AP Physics
A measure of the average kinetic energy of the particles in a
sample of matter, expressed in terms of units or degrees
designated on a standard scale
~American Heritage Dictionary
3 Scales
Scale
Fahrenheit
Celsius
Kelvin
Boiling Point of H2O
Freezing Point of H2O Absolute Zero
Two systems are in thermal equilibrium if and only if they
have the same temperature.
Two systems that are each in thermal equilibrium with a third
system are in thermal equilibrium with each other.
Most materials expand when their
temperature increases.
Linear Expansion
 Expansion in one dimension
(usually the length)
 change in length is proportional
to the change in temperature
Material
α (1/oC)
Aluminum
24 x 10-6
Brass & Bronze
19 x 10-6
Copper
17 x 10-6
Steel
11 x 10-6
Concrete
12 x 10-6
A steel railroad track has a length of 30.000 m when the
temperature is 0oC. What is its length on a hot day when
the temperature is 40oC?
α = 11 x 10-6 (oC)-1
James Joule discovered by various means that he could heat a body of
water by purely mechanical means:
a)
by lowering a weight and letting a paddle wheel stir the water
b)
by passing electric current through a resistor
c)
by compressing a piston immersed in the water
d)
by friction from rubbing blocks together.
He found that about 800 foot-pounds ( 1 kilo joule) of work could raise the
temperature of one pound (.45 kilograms) of water one Fahrenheit
degree (0.55oC).
Energy that is transferred from one object or system to
another object or system as a result of a temperature
difference.
(The concept of heat only has meaning as energy in transit.)
Heat flow is energy transfer.
Units: Joule (J), calorie (cal), Calorie (Cal), British thermal
unit (Btu)
Unit Relations
Joules
calories
Calories
Btu
1
0.239
2.39 x 10-4
9.48 x 10-4
4.186
1
0.001
3.97 x 10-3
4186
1000
1
3.97
1055
252
0.252
1
The calorie is defined as the
amount of heat required to raise
the temperature of 1 gram of
water from 14.5oC to 15.5oC.
One Btu is defined as the
quantity of heat required to
raise the temperature of 1
pound of water from 63oF to
64oF.
Quantity of heat needed to change the temperature of a system is
dependent on three things:
1.
The quantity of the temperature change.
2.
The mass of the system.
3.
The nature of the material.
The amount of heat Q needed for a certain temperature change ΔT is
proportional to the temperature change and to the mass m of
substance being heated;
During a bout with the flu, an 80 kg man ran a fever of 2.0 Co.
His body temperature was 39.0oC (102.2oF) instead of the normal
37oC. Assuming that the human body is mostly water, how much
heat was required to raise his temperature by that amount?
Material
c [J/(kg·K)]
Water
4.19 x 103
Ice (-25oC to 0oC)
2.01 x 103
Aluminum
0.91 x 103
Salt
0.88 x 103
Iron
0.47 x 103
Silver
0.23 x 103
You are an electric engineer designing an electronic circuit element
made of 23 mg of silicon. The electric current through it adds
energy at the rate of 7.4 mW (7.4 x 10-3 J/s). If your design
doesn’t allow for any heat transfer out of the element, at what
rate does its temperature increase? The specific heat capacity
of silicon is 705 J/(kg·K).
Ingot - metal that is cast in the shape of a block for convenient
handling
A 0.0500 kg ingot of metal is heated to 200.0oC and then dropped
into a beaker containing 0.400 kg of water that is initially at
20.0oC. If the final equilibrium temperature of the mixed system
is 22.4oC, find the specific heat of the metal.
Three familiar phases of matter include:
1.
Gases
2.
Liquids
3.
Solids
…but there are more than those three
including…
plasmas, superfluids, supersolids, Bose-Einstein
condensates, fermionic condensates, liquid
crystals, strange matter and quark-gluon
plasmas
Transition from one phase of matter to another
What goes on during a phase change?
At a given pressure, phase changes:
1.
usually occur at a definite temperature*
2.
heat is absorbed or released
3.
there is a change in volume of the matter
4.
there is a change in density of the matter
*At this temperature and pressure both phases of matter can coexist,
this is called phase equilibrium.
Phase Change
Process
Solid  Liquid
Example
Metal gallium melts in your
hand
Liquid  Solid
Water freezes
Liquid  Gas
Liquid Nitrogen boils at room
temp.
Gas  Liquid
Solid Gas
Gas  Solid
To melt a mass m of material that
has a heat of fusion Lf requires a
heat Q given by:
Water vapor condenses
Dry Ice (CO2) sublimes to its
gaseous phase at room temp
Frost forms on cold objects
Heat Required
per unit mass
Latent Heat of
Fusion (Lf)
Latent Heat of
Vaporization
(Lv)
Latent Heat of
Sublimation
(Ls)
To freeze a mass m of material that
has a heat of fusion Lf requires a
heat Q given by:
To cause a phase change energy must be absorbed or released by the
object/system. During the phase change the heat energy does not
change the temperature of the object.
A physics student wants to cool 0.25 kg of Mountain Dew (mostly
water and high fructose corn syrup) initially at 20oC by adding ice
initially at -20oC. How much ice should she add so that the final
temperature will be 0oC with all the ice melted? Assume that
the heat capacity of the container may be neglected.
There are three mechanisms of heat transfer.
1. Radiation
2. Convection
3. Conduction
Energy transfer through electromagnetic
radiation (fancy words for light)
Ex.
Energy transfer of heat by the motion of a
mass of fluid
Ex.
heat transfer through a material from regions of
higher temperatures to regions of lower
temperatures
Good thermal conductors are metals because they
have many free electrons
Ex.
Rate of Heat Transfer…Heat Current (H)
The rate of heat transfer through a material with regions of
varying temperatures is:
 proportional to the cross sectional area A.
 proportional to the temperature difference ΔT.

inversely proportional to the distance between the regions L.
The constant of proportionality is the thermal conductivity of
the material k.
The StyrofoamTM box seen below is used to keep drinks cold at a
picnic (or wild beach party whatever suits your taste). The total
area of the sides, top, and bottom is 0.80 m2, and the wall
thickness is 2.0 cm. The box is filled with ice and Root Beer,
keeping the inner surface at 0oC. What is the rate of heat flow
into the box if the temperature of the outside surface is 30oC?
How much ice melts in one day (24 hr.)?
Material
k (W/(m·K)
Silver
406
Copper
385
StyrofoamTM
0.01
Wood
0.12 – 0.04
Air
0.024
A steel bar 10.0 cm long is welded end to end to a copper bar 20.0
cm long. Each bar has a square cross section, 2.00 cm on a side.
The free end of the steel bar is in contact with steam at 100oC,
and the free end of the copper bar is in contact with ice at 0oC.
Find the temperature at the junction of the two bars and the
total rate of heat flow.
Material
k (W/(m·K)
Steel
50.2
Copper
385