Download Wilson-Ch

Lecture Outline
Chapter 11
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
Chapter 11
Heat
© 2010 Pearson Education, Inc.
Units of Chapter 11
Definition and Units of Heat
Specific Heat and Calorimetry
Phase Changes and Latent Heat
Heat Transfer
© 2010 Pearson Education, Inc.
11.1 Definition and Units of Heat
Heat is a form of energy, and therefore is
measured in joules. There are other units of
heat, though; the most common one is the
kilocalorie:
One kilocalorie (kcal) is defined as the amount of heat
needed to raise the temperature of 1 kg of water by 1 C°
(from 14.5°C to 15.5°C).
One calorie is .001 kilocalorie.
© 2010 Pearson Education, Inc.
11.1 Definition and Units of Heat
Confusingly, the calories listed on nutrition
labels in the U.S. are really kilocalories
(sometimes called Calories). Some other labels
are more accurate (left, Australia; right,
Germany).
© 2010 Pearson Education, Inc.
11.1 Definition and Units of Heat
This figure illustrates the three most
common units of heat.
© 2010 Pearson Education, Inc.
11.1 Definition and Units of Heat
So, if heat is energy, how do kilocalories
convert to joules? Careful experimentation
using the apparatus below gives the answer, as
the work done by the falling weights raises the
temperature of the water.
This relationship is
called the mechanical
equivalent of heat.
© 2010 Pearson Education, Inc.
11.2 Specific Heat and Calorimetry
The amount of heat needed to increase the
temperature of a solid or liquid depends on
the amount of the substance, the
temperature change, and the properties of
the substance itself.
The constant c is called the specific heat of
the substance.
© 2010 Pearson Education, Inc.
11.2 Specific Heat and Calorimetry
© 2010 Pearson Education, Inc.
11.2 Specific Heat and Calorimetry
Calorimetry is the quantitative measurement of
heat exchange; it is done using a calorimeter.
A calorimeter is insulated from the
environment, minimizing heat exchange.
Therefore, heat lost by one object in the
calorimeter must be gained by another. This is
one way of measuring specific heats.
© 2010 Pearson Education, Inc.
11.2 Specific Heat and Calorimetry
Specific heat can be defined for gases as well,
but gases do not have constant volume or
pressure.
We therefore define two specific heats for
gases—one at constant volume (cV), and one at
constant pressure (cP).
For a particular gas, cP is always greater than cV.
© 2010 Pearson Education, Inc.
11.3 Phase Changes and Latent Heat
Three phases of matter: solid,
liquid, and gas.
Solid has a definite shape and
the strongest intermolecular
bonds.
Liquid flows but is relatively
incompressible, so it has a
definite volume.
Gas is compressible, and will
expand to fill a container.
© 2010 Pearson Education, Inc.
11.3 Phase Changes and Latent Heat
Phase changes:
Solid → liquid: melting
Liquid → gas: evaporating, boiling
Gas → liquid: condensing
Liquid → solid: freezing
Solid → gas: sublimating
Latent heat is the amount of heat absorbed or
released when a substance undergoes a
phase transition.
© 2010 Pearson Education, Inc.
11.3 Phase Changes and Latent Heat
During a phase transition, the heat energy
goes to changing the intermolecular bonds,
and the temperature does not change.
The heat needed for a phase change is:
Here, L is the latent heat; Lf is the latent
heat of fusion (solid ↔ liquid) and Lv the
latent heat of vaporization.
© 2010 Pearson Education, Inc.
11.3 Phase Changes and Latent Heat
Latent heat is a property of a particular
substance.
© 2010 Pearson Education, Inc.
11.3 Phase Changes and Latent Heat
Heat added as ice becomes steam:
© 2010 Pearson Education, Inc.
Example (similar to 11.4, p.392)
Students in a physics lab are to determine the
specific heat of copper experimentally. They
heat 0.150kg of cooper shot to 100 C and then
carefully pour the hot shot into a calorimeter
cup containing 0.200kg of water at 20.0 C. The
final temperature of the mixture in the cup is
measured to be 25 C. If the aluminum cup has a
mass of 0.0450kg, what is the specific heat of
cooper? (Assume that there is no heat
exchange with the surroundings.)
