Download Thermal Energy

WELCOME TO 1104
PERIOD 2
Homework Exercise #1 is due at the
beginning of class. Please put it on
the stool in the front of the classroom.
PHYSICS 1103 – PERIOD 2
•What are the fundamental forces?
•How is work done?
•How are forms of energy defined?
•What happens when one form of energy is
converted into another form?
Fundamental force – gravity
The gravitational force is an attractive force that exists
between all objects.
The gravitational force between the center of the
Earth and objects near it causes objects to fall toward
the surface of the Earth.
The gravitational force between the Earth and the Sun
holds the Earth in an orbit around the Sun.
Fundamental force – electromagnetic
The electromagnetic force holds matter together by
providing the force that binds atoms into molecules.
The force between particles of opposite charge is attractive.
+
The force between particles of the same charge is repulsive.
+
+
Since positive charges repel one another, how can
positively charged protons exist closely packed in an
atomic nucleus?
Fundamental force – strong nuclear
The strong nuclear force holds protons +P
and neutrons
together in atomic nuclei.
n
It is the strongest force.
It acts over very short distances inside a nucleus.
+P
n
n
n
+P
+P
n
+P
Fundamental force – weak nuclear
The weak nuclear force is responsible for some kinds
of radioactive decay.
The fission (splitting) of Uranium-235 nuclei into
smaller fragments in nuclear power plants is an
example of the weak force.
The weak force acts only when particles are close
together.
The weak force is the next to the weakest force.
(The gravitational force is the weakest of the forces.)
Summary of the fundamental forces
The gravitational force is an attractive force that
exists between all objects.
The electromagnetic force holds matter together by
providing the force that binds atoms into molecules.
The strong nuclear force holds protons and neutrons
together in atomic nuclei.
The weak nuclear force is responsible for some kinds
of radioactive decay.
Frictional force
The frictional force is NOT one of the fundamental
forces.
Friction arises from the electromagnetic force between
the atoms in an object and the atoms in the surface
the object moves across.
The gravitational force pressing an object down onto a
surface can increase the frictional force between the
object and the surface.
Friction results in wasted thermal energy.
Forces, work, and energy
Work is done when a force moves an object over some
distance in the direction of the force.
Work = Force x Distance
W = FxD
W = work (joules or foot-pounds)
F = force applied (newtons or pounds)
D = distance moved in the direction of the
force (meters or feet)
Work and energy are measured in units of joules.
1 joule = 1 kg m2/s2
Work done against friction
Work is done to move the box horizontally against
the force of friction.
The work done goes into thermal energy.
Applied
Force
Force of
friction
W=FxD
Work done against gravity
Work is done to raise the box vertically against the force
of gravity.
The work done goes into gravitational potential energy.
Applied Force equals the
weight of the box = Mg
Force of gravity
h = height the center
of gravity of the
object is raised
W = F x D can be written W = M g x h
Work and gravitational potential energy
Raising an object gives it gravitational potential
energy, which can be used to do work.
E pot
= M g h
M = mass of object (kilograms)
g = acceleration of gravity = 9.8 m/s2
h = change in vertical height (meters)
Neglecting the energy wasted by friction, the gain in
gravitational potential energy from raising an object
equals the work done to raise it.
Forms of energy: motion
Three forms of energy are related to energy of motion:
Kinetic Energy: All moving objects exhibit kinetic
energy.
Thermal Energy: The unorganized kinetic energy of
vibrating molecules and atoms within an object.
The higher the temperature of an object, the faster
its atoms and molecules vibrate.
 Sound Energy: When atoms and molecules vibrate
in an organized way, their vibrations may travel as a
sound wave.
Forms of energy: electric charge
Energy that results from the forces between electric
charges is called electromagnetic energy.
Electrical Energy: The result of forces between
charged particles.
 Magnetic Energy: Charges moving within some types
of materials produce magnetic forces.
Magnetic energy results from these forces.
Radiant Energy: Radiant energy results from
vibrations of charges.
Radiant energy (electromagnetic radiation)
Radiant energy results from vibrations of charges.
As the charges vibrate, they produce waves of energy.
The type of radiant energy depends on the wavelength.
Shorter wavelengths transmit more
energy than longer wavelengths.
Forms of stored (potential) energy
Stored energy, which can be used to do work, is called
potential energy.
Gravitational Potential Energy:
Energy stored in a raised object that has the
potential to fall, slide or roll to a lower position.
Strain Potential Energy:
Energy stored in a stretched or compressed spring
or elastic.
Electrical Potential Energy:
Energy stored when static positive and negative
charges are separated. Capacitors store electrical
potential energy.
Chemical Potential Energy:
Energy existing because atoms and molecules can
take in or give off energy when their chemical bonds
are formed or broken. Batteries store chemical
potential energy.
Nuclear Energy:
Energy available in the nuclei of atoms that are
radioactive and undergo nuclear changes.
Summary of the forms of energy
Kinetic
The energy exhibited by objects in motion.
Thermal
The unorganized energy of motion of vibrating atoms
and molecules.
Sound
The organized energy of motion of vibrating atoms and
molecules.
Electrical
The energy resulting from forces between charged
particles.
Magnetic
The energy resulting from the forces between magnets.
Radiant
The energy resulting from vibrations of charges, such as
radio waves, microwaves, or visible light.
Gravitational
The energy stored in raised objects that could fall.
Strain
The energy stored in a stretched or compressed spring.
Chemical
The energy available in the chemical bonds binding
atoms together.
Electrical
The energy stored by static electric charges.
Nuclear
Energy available in the nuclei of radioactive atoms.
Every conversion has some wasted energy!
Energy
Conversion
Useful energy out
Total energy in
Wasted energy out
Efficiency = Useful energy out < 100%
Total energy in
Law of conservation of energy
• Energy can be neither created nor destroyed.
• The total amount of energy we have now in the
Universe is the same amount that existed at its
beginning.
• All the energy that goes into a conversion process
must come out – no energy can be lost or destroyed.
• In every energy conversion, some energy is wasted.
• Wasted energy is often in the form of thermal energy
from the force of friction.
Efficiency of the water wheel
Efficiency =
Epot
Gravitational
potential
energy in
Useful work out
Total work in
= Mgh
Conversion
Process
Wasted energy out
Kinetic
energy out
BEFORE THE NEXT CLASS…
Read textbook chapter 3
Print out Activity Sheet 3 and bring it to class.
Complete Homework Exercise 2