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Energy and Efficiency Ch 12.3 & 12.4

Energy and Efficiency Ch 12.3 & 12.4. Energy and Work. Energy: is the ability to do work (to move a force a distance) is measured in Joules, just like work is present in all objects, but is observed when transferred from 1 object to another.

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Energy and Efficiency Ch 12.3 & 12.4

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  1. Energy and EfficiencyCh 12.3 & 12.4

  2. Energy and Work • Energy: • is the ability to do work (to move a force a distance) • is measured in Joules, just like work • is present in all objects, but is observed when transferred from 1 object to another

  3. Gravitational Potential Energy • Stored energy • Energy of position (due to where something is) • Depends on the mass of an object and its height. • GPE = m g h • = mass x accel. due to gravity x height kg x m/s2 x m = N x m = J !

  4. Gravitational Potential Energy • If two things have the same mass and one is higher than the other, which one hurts more if it hits you? Higher one • Why? Higher = more energy • So… the higher something is the more GPE it has.

  5. Gravitational Potential Energy • If two things have the same height and one is heavier than the other, which one hurts more if it hits you? Heavier one • Why? Heavier = more energy • So… the heavier something is the more GPE it has. • movie

  6. Elastic Potential Energy • stored by stretching or compressing something • EPE = ½ kx2 • k = a constant (“springiness” of the material) • x = displacement (how much it’s stretched or compressed) • More you stretch = more energy stored

  7. How does the energy “get there?” • In the case of potential energy, the energy has to come from somewhere. • Something did work on the object to store the energy • A roller coaster at the top of the hill was put there because of work done by a motor • A stretched rubber band has EPE because of work done by you • The work done is stored as energy…the ability to do work!

  8. Kinetic Energy • Energy of motion • Depends on mass and velocity • KE = ½ m v2 • = ½ mass x velocity2 kg x (m/s)2 = kg x m2/s2= J !

  9. Kinetic Energy • Two things have the same mass, but one is faster than the other. Which one hurts more if it hits you? Why? the faster one…it has more energy! • So, the faster it’s going, the more KE it has.

  10. Kinetic Energy • Two things have the same velocity, but one is heavier than the other. Which one hurts more if it hits you? Why? the heavier one…it has more energy! • So, the heavier it is, the more KE it has • movie

  11. GPE & KE • Where is the…. Most GPE? Least GPE? Most KE? Least KE? • How can you tell? • Does it have both GPE and KE anywhere? • What other forms of energy are involved? B…highest B. D…lowest D….fastest C. B…slowest A. YES! Everywhere! D.

  12. Other forms of energy • Mechanical energy: NOT energy of machines • KE + PE, Combination of KE andPE • Chemical energy: energy stored in chemical bonds • Electromagnetic energy: energy in light/waves • Electrical energy: energy of moving electrons • Thermal energy: heat (really KE of moving molecules) • Sound energy: compression/stretching of air molecules (travels in waves) • Nuclear energy: energy withinatoms (fission…splitting atoms or fusion…combining atoms)

  13. Energy nonmechanical mechanical GPE EPE KE thermal electrical nuclear chemical sound electromagnetic

  14. Other forms of energy • It’s not always clear what form of energy it is. It could be a combination. • Solar energy is it Nuclear (fusion)? is it Electricity (solar panels)? is it Electromagnetic (waves)? • Batteries: is it Chemical (chemicals in batteries)? is it Electrical (make electricity)?

  15. Energy Transformations • Energy can change from one form to another • Example: Car at top of hill, not moving (GPE). Rolls down the hill (KE). • Every time energy is transferred from one object to another or from one form toanother, work is done by a force. • The engine worked to put the car at the top of the hill (stored as GPE) • Gravity does work to transfer the energy to KE.

  16. Law of Conservation Energy • Energy can not be created or destroyed, it just changes form • Sometimes it appears we lose energy because it is turned into forms we can not use...heat, sound, etc, by the force of friction • Open system, energy is free to move in or out (e.g. boiling water…heat escapes). Some energy is “lost,” i.e. turned to forms we can’t use (heat, sound) • Closed system, energy does not move in and out. • Most of what we deal with are open systems, so we don’t always get the results we expect because friction changes some of our energy to useless forms and its “lost.” • Movie…law of cons.

  17. Efficiency • The amount of work that actually gets done by a machine (not “lost” due to friction, etc.) • Doesn’t really have any units, but is given as a percent. • Is never more than 100% Why? What happened to the rest of the energy you put in? some energy is always lost to “useless” forms like heat and sound • How can you improve the efficiency? Decrease friction…grease it, smooth it, etc.

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