Energy and Cost

Energy

Energy: the ability to do work all appliances need energy to work ex. a hair dryer needs to be plugged

in to get energy to work ex. a remote control needs energy

from batteries to work ex. a light bulb needs electrical

energy to shine bright ex. solar panels use energy from the

sun to heat a house

What is energy?

Power

Power: the rate at which energy is produced is the product of potential difference

(voltage) & current intensity

What is power?

Symbols and Units

Symbol =

Units =

Symbol =

Units =

E = P•t

Energy

Joules (J) Power

Watts (W)

Time

Seconds (s)

A stereo has a power rating of 200W & is used for 1800 seconds. How much energy is used?

E = P•t

E = (200)•(1800)

E = 360,000 J

360,000 Joules of energy were used to work the stereo

A video game has a power rating of 175W & is used for 3600 seconds. How much energy is used?

E = P•t

E = (175)•(3600)

E = 630,000 J

630,000 Joules of energy were used to work the video game

A hair dryer has a power rating of 425W & is used for 10 minutes. How much energy is used?

E = P•t

E = (425)•(600)

E = 255,000 J

255,000 Joules of energy were used to work the hair dryer

Time must be in seconds

10 min x 60 = 600 seconds

P = V•I

Power

Watts (W) Potential Difference

Volts (V)

Current

Amps (A)

A toaster requires 120V & 6A to work. What is its power?

P = V•I

P = (120)•(6)

P = 720 W

The toaster has 720 watts of power

An appliance requires 240V & 2A to work. What is its power?

P = V•I

P = (240)•(2)

P = 480 W

The appliance has a power of 480 watts

An appliance requires 120V & 12A to work. It is used for 900 seconds. How much energy is used?

P = V•I

P = (120)•(12)

P = 1440 W

Tricky Question

E = P•t

E =(1440)•(900)

E =1,296,000 J

An appliance uses 225V & 4A to work. It is used for 60 minutes. How much energy is used?

P = V•I

P = (225)•(4)

P = 900 W

Tricky Question

E = P•t

E =(900)•(3600)

E =3,240,000 J

60min x 60

Cost

Every time you use an appliance it costs money……how do you

calculate how much it will cost?

(Usually Given) Must be in kW

Must be in hours

Remember: 1000 watts = 1 kW

A 4800 watt television is used for 5 hours. If the cost of electricity is $0.04kW•h, what would your bill read? (How much would it cost?)

cost = $0.04 × 4.8kW× 5h

Cost = cost of electricity x Power x Time

cost = $ 0.96 4800 W = 4.8 kW

A 1500 watt hair dryer is used for 30 minutes. Electricity costs $0.05/kW•h. How much does it cost to use this hair dryer for the period indicated?

cost = 0.05 × 1.5kW × 0.5 hCost = cost of electricity x Power x Time

30 min ÷ 60 min = 0.5 hrs

cost=$ 0.0375

1500 W = 1.5 kW

A 3.9 kilowatts curling iron is used for 10 minutes once a day. Electricity costs $0.05/kW•h. How much does it cost to use this curling iron for a week?

cost = 0.05 × 3.9 kW × 0.167 hCost = cost of electricity x Power x Time

10 min ÷ 60 min = 0.167 hrs

cost=$ 0.0325 X 7 days

cost=$ 0.23

Cost & Rating Plates

Every electrical appliance has a rating plate.

Rating plates allow us to calculate the electrical cost of an appliance.

Model: VCANN-1977

Serial No: 2005-13

240 V 60 Hz 4000 W

POWER

Look for

power

The rating plate for an electric hair dryer is given below.

Model: LHOPE-1979

1500 W 200V 40Hz

If the dryer is used for 30 minutes a day & the cost of electricity is $0.04/kW•h, how much would it cost to use every day?

Cost = cost of electricity x power x time

Cost =($0.04/kW•h) x (1.5kW) x (0.5h)

Cost =$0.03 or 3¢

The rating plate for a television is given below.

Model: CANNHOPE-2005

2500 W 250V 60Hz

If the TV is used for 2 hours a day & the cost of electricity is $0.05/kW•h, how much would it cost to use for a 30-day month?

Cost = cost of electricity x power x time

Cost =($0.05/kW•h) x (2.5kW) x (2h)

Cost =$0.25 x 30 days Cost =$7.50

If the appliance is used for 1 hour a day & the cost of electricity is $0.0475/kW•h, how much would it cost to use every day?

The rating plate for an electric appliance is given below.

Model: NOV-05

30Hz 200V 6A

Cost = cost of electricity x power x time

Cost =($0.0475/kW•h) x (1.2kW) x (1h)

Cost =$0.057 or $0.06

No power!! Recall P = VI

P = (200)(6) P = 1200 W

?

If the appliance is used for 3 hours a day & the cost of electricity is $0.05/kW•h, how much would it cost to use for a week?

The rating plate for an electric appliance is given below.

Model: RAF-2005

375V 0.2A

Cost = cost of electricity x power x time

Cost =($0.05/kW•h) x (0.075kW) x (3h)

Cost =$0.01125

No power!! Recall P = VI

P = (375)(0.2) P = 75 W

?

x 7 days Cost =$0.08

The rating plates of four different electric appliances are given below

GIN-4445

MAC-2453

I8JUL-6712

MANDANE-2445

860 W 120V

60 HZ

2 A 250 V

66 HZ

10 A

125 V

1400 W 15 A

140 V

Each appliance is used for 4 hours.

Which of these appliances would be the most expensive to use?

A)

D)

C)

B)Since the cost and

time are the same you must just look

for power…P = V x I

P = 10 x 125

P = 1250 W

P = V x I

P = 2 x 250

P = 500 W

By comparing Power it is clear to see that appliance “C” would be the most expensive!

Joule : a unit of energy Ex.: 25 g of candy contains 276 000 J

ampere : a quantity of charges per time Ex.: 15 ampere breaker

Watt : a unit of power Ex.: 100 W light bulb

Volt : energy per unit charge Ex.: AA battery has 1.5 volts

Recalling Units

J

A

W

V

Important Units to Remember

Joules Watts

W•s kW

kw•h

V•A•s

It is important to remember the following units for energy and power.

Electricity uses the following units:1. Watt 4. Kilowatt-hour2. Volt 5. Joule3. Ampere

Which of the above are units of energy?

A) 1 and 3 B) 2 and 4

C) 1 and 5 D) 4 and 5

Energy Power

Joules Watts

W•s kW

kw•h

V•A•s

Which of the following units are not units of energy?

1. W2. V•A•s3. J4. A5. kW•h6. W•s

A) 1, 3, and 4 B) 1 and 4

C) 1, 4, and 6 D) 4 and 5

Energy Power

Joules Watts

W•s kW

kw•h

V•A•s

KEY POINTS

- FORMULASE = P x tP = V x ICost = cost of electricity x P x t

**make sure you have the correct units in these formulas**

- UNITSEnergy Power

Joules Watts

W•s kW

kw•h

V•A•s