Download I) Work II) Power III) Mechanical Energy

Chapter 9 Energy
I)
Work
A) Work is done when a force acts on an object and the object moves in the direction
of the force.
B) Work is the “product” of the force on an object and the distance through which
the object is moved.
1) In the simplest case, when: 1- the force is constant, and 2-the motion takes
place in a straight line in the direction of the force:
(a) work = force × distance.
(b) In equation form, W = Fd.
C) Work generally falls into two categories:
1) Work done against another force and
2) work done to change the speed of an object.
D) The unit of work is the newton-meter (N·m), also called the joule (J).
1) One joule of work is done when a force of 1 N is exerted over a distance of 1
m.
E) Work always involves a transfer of energy between something and its
surroundings
II) Power
A) Power is the rate at which work is done:
B) Power equals the amount of work done divided by the time interval during which the
work is done.
1) power = work done / time interval
C) A high-power engine does work rapidly. If an engine has twice the power of
another engine, this means that it can do twice the work in the same amount of
time
1) or the same amount of work in half the time.
D) The unit of power is the joule per second (J/s), which is also known as the watt
(W).
One watt of power is expended when one joule of work is done in one second.
E) In the United States, we customarily
1) rate engines in units of horsepower and
2) electricity in kilowatts, but either may be used.
(a) One horsepower (hp) is the same as 0.75 kW.
III) Mechanical Energy the energy due to the position or the movement of
something.
A) The property of an object or system that enables it to do work is energy.
1) The two forms of mechanical energy are kinetic energy and potential energy.
B) Like work, energy is measured in joules.
C) Energy is the ability to DO WORK
IV) Potential Energy is energy that is stored and held
A) (PE) because in the stored state it has the potential for doing work.
B) Three examples of potential energy are elastic potential energy, chemical
energy, and gravitational potential energy.
1) A stretched or compressed spring, a bow that is drawn back, and a
stretched rubber band have elastic potential energy.
2) The chemical energy in fuels is potential energy at the submicroscopic level.
(a) The energy is stored in bonds. When they are broken, energy is released.
3) The potential energy due to the elevated position of an object is
gravitational potential energy.
(a) The amount of gravitational potential energy is equal to the work done
against gravity in lifting it.
(b) Gravitational potential energy = weight × height.
• PE = mgh.
• The height in this equation is the distance above some chosen
reference level.
V) Kinetic Energy = energy of motion
A) The kinetic energy of a moving object is equal to the work required to bring it
to its speed from rest, or the work the object can do while being brought to
rest.
B) The energy of motion is kinetic energy (KE).
1) KE = 1/2mv2.
VI) Work-Energy Theorem
A) The work-energy theorem states that whenever work is done, energy changes.
B) Work equals change in kinetic energy.
1) Work = ΔKE,
2) Since W = Fd and W = KE
(a) Fd = ½ mv2.
(b) where the delta symbol, Δ, means “change in.”
3) ∆PE = Work
C) Kinetic energy often appears hidden in different forms of energy.
1) Random molecular motion is sensed as heat.
2) Sound consists of molecules vibrating in rhythmic patterns.
3) Light energy originates in the motion of electrons in atoms.
4) Electrons in motion make electric currents.
VII) Law of Conservation of Energy - energy cannot be created or destroyed
A) Energy can be transformed from one form into another, but the total amount of
energy never changes.
1) If all energy is PE to start, and KE to end, than starting PE = ending KE
(a) Total E = PE + KE
B) Examples:
1) Everywhere along the path of a pendulum, the sum of potential energy and
kinetic energy is the same.
(a) At the highest points, the energy is only potential energy.
(b) At the lowest point, the energy is only kinetic energy.
2) The sun shines because some of its nuclear energy is transformed into
radiant (EM) energy. In nuclear reactors, nuclear energy is transformed into
heat.
3) Some electric-generating plants transform the energy of falling water into
electrical energy. Electrical energy then travels through wires to homes.
VIII) Machines transfer energy
A) A machine is a device used to multiply forces or to change the direction of
forces.
1) A machine cannot put out more energy than is put into it.
B) work input = work output
1) (force × distance)input = (force × distance)output
2) (only if energy lost to heat is small, which we usually assume)
C) The ratio of output force to input force for a machine is called the mechanical
advantage.
1) MA = (Output force) / (Input Force)
D) A lever is a simple machine made of a bar that turns about a fixed point.
1) The pivot point of a lever is the fulcrum.
E) Types
1) A type 1 lever has the fulcrum between the input force and the load.
2) For a type 2 lever, the load is between the fulcrum and the input force.
3) In a type 3 lever, the fulcrum is at one end and the load is at the other.
F) Effects
(a) Increasing the distance travelled allows you to decrease the force
needed
(b) The directions of input and output can be the same or opposite.
G) A pulley is a kind of lever that can be used to change the direction and multiply
a force.
