Presentation on theme: "Conservation and Exchange of Energy"— Presentation transcript:
1Conservation and Exchange of Energy Nothing Comes for Free
2Energy is ConservedConservation of Energy is different from Energy Conservation, the latter being about using energy wiselyConservation of Energy means energy is neither created nor destroyed. The amount of energy in the Universe is constant!!Don’t we create energy at a power plant?Oh if only that were true—no, we simply transform energy at our power plantsDoesn’t the sun create energy?Nope—it exchanges mass for energy
3Energy ExchangeThough the total energy of a system is constant, the form of the energy can changeA simple example is that of a simple pendulum, in which a continual exchange goes on between kinetic and potential energypivotK.E. = 0; P. E. = mghK.E. = 0; P. E. = mghheight referencehP.E. = 0; K.E. = mgh
4Perpetual Motion Why won’t the pendulum swing forever? It’s hard to design a system free of energy pathsThe pendulum slows down by several mechanismsFriction at the contact point: requires force to oppose; force acts through distance work is doneAir resistance: must push through air with a force (through a distance) work is doneGets some air swirling: puts kinetic energy into air (not really fair to separate these last two)Perpetual motion means no loss of energysolar system orbits come very close
5Some Energy Chains:A coffee mug with some gravitational potential energy is droppedpotential energy turns into kinetic energykinetic energy of the mug goes into:ripping the mug apart (chemical: breaking bonds)sending the pieces flying (kinetic)into soundinto heating the floor and pieces through friction as the pieces slide to a stopIn the end, the room is slightly warmer
6Gasoline Example Put gas in your car, containing 9 Cal/g Combust gas, turning 9 Cal/g into kinetic energy of explosionTransfer kinetic energy of gas to piston to crankshaft to drive shaft to wheel to car as a wholeThat which doesn’t go into kinetic energy of the car goes into heating the engine block (and radiator water and surrounding air), and friction of transmission system (heat)Much of energy goes into stirring the air (ends up as heat)Apply the brakes and convert kinetic energy into heatIt all ends up as waste heat, ultimately
7Bouncing Ball Superball has gravitational potential energy Drop the ball and this becomes kinetic energyBall hits ground and compresses (force times distance), storing energy in the springBall releases this mechanically stored energy and it goes back into kinetic form (bounces up)Inefficiencies in “spring” end up heating the ball and the floor, and stirring the air a bitIn the end, all is heat
8Why don’t we get hotter and hotter If all these processes end up as heat, why aren’t we continually getting hotter?If earth retained all its heat, we would get hotterAll of earth’s heat is radiated away
9No Energy for FreeNo matter what, you can’t create energy out of nothing: it has to come from somewhereWe can transform energy from one form to another; we can store energy, we can utilize energy being conveyed from natural sourcesThe net energy of the entire Universe is constantThe best we can do is scrape up some useful crumbs2Q
10Perpetual Machine Attempts Bhāskara’s model one of the first known attempts at a perpetual motion machine.
11Perpetual Machine Attempts Villard de Honnecourt (1230)
12Perpetual Machine Attempts Leonardo da Vinci’s Model
13Perpetual Machine Attempts Another Leonardo da Vinci’s Model
14Perpetual Machine Attempts Robert Fludd 1618 – accredited as the first device to produce work – it failed.
15Perpetual Machine Attempts There have been thousands attempts to create a perpetual motion machine or generator. All attempts have failed, not because of lack of understanding, but because the individuals have overlooked several important laws of physics, such asLaw of conservation of energy1st Law of Thermodynamics2nd Law of ThermodynamicsOne last thing, entropy – the thermodynamic property that is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. (We will get further in depth in this later in the semester.)