© 2010 Pearson Education, Inc.
Problem
A 0.30 kg piece of ice at 0 C is placed in a
liter of water at room temperature (20 C) in
an insulated container. Assuming that no
heat is lost to the container, what is the final
temperature of the water?
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Heat transfer takes place via three
mechanisms:
1. Conduction
2. Convection
3. Radiation
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Conduction is the transfer of heat through a
substance, such as the disposable cup that
holds your hot coffee. If the cup is a good
conductor of heat, you will need a sleeve to
keep from burning your hand.
Typically, metals are good conductors of
heat—they have electrons that are free to
move throughout the material—and
nonmetals are not.
Nonconductors of heat are also called
insulators.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
The heat flow rate through a slab of material is
proportional to its surface area and to the
temperature difference, and inversely
proportional to its thickness.
The constant k is called the thermal
conductivity.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
This diagram illustrates
the geometry of heat
transfer by conduction.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
The thermal
conductivities of
substances vary
widely.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
When insulating a house, we want materials
whose thermal conductivity is as low as
possible.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Heat transfer in fluids is mostly by convection,
which is the result of mass transfer; that is,
heat is transferred as warmer fluid moves to
replace cooler fluid. Convection may be
spontaneous (as below) or forced.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Many homes are heated
using forced hot air; this is
an example of forced
convection.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Radiation is the only type of heat transfer that
can take place through a vacuum. You can feel
the radiation of heat when you stand near a
fireplace. This radiation is in the form of
electromagnetic waves, in the infrared part of
the spectrum.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
The rate of energy radiation is given by
Stefan’s law:
A is the object’s surface area, T is its
temperature, and e is a number between 0 and
1 called the emissivity. σ is the Stefan–
Boltzmann constant:
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
A good emitter of radiation is also a good
absorber:
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Here we see the three types of heat transfer:
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
A Thermos bottle minimizes all three types of
heat transfer.
© 2010 Pearson Education, Inc.
11.4 Heat Transfer
Passive solar heating uses the changing angle
of the Sun to warm buildings in the winter but
not in the summer.
© 2010 Pearson Education, Inc.
Summary of Chapter 11
Heat is the energy exchanged between
objects due to a temperature difference.
Specific heat tells the energy needed to raise
1 kg of a substance by 1 C°.
Calorimetry uses the heat transfer between
objects to measure specific heats.
Latent heat is the heat needed per kilogram to
change the phase of a substance.
© 2010 Pearson Education, Inc.
Summary of Chapter 11
Conduction is the transfer of heat between
objects that are in direct contact.
Convection is the transfer of heat via mass
movement of the molecules of a fluid.
Radiation is the transfer of heat via
electromagnetic radiation.
© 2010 Pearson Education, Inc.
Chapter 11, Problem
A volume of 0.50 L of water at 16 C is put
into an aluminum ice cube tray of mass
0.250 kg at the same temperature. How
much energy must be removed from this
system by the refrigerator to turn the water
into ice at -8.0 C?
© 2010 Pearson Education, Inc.
Chapter 11, Problem
An electric immersion heater has a power
rating of 1500 W. If the heater is placed in a
liter of water at 20 C, how many minutes will
it take to bring the water to a boil?
(Assume that there is no heat loss except to
the water itself.)
© 2010 Pearson Education, Inc.
Chapter 11, Problem
(a) A 0.500 kg piece of ice at -20 C is
converted to steam at 115 C. How much
heat must be supplied to this?
(b) To convert the steam back to ice at -5 C,
how much heat would have to be
removed?
© 2010 Pearson Education, Inc.
Chapter 11, Problem 49
An aluminum bar and a copper bar of
identical cross-sectional area have the
same temperature difference between their
ends and conduct heat at the same rate.
Which bar is longer, and how many times
longer?
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.