Download Energy powerpoint - River Dell Regional School District

Presentation on
Energy and Work
SYSTEM
Energy transfer
ENVIRONMENT
Work = W
Motion energy = “kinetic energy” = KE
Stored energy = “potential energy” = PE
Mechanical energy = ME = KE + PE
System boundary
Diagram adapted from pg 133 Five Easy Lessons by Randall D Knight
Energy is like Money
• Money, like energy, has no single unique
definition.
• A simple, yet inadequate definition of energy as
“the ability to do work” is analogous to a
definition of money as “the ability to purchase
goods”
• Energy, as a quantity can be either transferred
to or from the system or transformed within the
system.
Adapted from Five Easy Lessons by Randall D Knight
Money can be transformed
in various ways.
• Dollars can be transformed to
coins or vice versa.
• Yen can be transformed to euro.
• I can transform some of my
money into stocks or bonds.
Energy can be transformed
in various ways.
• I can transform some of the chemical
energy stored in my body into kinetic
energy by moving my body.
• A wind farm can transform kinetic
energy of air molecules into electrical
energy.
Money can transferred
in various ways.
•By working here, I get money
automatically transferred from the school
into my savings account at the bank.
•Then, every week, I transfer that money
to gas stations, grocery stores, etc. etc.
Energy can transferred
in various ways.
• Energy can be transferred from one object to
another. A stretched bowstring (Elastic Potential
Energy) can launch an arrow. (Kinetic Energy)
• A moving cue ball (Kinetic Energy) hits a
stationary billiard ball. The cue ball stops, and
the billiard ball moves (Kinetic Energy).
Again. . .
• Energy can be transferred to or from a
system.
OR
• Energy can be transformed within a
system.
Work : energy in transfer
• Work is only present when energy is
being transferred to or from a system.
• Work can be (+) positive or
(-) negative.
• + Work : earn $$ at a job = $$ coming in
• - Work : pay bills
= $$ going out
Kinetic Energy
• This is motion energy.
• Like “Fast Cash”, $ in your
pocket, it is going to be spent.
• KE = ½ mv2
Potential Energy
• Stored Energy – due to position,
location, or chemicals.
• “Stored Cash”, $ in bank, Savings
Accounts
• PEgrav = mgh
• PEspring = ½ kx2
Units are Joules
• The units for both work and
energy are Joules.
(Hint: think “jewels”, as in you earn
“jewels” in the money analogy)
Energy is conserved.
• Like momentum, the total energy
in an isolated system is
conserved.
• Be careful about whether your
system is truly closed or not.
Energy Graveyard
R.I.P
You can’t
get it
back . . .
Disapated,
but not
forgotten.
Energy appears to be “lost”, when it is
transferred to its surroundings in the form
of heat due to friction, or sound,
or vibrations. . . etc. etc.
Last, but not least, . . . Power!
• Power is simply the rate at which
work is done or energy is transferred
or transformed.
• Power = Work
time
• Power is measured in Watts
• Watt = Joule/sec