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Per9_over - Preview of Period 9 Power 9.1 Measuring the...

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Preview of Period 9: Power 9.1 Measuring the Power of Appliances How much power do appliances use? 9.2 Measuring Your Power How much power do you use for daily activities? 9.3 Measuring the Cost of Electricity How much does electricity cost? 9.4 Growth in Electricity Use How has electricity use increased over the past century? 9-1
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Review of Work and Energy Work is done on an object against a force. An object is lifted against the force of gravity A box is pushed across the floor against the force of friction. W = F D Objects gain energy when work is done on them. A raised object gains gravitational potential energy. A box pushed across the floor gains kinetic energy of motion. E pot = M g h E kin = ½ M v 2 Machines make tasks easier by reducing the force we must apply ( F in ), but they cannot reduce the amount of work needed. 9-2
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What is Power? Power is a rate describing the amount of work done per unit of time: Power = Work/time elapsed P = W t We can also think of power as the amount of energy transferred per unit time: Power = Energy transferred/time elapsed P = E t Power is measured in units of joules/second. One joule/second = 1 watt Power is also measured in ft-lbs/sec or horsepower (hsp). One horsepower = 746 watts One ft-lb/sec = 0.0018 hsp = 1.36 watts 9-3
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Act. 9.1- 9.2: Power Requirements Light the bulbs using power generated by your muscles. Which energy conversions take place as you crank the generator? Do the power requirements of the appliances match their wattages? Which has the largest power requirement? How does increasing the “load” on the drill affect its power requirement? Time yourself climbing the stairs. How much gravitational potential energy do you gain?
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