Section 3.3 Measuring Section 3.3 Measuring Energy Input and OutputEnergy Input and Output

11

p. 332 - 338p. 332 - 338

We use energy in every aspect of our life.We use energy in every aspect of our life.

Driving, ipods, heating our homes, cooking Driving, ipods, heating our homes, cooking food, watching tv.food, watching tv.

How can we choose which type of energy How can we choose which type of energy to use? (ex: we could use electricity, to use? (ex: we could use electricity, gasoline, propane, hydrogen)gasoline, propane, hydrogen) We need to measure the types and We need to measure the types and

amounts of energy going into and coming amounts of energy going into and coming out of the devices.out of the devices.

PowerPowerPower is the rate at which a device converts energy.

Unit of power is the watt (W).

1 watt is equal to 1 joule per second

1W = 1J/s Ex: 44 W = 44 J/s

The faster a device converts energy, the greater its power rating.

Is an appliance with high watts or low Is an appliance with high watts or low watts better? Why?watts better? Why?

Is an appliance with high watts or low Is an appliance with high watts or low watts better? Why?watts better? Why? Depends on the way you look at it.Depends on the way you look at it.

High watts, is more powerful, stronger, might not High watts, is more powerful, stronger, might not have to use it as longhave to use it as long

However, low watts is more energy efficient. Might However, low watts is more energy efficient. Might save you money in the long run.save you money in the long run.

Power formulaPower formula

For an electrical device, the power is the For an electrical device, the power is the current multiplied by the voltage.current multiplied by the voltage.

P = I x V (watts = amps x volts)P = I x V (watts = amps x volts)

V

P

I

Power Calculation ExamplePower Calculation ExampleA hair dryer has a power rating of 1000 W. It A hair dryer has a power rating of 1000 W. It

is plugged into a 120-V outlet. What is the is plugged into a 120-V outlet. What is the current flowing through the hair dryer?current flowing through the hair dryer?

Known: Power (P) = 1000 W, Voltage (V) = 120 VKnown: Power (P) = 1000 W, Voltage (V) = 120 VUnknown: Current (I) = ?Unknown: Current (I) = ?Formula: I = P/VFormula: I = P/VSolve: I = 1000 W/ 120 VSolve: I = 1000 W/ 120 V

I = 8.33AI = 8.33A

EnergyEnergyYou can use the power rating of a device You can use the power rating of a device to determine the amount of energy the to determine the amount of energy the device uses (power is the rate at which a device uses (power is the rate at which a device converts energy).device converts energy).

Energy formulaEnergy formula

Energy consumption of an electrical Energy consumption of an electrical device is its input power multiplied by device is its input power multiplied by the time the device is used.the time the device is used.

E = P x t (joules = watts x seconds)E = P x t (joules = watts x seconds)

t

E

P

Energy Calculation ExampleEnergy Calculation ExampleA microwave oven has a power rating of 800W. If you A microwave oven has a power rating of 800W. If you

cook a roast in the oven for 30 min at high, how cook a roast in the oven for 30 min at high, how many joules of electrical energy are converted into many joules of electrical energy are converted into heat by the microwave?heat by the microwave?

*Remember that W = J/s*Remember that W = J/s

• Known: P = 800 W (J/s), t = 30 minKnown: P = 800 W (J/s), t = 30 min30 min = ? s30 min = ? s

30min x 60 s/min = 30min x 60 s/min = 1800s1800s

• Unknown: E = ?Unknown: E = ?• Formula: E = P x tFormula: E = P x t• Solve: E = 800 W x 1800sSolve: E = 800 W x 1800s

E = 1 440 000 J = 1.4 MJE = 1 440 000 J = 1.4 MJ

Write this at the bottom of the Write this at the bottom of the page…page…

Kilowatt HoursKilowatt HoursIt doesn’t take common electrical devices It doesn’t take common electrical devices long to consume a long to consume a large number of large number of joulesjoules..Because of this, the Because of this, the kilowatt hourkilowatt hour is often is often used.used.Energy calculation is the same, except you Energy calculation is the same, except you use hours instead of seconds, and use hours instead of seconds, and kilowatts instead of watts.kilowatts instead of watts.1 kW = 1000 W1 kW = 1000 W