Transcript Chapter 4 - "Energy"

•Energy
• Work.
• Introduction
– A force is a result of an interaction between
objects that can change the state of motion of
an object.
– Work is the result of the force applied to an
object and the distance the object moves as a
result of the force.
This is Glen Canyon Dam on the Colorado River
between Utah and Arizona.
The dam is among the tallest in America, so you can
imagine the tremendous pressure on the water as it
moves to generators at the bottom of the dam.
– The work done on an object is the magnitude
of the applied force multiplied by the parallel
distance through which the force acts.
• Work = force X distance
• W = Fd
– Something must move when work is done.
– The movement must be in the same direction
as the applied force.
– A force is a vector that can be resolved into the
component force that acts in the same direction
as the movement.
• Units of Work.
– The units of force is the Newton
– The distance moved is in meters.
– So the units of work in the metric system is
• W = Fd
• W = (Newton)(Meter)
• W = Nm
The force on the
book moves it
through the vertical
distance from the
second shelf to the
fifth shelf, and the
work is done,
W = Fd.
It is true that a force is exerted simply to hold a book,
but the book does not move through a distance.
Therefore the distance moved is 0, and the work
accomplished is also 0.
If the direction of
movement (the distance
moved) is perpendicular
to the direction of the
force, no work is done.
This person is doing no
work by carrying a book
across a room.
– The unit of work in the metric system is
the Nm.
• The Nm is called a joule
• The units for the Newton are kgm/s2
• The units for distance is meter, therefore.
• The units for the joule are kgm2/s2
• The unit of work in the English system is the
ftlb
A force at some angle (not 90O) to the direction of
movement can be resolved into the horizontal
component to calculate the work done.
Work is done against
gravity when lifting
an object. Work is
measured in joules
or foot-pounds.
– Example:
• How much work is needed to lift a 1,000.0 kg
boulder to 1,000.0 m above sea level.
• Since weight is really a force we can first
calculate the weight (force) necessary to lift
the boulder.
• w = mg
• (1,000 kg)(9.8m/s2)
• (1,000 X 9.8)(kg X m/s2)
• 9,800 kg  m/s2
• 9,800 N
• Work = Force X distance W = Fd
• (9,800N)(1,000m)
• 9,800,000 Nm
• 9.8000 X 106 J
• Power.
– Power is the rate at which work is done.
– Power is defined as work per unit of time.
• Power = work/time
• P = W/t
– Horsepower is defined as a power rating of 550
ftlb
• To convert from ftlb to horsepower, divide
by 550 ftlb/s/hp
(A) The work
accomplished in
climbing a stairway
is the person's
weight times the
vertical distance.
(B) The power level
is the work
accomplished per
unit of time.
(A) A horsepower is
defined as a power
rating of 550 ft?lb/s.
(B) A watt is defined
as a newton-meter per
second, or joule per
second.
If moving a book from
the floor to a high shelf
requires 10J of work
then the book will do
10 J of work on an
object of the same mass
when the book falls
from the shelf.
– In the metric system power is measured
in J/s.
• This is called a watt (W).
• Since a J is kgm2/s2, it follows that a watt is
actually kgm2/s2/s, or kgm2/s3.
• Metric prefixes can be used to show very
large measures of power, such as M, G, etc…
• 746 W = 1 hp
• Motion, Position, and Energy.
• Energy is the ability to do work
• Potential Energy.
– Potential energy is the energy that an object
has due to its position
– Most potential energy is actually gravitational
potential energy, since it is due to the
gravitational attraction of the Earth for an
object.
– For the metric unit of mass, weight is the
product of the mass of an object times g, the
acceleration due to gravity.
– Potential energy = weight X height
• PE = mgh
The zero referenced level for potential energy is
chosen for convenience. Here the reference position
chosen is the third floor, so the book will have a
negative potential energy at ground level.
– Example
• What is the potential energy of a 2000.0 lb
boulder 12176 ft above sea level, perched
above the city of Denver (elevation 5260.0 ft)
• PE = mgh = wh
• = (2,000.0 lb)( 12176 ft - 5260.0 ft)
• = (2,000 lb) (6916ft)
• = 13832000 ftlb
• = 1.3832 X 10 7
• Kinetic Energy.
