Presentation on Energy and Work

ENVIRONMENT

Diagram adapted from pg 133 Five Easy Lessons by Randall D Knight

Energy transfer

Work = W

SYSTEM

Motion energy = “kinetic energy” = KEStored energy = “potential energy” = PE

Mechanical energy = ME = KE + PE

System boundary

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.PYou 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