POWERChapter 9.2

What is Power?

What is Power?

Energy is what costs money

Power is the rate at which energy is being transformed (or consumed)

Energy is measured in Joules (J)

Power is measured in Watts (W = J/s)James Prescott JouleJames Watt

Electrical Power

The rate of change in electrical energy

A 60W light bulb uses 60J of electrical

energy every second and produces 60J

of heat and light energy

Power

Power is equal to voltage multiplied by current

P = IV

P=IV

P

I V

PVI= PIV=

Example 1

P = ?I = 8A

V = 120V

P

I VHousehold sockets supply 120V. If the current drawn by the toaster is 8A, what is the power rating of the toaster?

P = IVP = 8A x 120VP = 960W

Electronics WattsDesktop Computer 120

Laptop 35

PS3 184

XBOX 102

iPhone (charging) 5

40” LCD 400

Electronic Clock 3

Fan 25

Blow Dryer 1000

Shaver 15

CD Player 35

Note: Yours may be different

Appliance WattsWashing Machine 500

Electric Clothes Dryer 4000

Air Conditioner (room) 1000

Dishwasher 1400

Microwave 1100

Toaster 950

Kettle 1500

Vacuum 500

Iron 1000

Refrigerator 500

Sewing Machine 100

Various Power Ratings

A Comparison on Light Bulbs

Incandescent Compact Fluorescent

40W

60W

75W

100W

11W

16W

20W

30W

VS

Energy Consumption

Energy costs money So, we are interested in how much

energy we use powering our different appliances and electronics

We know Power has units (J/s) and Energy has units (J)

Therefore…

EtP =

Energy Consumption

Re-work that equation…

EtP = tt

E = Pt

E=Pt

E

P t

EtP= EPt=

Example 2

E = ?P = 960W

t = 1.5min

E

P tUsing the same toaster from before (960W), it takes 1 and a half minutes to turn bread into delicious toast. How much energy is used?

E = PtE = 960W x 90sE = 86400J

t = 90s

Do BLM 3-38 Electrical Power & Energy Do BLM 3-39 Power Problems

A Larger Unit for Energy

With the units we are using (Watts and seconds), our calculated values for energy can get fairly large

Instead of using…

The following is often used:

1.0 joule = 1.0 watt x 1.0 second

1.0 kW•h = 1.0 kilowatt x 1.0 hour

A Larger Unit for Energy

Appliances and energy companies like BC Hydro work with large quantities of energy and will therefore tend to show energy consumption using kilowatt hours instead of joules

kW•h

Example 3 Refrigerators are turned ON every single

moment of the year. A refrigerator has a power rating of 500W. How much energy, in kW•h, is used by the refrigerator in 1 year?

P = 500W = 0.5kW

t = 1year = 8760hThere are 1000W in

1kW. Therefore, divide by 1000

There are 365 days in 1 year and 24 hours in 1 day.

Therefore multiply

365 d x 24h/d = 8760h

E

P tE = Pt = 0.5kW x 8760h

E = 4380 kW•h

E = ?

The Cost of Energy

Yes, every month or so your parents have to pay the electricity bill

=

BC Hydro Residential Rates (2011) Under the Residential Conservation Rate, customers pay

6.67 cents per kWh for the first 1,350 kWh they use over an average two-month billing period. Above that amount, customers pay 9.62 cents per kWh for the balance of the electricity used during the billing period. http://www.bchydro.com

STEP 1STEP 2

6.67 cents per kWh

9.62 cents per kWh

TWO-STEP CONSERVATION RATE (2 MONTH PERIOD)

1,350 kWh – Step 1 threshold

Calculating Cost

BC Hydro charges 6.67 cents per kW•h Steps

1. Determine the amount of energy in kW•h

2. Multiply energy by the rate (6.67 cents/kW•h)

Cost = Rate x E

Example 4

The power company charges 6.67 cents per kW•h. How much does it cost (rounded to the nearest cent) to watch 2 hours of TV on a big screen 40” LCD (400W)?

Cost = Rate x E

We know the rate, we need to find

energy

E = Pt

E = 400W x 2h

E = 0.4kW x 2h

E = 0.8kW•h

= 0.0667$/kW•h x 0.8kW•h

Cost = $0.05

Try This

An air conditioner has a power rating of 1000W per room and the energy company charges 6.67 cents per kW•h. How much does it cost a family to use the air conditioner in their living room during July and August for 10 hours a day?

Cost = Rate x E

We know the rate, we need to find

energy

E = Pt

E = 1000W x t

E = 1.0kW x 620h

E = 620kW•h

= 0.0667$/kW•h x 620kW•h

Cost = $41.3562 days in July and August multiplied by

10 hours a day

Try This ChallengeA family uses a dishwasher that draws 10A from a 120V source. The dishwasher is run 1 hour a week. What rate is the power company charging the family if the family if at the end of 1 year the family is charged $4.99.

P = IV

P = 10A x 120V

P = 1200W

P = 1.2kW

E = Pt

E = 1.2kW x 52h

E = 62.4kW•h

Cost = Rate x E

Cost = Rate x EE E

Rate = CostE

Rate = $4.9962.4kW•h

Rate = 0.08$/kW•h1 year has 52 weeks and

the dishwasher is used for 1 hour each week

Summary of FormulaeRemember, you don’t have to memorize formulas.

However, you must know how to use them!

V = IR

P = IV

E = Pt

Cost = Rate x E

V = Voltage (V)

I = Current (A)

R = Resistance (Ω)

P = Power (W)

E = Energy (J or kW•h)

t = time (s)

~ fin~

Do BLM 3-40 The Price of Energy