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### Ppt Power

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