Electricity & Safety in the Home

Electrical Devices

Electrical devices in the home are grouped into 4 different categories

1) Light-producing (i.e. lamps & flashlights)

2) Heat-producing (i.e. hair dryers)

3) Mechanical (i.e. vacuum cleaners & drills)

4) Audio-visual (i.e. TV & computers)

Measuring Electricity

A unit of energy is called a joule (J)

A watt (W) is a unit used to measure electrical power

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)

JsWors

JW 11

1

11

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)

Measuring Electricity

Example : One kilowatt equal 1000W, and

one hour equals 3600s

1 kW∙h = 1000W∙3600s

1000 kW∙h x 3600s = 3 600 000 W∙s

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?

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 = $ 2.10Monthly cost for TV power

Homework

On one day, a household used a dryer for 2 hours,a colour TV for 4 hours, a vacuum cleaner for 1hour, 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?