Download Chapter 6 Energy

ENERGY
• Energy- the ability to do work
– SI unit for energy is Joules (J)
– 2 basic forms
• Potential energy- energy of position
• Kinetic energy- energy of motion
• Energy is classified by its source
– Mechanical potential energy
• Common example is gravitational potential energy
Energy
• Gravitational potential energy
– GPE= mgh
– EX. How high has a 7.26 kg shot put been thrown if it
has a potential energy of 240.2 J?
– h= GPE/(mg)
– GPE=240.2 J
– m= 7.26 kg
– g= 9.81 m/s2
– h=?
– h= 240.2 J/ [(7.26 kg)(9.81 m/s2)]
– h= 3.37 m
Kinetic Energy
• Kinetic energy- energy of motion
– KE= ½ mv2
• What is the kinetic energy of a 14.5 g rifle bullet
moving at 625 m/s?
–
–
–
–
–
–
KE=?
m= 14.5 g (convert to kg)= 0.0145 kg
v= 625 m/s
KE= ½(0.0145 kg)(625 m/s)2
KE= ½(0.0145 kg)(390,625 m2/s2)
KE= 2832 J or 2830 J
Chapter 6 Energy
•Thermal Energy- the sum of all of the kinetic
energies of all the particles in an object
•Theoretically, At 0 Kelvin all particle movement
stops, but this is not achievable on earth
•Acoustic Energy- involves random vibrations
and motion of particles
•Produces sound
•must travel through matter-cannot exist in a
vacuum
•Moves in periodic motion- back and forth
vibrations
Energy
• Electrical Energy- forces acting on other electrical
charges
• Opposite attract, like repel
• Natural sources- lightning, electric rays and eels
• Man made sources- batteries, generators, etc
• Magnetic Energy-ability to do work through the
influence of a magnetic field
– Opposite attract, like repel
• Radiant Energy- also called electromagnetic
energy- does not require matter through which
to travel, can travel in a vacuum
– Most common form is visible light
Energy
• Chemical Energy- potential energy stored in
chemical bonds
– During chemical reactions atoms rearrange their
bonds and energy is released or absorbed
– Energy can be released as thermal, radiant, or
acoustic energy
– Note: we will go over this in more detail in
Chapters 18 and 19
Energy
• Nuclear Energy- this is the energy associated
with the nucleus of an atom
– Can be released in 2 ways
• Fusion- is the combining of 2 or more nuclei to form a
larger nuclei
– stars 2 H atoms combine to form He
• Fission- is the splitting apart of a nucleus into 2 more
smaller nuclei
– Man made nuclear power plants to generate power
Nuclear energy always releases radiation-DANGER
REMEMBER- energy cannot be created nor
destroyed only transferred- all energies in the
end must equal that with which you started!!
Energy
• Mass Energy- this is the largest source of
potential energy in the universe
– It is the energy equivalent to all matter itself
– This is where Einstein’s theory is applied
• E= mc2
• Energy is equal to the mass times the speed of light
squared
E=mc2
• What is the energy equivalent of the mass of a
0.01 g plant mite, a tiny spider-like animal?
(remember that c= 3.0 x 108 m/s)
–
–
–
–
–
–
–
–
–
E=mc2
E=?
m= 0.01g (you have to convert to kg, divide by 1000)
m= 0.00001 kg or 1.0 x 10-5 kg
c= 3.0 x 108 m/s
E= 1.0 x 10-5 kg (3.0 x 108 m/s)2
E= 1.0 x 10-5 kg (9.0 x 1016 m2/s2)
E= 9.0 x 1011 kg m2/s2
E= 9.0 x 1011 J
Example 6-3
• How much mass energy, in joules, could be
obtained from the complete conversion of a
compact 138 g media player?
– m= 138 g = 0.138 kg
– c= 3.00 x 108 m/s2
– E= ?
– E= mc2
E= 0.138 kg (3.00 x 108 m/s)2
E= 1.242 x 1016 kg* m2/s2
E= 1.24 x 1016 J
Energy Conservation
• 1st Law of Thermodynamics- conservation of
energy
• energy cannot be created nor destroyed only
transferred, therefore the energy before the
transformation must equal the energy after
– Sometimes when energy is transferred some
energy is ‘lost’ to the surrounding,
• Because of this we measure the efficiency
– This a comparison of the measurement of the amount of
usable energy produced with the amount available before the
transformation
Energy Conservation
• Energy can be transferred to one or more
forms of energy
– Chemical energy from food becomes thermal
energy in our cells and mechanical energy in our
muscles
• The 1st Law of thermodynamics is easily
demonstrated by a pendulum
– Consists of a heavy weight at the end of an arm
that swings back and forth on a pivot point at its
upper end
Collisions and Energy
• As we have discussed before momentum and
kinetic energy are properties of motion
• Momentum is a vector defining a system’s
quantity of motion
• Kinetic energy is a scalar quantity describing
the mechanical energy of a moving system
• Collisions……………………….
Collisons
• There are 3 types of collisions
– 1. Elastic collisions- occurs when 2 objects collide
and bounce (rebound) off from one another
• The sum of their momentums and the sum of their kinetic
energies are the same before and after the collision
– 2. Partially elastic collisions – occurs when one or
both objects in a collision are deformed before
rebounding
– 3. Inelastic collision- occurs when two objects collide
and stick together
REMEMBER ENERGY IS NOT LOST BUT TRANSFERRED