– Kinetic energy is the energy that an object
contains due to its motion.
– Kinetic energy can be measured:
• In terms of the work done to put the object
in motion.
• In terms of the work the moving object will
do in coming to rest (transfer of energy to
another object).
– Kinetic energy is proportional to the mass of a
moving object, but the velocity of the object
has a greater influence.
(A) Work is done on the bowling ball as a force (FB)
moves it through a distance. (B) This gives the ball a
kinetic energy equal to the amount of work done on it.
(C) The ball does work on the pins and has enough
remaining energy to crash into the wall behind the
pins.
– Kinetic energy is proportional to the square of
the velocity.
• The kinetic energy of an object is kinetic
energy = 1/2 (mass) (velocity)2
• KE = 1/2mv2
• The unit of mass is the kg and the unit of
velocity is m/s.
• Therefor, the unit of kinetic energy is:
• KE = (kg)(m/s)2
• = (kg)(m2/s2)
• = kgm2/s2
• Which is the same as
–(kgm/s2)(m)
–or
–Nm
–Or
–Joule (J)
– Example
• A 3500 kg automobile is moving down the
interstate with a velocity of 83 km/hr, what is
the kinetic energy of the automobile?
• 83 km/hr X 1 hr/60min X 1 min/60s X 1000
m/km = 23.056 m/s
• Use KE = 1/2mv2
• KE = 1/2(3500.0 kg)(23.056m/s)2
• KE = 1/2 (3500.0 kg)(531.56 m2/s2)
• KE = 930227.62346
• KE = 9.3 X 10 5 kgm2/s2
• KE = 9.3 X 10 5 J
• Energy Flow.
• Work and Energy.
– Energy is used to do work on an object,
exerting a force through a distance.
– This force is usually against something and
there are five main groups of resistance.
• Work against inertia.
–Since inertia is an objects resistance to
change of motion, it naturally follows that
this would resist forces acting upon it.
• Work against fundamental forces.
–Gravitational attraction.
–Electromagnetic forces.
–Nuclear forces.
• Work against friction
–Friction is always present when two
objects are in contact with each other.
–Friction is always a force in the opposite
direction of the applied force.
• Work against shape.
–Work is needed to stretch or compress an
object.
–This is what happens when we work
against the shape of a spring.
• Work against any combination of inertia,
fundamental forces, friction, or shape.
Examples of
working
against:
(A) inertia,
(B) gravity,
(C) friction,
and
(D) shape.
– Some kind of energy change has taken
place, which may include one of the
following:
• Increased kinetic energy.
–Work against inertia results in energy of
motion for an object.
• Increased potential energy.
–Work against fundamental forces and
work against shape result in an increase in
energy of position (potential energy)
• Increased temperature.
–Work against friction always results in an
increase in temperature.
• Increased combination of kinetic energy,
potential energy, and/or temperature.
• Energy Forms. (five forms).
– Mechanical energy.
• Usually associated with the kinetic energy of
everyday objects and potential energy that
results from the effect of gravity.
Mechanical energy is the energy of motion, or the
energy of position, of many familiar objects. This boat
has energy of motion.
– Chemical energy.
• Chemical energy is the form of energy
associated with chemical reactions.
• Chemical energy is released during the
process known as oxidation.
• Chemical energy is potential energy that is
released when chemical reactions break
bonds in molecules.
Chemical energy is a form of potential energy that is
released during a chemical reaction. Both (A) wood
and (B) coal have chemical energy that has been
stored through the process of photosynthesis. The pile
of wood may provide fuel for a small fireplace for
several days. The pile of coal might provide fuel for a
power plant for a hundred days.
– Radiant energy.
• Radiant energy is the form of energy that
travels through space.
• Also called electromagnetic radiation.
• Visible light is one small part of the
electromagnetic radiation.
Radiant energy is energy that travels through space..
(A) This demonstration solar cell array converts
radiant energy from the sun to electrical energy,
producing an average of 200,000 watts of electric
power (after conversion). (B) Solar panels are
mounted on the roof of this house.
