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**Electricity & Safety in the Home**

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Electrical Devices Electrical devices in the home are grouped into 4 different categories Light-producing (i.e. lamps & flashlights) Heat-producing (i.e. hair dryers) Mechanical (i.e. vacuum cleaners & drills) Audio-visual (i.e. TV & computers)

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**Measuring Electricity**

A unit of energy is called a joule (J) A watt (W) is a unit used to measure electrical power

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**Measuring Electricity**

If a device uses energy at the rate of one joule per second, then it’s power rating is one watt (W) Thus, energy can be measured in either joules (J) or watt seconds (W∙s)

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**Measuring Electricity**

To measure the electrical energy consumed in the household (W∙s) is too small a unit Household electrical energy is commonly measured in the kilowatt hour (kW∙h)

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**Measuring Electricity**

Example : One kilowatt equal 1000W, and one hour equals 3600s 1 kW∙h = 1000W∙3600s 1000 kW∙h x 3600s = W∙s

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Calculating Cost Suppose your local electric company charges 7¢ per kilowatt hour of electricity. If your TV consumes (uses) at a rate of 200 watts, and the TV is on for 150 hours on average a month, how much would the monthly cost of power for the television be?

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**Calculating Cost First, change the watts into kilowatts.**

200 W = 0.2 kW Divide by 1000 Next, multiply the number of kilowatts by the number of hours the TV runs. 0.2 kW x 150 h = 30 kWh Last, multiply the number of kilowatt hours by the cost per kilowatt hour. 30 kWh x $0.07/ kWh = $ 2.10 Monthly cost for TV power

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**Homework On one day, a household used a dryer for 2 hours,**

a colour TV for 4 hours, a vacuum cleaner for 1 hour, and a toaster for 0.2 hours. a) What was the total energy these appliances consumed that day? b) If electrical energy costs 7¢/(kWh), what was the total cost of the energy used by these appliances on that day?

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© Boardworks Ltd 2005 1 of 43 KS4 Physics Mains Electricity.

© Boardworks Ltd 2005 1 of 43 KS4 Physics Mains Electricity.

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