1) There are several types:
(a) A single pulley with a
• MA = 1
fixed axis behaves like a type 1 lever.
(b) A single pulley with an axis that
• MA = 2
moves behaves like a type 2 lever.
(c) A system of pulleys multiplies the force.
• The mechanical advantage for a simple pulley system is the same as
the number of strands of rope that actually support the load.
H) An inclined plane is a machine.
1) Its theoretical mechanical advantage is the length of the incline divided by
the height of the inclined plane.
(a) MA = length / height
2) A screw is an inclined plane wrapped around a cylinder
IX) Efficiency -The efficiency of a machine is the ratio of useful energy output to
total energy input
A) or the percentage of the work input that is actually converted to work output.
1) No real machine can be 100% efficient.
2) The wasted energy is dissipated as heat.
(a) Lost energy is transformed into atomic or molecular kinetic energy—making
the machine warmer.
B) Efficiency can also be expressed as the ratio of actual mechanical advantage to
the theoretical mechanical advantage.
1) Efficiency = actual MA / theoretical MA
2) To convert efficiency to percent, express it as a decimal and multiply by
100%.
X) Energy for Life
A) Consumers eat food and change the chemical energy to maintain life processes
1) There is more energy stored in the molecules in food than there is in the
reaction products after the food is metabolized.
2) In metabolism of food in the body, carbon combines with oxygen to form
carbon dioxide.
(a) Carbon (w/ O & H) is the primary component of carbohydrates and fats,
the main sources of energy for all living things
(b) Carbon chain + O2  CO2 + Energy
• (this reaction occurs in the Mitochondria)
B) Only green plants and certain one-celled organisms can make carbon dioxide
combine with water to produce hydrocarbon compounds such as sugar.
1) This process is called photosynthesis and requires an energy input, which
normally comes from sunlight.
XI) Energy In Life
A) The sun is the source of practically all our energy on Earth.
1) Sunlight moves through space as EM energy and changes forms when it
reaches earth
2) Sunlight is directly transformed into electricity by photovoltaic cells
3) Geothermal & Nuclear energy are not derived from solar energy
B) Wind, caused by unequal warming of Earth’s surface,
1) Wind can be used to turn generator turbines
C) Hydrogen is the least polluting of all fuels.
1) Because it takes energy to make hydrogen (to extract it from water and
carbon compounds), it is not a source of energy.
2) In a fuel cell, hydrogen and oxygen gas are combined at electrodes to
produce water and electric current.
D) The most concentrated form of usable energy comes from nuclear power.
1) Nuclear power manipulates Einstein’s equation E = MC2 to produce huge
amounts of energy
2) Nuclei of atoms are split (fission) or combined (fusion).
(a) The new nucleus will have less mass than expected. (lost mass becomes
energy)
3) Nuclear stability is based on the isotopic ratio (p+:n0) and nuclear mass of an
atom
(a) iron-54 is the most stable isotope known (1:1)
(b) U-235 is a common source of nuclear fuel
• products of uranium decomposition are radioactive (dangerous) for
millions of years
(c) HHe fusion is the ideal nuclear energy source (safe products)
• Not yet feasible b/c reaction requires high temperatures
• This is the main reaction occurring in the sun
4) Fission reactors use fuel separated by lead bars to maintain a controlled
chain reaction
(a) A neutron can cause a U-235 to split. When t splits it releases 3 more n0
which will cause another 3 atoms to split (if they reach them)
(b) The lead bars catch n0 allowing us to control how many nuclei split
(c) The energy released heats water which then spins turbines to make
electricity
E) Earth’s interior is still hot from the massive energy stored in formation. The
crust acts as blanket keeping it warm
1) Geothermal energy is held in underground reservoirs of hot water which can
be used directly to heat homes, or to turn turbines and make electricity
XII) Renewable Energy
A) Currently most energy comes from fossil fuel
1) These include oil, coal, & natural gas
2) They are NOT renewable
3) They release CO2
4) They are thought to contribute to the Greenhouse effect
(a) EM energy comes from the sun, becomes thermal energy, and is trapped
in the atmosphere (like a green house)
B) Renewable energies include wind, hydroelectric and geothermal
1) These technologies use turbines to create electricity
(a) magnets interact with electrons. When a magnet moves it causes a force
on any charge near it
(b) by making many loops of wire (or coils) this interaction can be multiplied
(c) Turbines (and generators) spin a magnet inside of a coil causing
alternating current electricity
C) Some renewables use new technology
1) photovoltaic cells (solar panels) use light to ‘push’ electrons (photoelectric
effect)
2) electrons are collected and create a direct current
3) these technologies are very expensive
D) All renewable have drawbacks
1) wind & solar are inconsistent
2) hydroelectric damages ecosystems
3) drilling to use geothermal energy can trigger earthquakes
(a) (so can drilling for fossil fuels)