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Measuring Electrical Energy Lesson 11

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Measuring Electrical Energy What is Energy? Energy- The ability to do work Electrical Energy- The energy transferred to an electrical load by moving charges.

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The symbol for electrical energy is E and the SI unit for measuring energy is called the joule. 1 joule is a very small amount of energy so we use Watt hour and Kilowatt hour.

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1 watt hour = 3600 joules. 1 kilowatt hour = 1000 watt hours.

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Observing sample circuits Circuit 1 has two 1.5 volt batteries in series producing a total of 3.0 volts. The circuit can keep the bulb lit for 5 hours.

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Circuit two has two 1.5 volt batteries in series producing a total of 3.0 volts. There are two bulbs in parallel in this circuit which can stay lit for 2.5 hours.

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Why does circuit two 2 last half of the time that circuit 1 does? It is using more energy.

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Calculating Electrical Energy We can use a formula to calculate the amount of energy used in household appliances. Energy = Power x Time kWh kW h Total Energy Used = Speed That Electricity is used x How long electricity is used

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How much energy is that? Examples : How many kilowatt hours of electrical energy are used by a kettle that has a power rating of 1.5kW and is operated for 1.0 hours? Energy = Power x Time E = P x T E = 1.5 kW x 1.0 hour E = 1.5 kWh

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Using a vacuum cleaner runs 2 for hours with a power rating of 1.6 kW. E = P x t E = 1.6 kW x 2 h E = 3.2 kWh

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Mowing a lawn for 3.5 hours with a mower that has a rating of 1.65 kW. E = P x t E = 1.65 kW x 3.5 h E = 5.78 kWh

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Something to remember Power must be in kW 1000 W = 1 kW To change from W to kW divide by 1000

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Example Example 1 400 W = ? kW 400 ÷ 1000 = 0.4 kW

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Example Example 2 15 W = ? kW 15 ÷ 1000 = 0.015 kW

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Example Example 3 3750 W = ? kW 3750 ÷ 1000 = 3.75 kW

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Time must be in hours 60 minutes = 1 hour To change from min to hours divide by 60

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Example 1 45 min = ? h 45 ÷ 60 = 0.75 hours

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Example 2 5 min = ? h 5 ÷ 60 = 0.08 hours

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Example 3 90 = ? h 90 ÷ 60 = 1.5 hours

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More examples Using a stereo with a rating of 30 W for 2 hours. E = P x t E = (30W ÷ 1000) x 2 h E = 0.03 kW x 2h E = 0.06 kWh

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Using a blender with a rating of 390 W for 20 minutes. E = P x t E = (390 W ÷ 1000) x (20 min ÷ 60) E = 0.39 kW x 0.33 h E = 0.13 kWh

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Word Problems I (5)

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