 mm
thick if the temps at the inner and outer
surfaces are 15.0°C and 14.0°C
respectively.
Ex 14-11 Solution
• Since A = (2.0 m)(1.5 m) = 3.0 m2, l= 3.2 x 103m and we get k from the table, so
Q
T1  T2 (0.84J /s  m  C)(3.0m 2 )(15.0C 14.0C)
 kA

T
l
(3.2x103 m)
 790J /s
 (790J /s)(4.19x10 3 J /kcal)  0.19kcal /s
 680kcal /h
Double pane windows increase the air layer thickness to reduce heat loss
more than a single pane of glass, since the thermal conductivity of air is much
less than that for glass.
Insulators
Clothing helps keep us warm
by trapping air around our
bodies so it cannot move and
be replaced by colder air. (So
it’s not the cloth that keeps us
warm, but the air we trap).
• Materials like down and “thermals” fluff air and
trap them around our bodies to prevent loss of
heat by conduction. (What if feathers get wet?)
• Insulating materials are specified by R-values
depending on k and thickness, l. R=l/k.
• Higher R values are better insulators.
Heat Transfer: Convection
• Heat transfer by
movement of molecules
over long distances. This
usually happens in gases
and liquids and is called
Convection currents.
• As air or water is heated,
it expands. Less dense
material rises.
• Warmer materials rise
and cooler materials sink.
This gives inland day breezes and outland
evening breezes near lakes / beaches.
Heat Transfer: Radiation
• While conduction and convection require a
medium to carry heat from hot to cold matter,
radiation requires NO medium.
• Radiation travels through empty space by EM
waves (heat is IR).
• Objects radiate heat at a rate proportional to
their Kelvin temperature to the 4th power and
their surface area, A.
Radiation cont’d
• The Stefan-Boltzmann equation gives the rate of
heat radiation by:
Q
4
 eAT
T
• Here e is the emissivity (a # between 0 and 1 like
black substances are close to 1 and shiny stuff is
close to 0 radiation), and σ is the Stefan-Boltzmann
 -8
constant, 5.67 x 10 W/m2*K4
• Shiny surface neither absorb nor emit much
radiation.
• A good absorber, like dark objects, is also a good
emitter.
Solar Radiation
• Although the Sun’s energy travels to Earth by
radiant waves, heating by the Sun is calculated
considering about 1350J of energy strike the
atmosphere of Earth per second per square meter
at right angles to the Sun’s rays.
• 1350W/m2 is called the Solar constant. Our
atmosphere absorbs about 70% of the Sun’s energy
so we tend to receive about 1000W/m2.
Earth’s axis and seasons
• An object of emissivity, e, with area A facing
the sun absorbs heat at a rate of
Q
 (1000W /m 2 )eA cos
T

Your turn to Practice
• Please do Ch 14 Rev pgs 439-440 #s 1, 2, 15,
16, 30, 33, 35, 39, and 40.