Download Pearson Prentice Hall Physical Science: Concepts in Action

Chapter 16
Thermal Energy
& Heat





Objectives:
1. Explain how heat and work transfer
energy
2. Relate thermal energy to the motion of
particles that make up a material
3. Relate temperature to thermal energy
and to thermal expansion
4. Calculate thermal energy, temperature
change, or mass using the specific heat
equation
•
•
•
•
Def: heat is the transfer of thermal energy
from one object to another because of
temperature difference
Heat flows spontaneously from hot
objects to cold objects
Def: temperature is a measure of how hot
or cold an object is compared to a
reference point
On the Celsius scale, the reference points
are the freezing point and boiling point of
water





Another reference point is called
absolute zero
Absolute zero is on the Kelvin scale
It is the lowest temperature known
There are no negative values on the
Kelvin scale
Temperature is related to the average
kinetic energy of particles in an object
due to their random motions through
space
•
•
•
•
Recall that thermal energy is the total potential
and kinetic energy of all the particles in an object
Thermal energy depends on the mass,
temperature and phase (solid, liquid, gas) of an
object
Def: thermal expansion is an increase in the
volume of a material due to a temperature
increase
Thermal expansion occurs when particles of
matter move farther apart as temperature
increases




Def: Specific heat is the amount of energy
transferred as heat that will raise the
temperature of 1 kg of substance by 1 K
Little c is the symbol for specific heat, &
specific heat is a physical property
The units for specific heat are J/kg-K
We will assume that volume & pressure do
not change in order to do calculations
involving the formula energy = specific
heat x mass x temperature change OR
energy = cm∆t (the ∆ means change & is
the final temperature minus the initial
temperature)





Objectives:
1. Describe conduction, convection and
radiation & identify which one of these
is occurring in a given situation
2. Classify materials as thermal
conductors or thermal insulators
3. State the 3 laws of thermodynamics
4. Apply the first & second laws to
given situations



Def: conduction is the transfer of thermal
energy with no overall transfer of matter
Conduction is the heat energy transfer between
particles as they collide within a substance or
between objects in contact (ex: roasting
marshmallows using a wire coat hanger)
Conduction in gases is slower than in liquids
and solids because the particles of gas collide
less often




Def: a thermal conductor is a material
that conducts thermal energy well
Def: a thermal insulator is a material that
conducts thermal energy poorly
Def: convection is the transfer of thermal
energy when particles of a fluid move
from one place to another
Convection is the heat energy transfer by
movement of fluids with different
temperatures (ex: warm air rising, cools,
condenses, becomes dense and sinks)




A convection current is the flow of
liquid due to heated expansion
followed by cooling and contraction
Convection currents are important in
many natural cycles such as ocean
currents, weather systems, and
movements of hot rock in Earth’s
interior in the asthenosphere under the
lithospheric plates
Def: radiation is the transfer of energy
by waves moving through space
Radiation does not require physical
contact between objects
Radiation does not involve
movement of matter & can take
place in a vacuum (ex: the radiation
we receive from the sun is in a
vacuum)
 All objects radiate energy
 As an object’s temperature increases,
the rate at which it radiates energy
increases
 Q: What is radiation?

Def: thermodynamics is the study of
conversions between thermal energy
and other forms of energy
 The First Law of Thermodynamics
states that energy is conserved
 The Second Law of Thermodynamics
states that thermal energy can flow
from colder objects to hotter objects
only if work is done on the system

Def: waste heat is thermal energy
that is not converted into work
 Spontaneous changes and waste
heat always make a system less
orderly
 The Third Law of
Thermodynamics states that
absolute zero cannot be reached
 Scientists have gotten close, but
have not been able to do this

Objectives:
 1. Describe heat engines and
explain how they convert thermal
energy to mechanical energy
 2. Describe how different heating
systems operate
 3. Describe how cooling systems
operate

Def: a heat engine is any device that
converts heat into work
 The two main types of heat engines
are the external combustion engine
and the internal combustion engine
 Def: the external combustion engine
is an engine that burns fuel outside
the engine
 Ex: steam engine

 Def:
an internal combustion is a
heat engine in which fuel burns
inside the engine
 Ex: your car
 Both types of engines have
pistons that move either back
and forth or up and down
 Some waste energy is
discharged




The principles of transfer of heat allow us to
design buildings that can be heated and
cooled efficiently
Recall that total energy is conserved whether
it is transferred due to work, heat or both
This is called the first law of
thermodynamics
Secondly, energy transferred as heat always
moves from an object of high temperature to
an object at low temperature





A heating system is any device or
process that transfers energy to a
substance to raise the temperature of
the substance
Work can be done to raise
temperatures
Mechanical processes are energy
transfer done by work
Def: a central heating system heats
many room from one central location
(often the basement)
Most heating systems use convection
to distribute thermal energy






Hot water heating is usually by boiler,
heating oil or natural gas
A pump circulates the water to radiators in
each room that transfer energy by
conduction
Most heating systems use these principles
A refrigerator is a heat pump
It transfer thermal energy from the cold
food compartment to the warm room
A cooling system is a device that transfers
heat energy out of an object to lower its
temperature




Air conditioners use a liquid
refrigerant to decrease temperature
and then transfer the warm air out
Def: A refrigerant is a substance used
in cooling systems that transfers large
amounts of energy as it changes state
Condensation transfers energy to the
surroundings
Condensation uses a refrigerant as
part of the process of transferring heat
away