Transcript 5.4 Thermal Energy Notes

Chapter
5.4 Notes
Thermal Energy
• Thermal Energy is the
total kinetic energy
of the motion of
atoms in an object.
• Molecules in an
object are constantly
moving in a random
motion.
All molecules have 3
types of motion:
Translational movement
– forward or backward
movement
•Rotational
movement –
spinning motion
•Vibration – small,
fast movements
back and forth
• Temperature is decided by
how much a molecule or
atom bounces around a
container and hit another
molecule and atom. Which
type of movement is this?
• Translational
• Temperature of an object is
dependent upon translational
movement only.
• The total energy of an
object is called the internal
energy.
• As atoms collide, the
Potential energy and Kinetic
energy is constantly being
transferred. However, the
total energy or the internal
energy of the object always
stays the same.
• A body’s internal energy
depends on:
• Material Composition
• Mass
• Starting Temperature
• Physical State – Solid,
liquid, gas, or plasma
• Internal energy can be
transferred from one
object to another with a
temperature difference
or by heat.
• Another way to transfer
energy is by doing work
to produce friction.
Rubbing hands together.
• Other forms of energy
can also be converted
into internal energy.
Example –
• Electric stove uses
resistance to convert
electrical energy to
thermal energy
• The science dealing
with relationships
between internal
energy, heat, and
work is called
thermodynamics.
• There are 3 laws of
thermodynamics.
• The law of conservation
of energy says that
energy cannot be
created or destroyed but
can change from one
form of energy to
another form. This law
is also called the 1st Law
of thermodynamics.
• Internal energy is
represented by what
letter? U
• Heat is represented by
what letter? Q
• Equation for 1st law of
thermodynamics = Internal
energy = heat – work
• U=Q-W
• Heat is positive if it
enters the system and
negative if it leaves the
system.
• Work is positive when
the system does work
and work is negative
when work is done on
the system.
• A system that has 50 units of
heat entering the system and
40 units of work are done on
the system, what is the
internal energy?
• Heat is positive; work is
negative
• U=Q-W
• U = 50 –(-40) = 50 + 40
• U = 90
• A system that has 50 units of
heat entering the system and
the system does 40 units of
work, what is the internal
energy?
• Heat is positive; work is
positive
• U=Q-W
• U = 50 – (+40) = 50 - 40
• U = 10
• A system that has 50 units
of heat leaving the system
and 40 units of work are
done on the system, what is
the internal energy?
• Heat is negative; work is
negative
• U=Q-W
• U = -50 – (-40) = -50 +40
• U = -10
• A system that has 50 units
of heat leaving the system
and the system does 40
units of work, what is the
internal energy?
• Heat is negative; work is
positive
• U=Q-W
• U = -50 – (+40) = -50 – 40
• U = -90
• A process in which there is
no heat transfer to or from
the system is called an
adiabatic process.
• 2 ways to create an
adiabatic process
• Isolate the system with
insulation
• Do work quickly so there is
no time for heat transfer
•A device that
converts thermal
energy into
mechanical
energy is called
a heat engine.
• Example of heat engine
that burns gas – internal
combustion engine
•Example of heat engine
that uses steam - Train
• Example of heat engine that uses
chemical reactions - rocket
Every heat engine must:
• Absorb thermal energy
from a high-temperature
source
• Convert some of the
thermal energy to work
• Discard the remaining
thermal energy into a lowtemperature reservoir
• Reservoirs are usually the
Earth, the Earth’s
Atmosphere, or bodies of
water.
Thermal Pollution
nd
2
• The
Law of
Thermodynamics
says that heat will
flow from a body with
a higher temperature
to a body at a lower
temperature.