The electromagnetic spectrum
includes many forms of radiant
energy. Note that visible light
occupies only a tiny part of the
entire spectrum.
– Electrical energy.
• Electrical energy is a form of energy that
comes from electromagnetic interactions.
• Electrical energy that travels through the
wires in our homes to light or houses is a
familiar form of electrical energy.
– Nuclear energy.
• this is the form of energy generated in
nuclear power plants.
The blades of a steam turbine. In a power plant,
chemical or nuclear energy is used to heat water to
steam, which is directed against the turbine blades.
The mechanical energy of the turbine turns an electric
generator. Thus a power plant converts chemical or
nuclear energy to mechanical energy, which is then
converted to electrical energy.
• Energy Conversion.
– Energy can be converted from one form to
another.
– For example, during a fall PE lost = KE gained
– mgh = 1/2mv2
– Solving for vf
– vf = 2gh
– This allows you to calculate the final velocity of
a falling object after its potential energy is
converted into kinetic energy.
This pendulum bob loses potential energy (PE) and
gains an equal amount of kinetic energy (KE) as it
falls through as distance h. The process reverses as the
bob moves up the other side of its swing.
The ball trades potential energy for kinetic energy as it
falls. Notice that the ball had 98 J of potential energy
when dropped and has a kinetic energy of 98 J just as
it hits the ground.
The energy
forms and
some
conversion
pathways.
Energy arrives from the sun, goes through a number of
conversions, then radiates back into space. The total
sum eventually equals the original amount that
arrived.
– Example
• a 11.1 kg rock falls from a height of 10.1 m.
What is its velocity as it hits the floor.
• vf = 2gh
• = 2(9.8m/s2)(10.1m)
• = 197.96 m2/s2
• = 14.1 m/s
• Energy Conservation.
– Any form of energy can be converted into
another form.
– The total amount of energy remains constant.
– Law of Conservation of Energy:
• Energy is never created or destroyed.
Energy can be converted from one form to
another, but the total energy remains
constant.
• Energy Transfer.
– Any time energy is transferred, either work or
heat is involved.
• Energy Sources Today.
• Introduction
– Petroleum is our most widely used source of
energy.
• Petroleum provides about 40 percent of the
energy used by the US.
– Natural gas provides about 23 percent of our
energy needs.
– Coal provides about 23 percent of our energy
needs.
– Alternative energies (solar, wind, geothermal)
provide less than 0.5 percent of the total.
– Over 99 percent of our energy needs are
supplied by 4 sources:
• Petroleum.
• Coal.
• Hydropower.
• Nuclear.
• Petroleum.
– Petroleum is oil that comes from oil bearing
rocks.
– Petroleum and natural gas come from organic
sediments, material that have settled out of
water.
– Most of the organic material comes from
plankton (phyto- and zoo-)
– The process of converted live organisms into
petroleum and natural gas takes millions of
years.
– Natural gas forms under higher temperatures
that petroleum.
• Coal.
– Coal forms from an accumulation of plant
materials that collected millions of years ago.
– Carbon rich decayed plant material is called
peat.
– Pressure, compaction, and heating are brought
about by movement of the Earth's crust
eventually change the water content and
release the carbon in the materials, it has now
begun the process toward coal formation.
– Coal is ranked according to how long it took to
form and how hard it is.
• Lignite is the lowest ranked and is softest,
took the least time to form, and burns
quickest so contains the least amount of
usable energy.
• Bituminous is the next highest raking.
• Anthracite is the hardest and took the
longest to form and so contains the most
usable energy.
• Softer coal also has more impurities which
contribute to increased pollution levels.
• Water Power.
– Moving water is a source of renewable energy
that has been used for thousands of years.
– At present in the US we have built about all of
the hydropower plants that we can as we have
no usable sources of moving water left.
• Nuclear Power.
– Nuclear power plants use the energy that is
release from the splitting of uranium atoms
and plutonium atoms to produce electrical
energy.
(A) The sources of
energy and
(B) the uses of energy
during the 1990s.