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Transcript

  • 1. POWER
  • 2. Power
    • Suppose you and a friend are pushing boxes of books up a ramp and into a van.
    • Each box is the same weight.
    • You make a game…who can push the box up faster?
  • 3. Power
    • The friend moves the box up faster than you. She does it in 30s you do it in 45s.
    • You both do the same amount of work.
    • Why? How does this happen? She’s faster.
    • She was faster, and had more power, but you did the same amount of work.
  • 4. Power
    • Power is the amount of work done in a certain amount of time.
    • Power is a rate –
    • The rate at which work is done.
    • Power = Work/time
    • P = W/t
  • 5. Power
    • It took 20 minutes (1,200 sec) to move a box. You did 40,000 Joules of work in the process. How much power did you use?
    • t = 20min = 1,200 sec
    • W = 40,000 J
    • P = ?
    • Remember = Joules / second = Watt
    • Power is always measured in Watts
    • P = W/t
    • P = 40,000 J/1,200 sec
    • P = 33.33 J/sec
    • P = 33.33 W
  • 6. Calculating Power
    • Power is measured in
      • Watts (W)
    • The watt is named after James Watt. He helped to develop the steam engine.
    • A watt (W) is 1 J/s
    • A watt is a pretty small unit.
  • 7. Calculating Power
    • Large amounts of watts are usually shown in kilowatts.
    • One kilowatt (kW) is equal to how many watts?
    • 1,000 W
    • If you were to run up one flight of stairs it would use about 1 kW of power.
  • 8. Power & Energy
    • Doing work transfers energy from one object to another.
    • Energy can be transferred in other ways that don’t involve something moving.
    • Lightbulbs…
  • 9. Power & Energy
    • A lightbulb uses electrical energy to produce heat and light…but no work is done.
    • Power is made or used any time energy is transferred divided by the time needed for the transfer to happen.
    • Anytime energy is transferred from one object to another, power can be calculated.
    • Power = Energy/time
    • P=E/t
  • 10. Changing Energy by Doing Work
    • What happens to the energy of the book when you lift it off your desk?
    • You changed the height of the book so it changed the potential energy.
    • Where did this increase of energy come from?
    • You transferred energy to the book by lifting it up.
  • 11. Changing Energy by Doing Work
    • You can also increase the kinetic energy of something by doing work on it.
    • For example, pushing a chair…
    • When you push a chair, you do work on it and it now has kinetic energy.
  • 12. Changing Energy by Doing Work
    • Rub your two hands together. What happens?
    • They feel warm.
    • Why?
    • The energy of your hands increased in the form of heat caused by friction.
    • Anytime you do work on an object, you cause its energy to increase.