Section 3.3 Measuring Energy Input and Output 1

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Section 3.3 Measuring Energy Input and Output1

• p. 332 - 338

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• We use energy in every aspect of our life.
• Driving, ipods, heating our homes, cooking food,
  watching tv.
• How can we choose which type of energy to use?
  (ex we could use electricity, gasoline, propane,
  hydrogen)
• We need to measure the types and amounts of
  energy going into and coming out of the devices.

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Power

• Power 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
• 1Watt 1Joule/second
• 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 watts
  better? Why?

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

• Depends on the way you look at it.
• High watts, is more powerful, stronger, might not
  have to use it as long
• However, low watts is more energy efficient.
  Might save you money in the long run.

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Power formula

• For an electrical device, the power is the
  current multiplied by the voltage.
• Power I x V (watts amps x volts)

P
I
V
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Power Calculation Example

• A hair dryer has a power rating of 1000 W. It is
  plugged into a 120-V outlet. What is the current
  flowing through the hair dryer?
• Known Power (P) 1000 W, Voltage (V) 120 V
• Unknown Current (I) ?
• Formula I P/V
• Solve I 1000 W/ 120 V
• I 8.33A

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Energy

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

Energy formula

• Energy consumption of an electrical device is its
  input power multiplied by the time the device is
  used.
• E P x t (joules watts x seconds)

E
P
t
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Energy Calculation Example

• A microwave oven has a power rating of 800W. If
  you cook a roast in the oven for 30 min at high,
  how many joules of electrical energy are
  converted into heat by the microwave?
• Remember that W J/s
• Known P 800 W (J/s), t 30 min
• 30 min ? s
• 30min x 60 s/min 1800s
• Unknown E ?
• Formula E P x t
• Solve E 800 W x 1800s
• E 1 440 000 J 1.4 MJ

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Kilowatt Hours

• in our households, it doesnt take long to use up
  a lot of Joules of energy
• we use a more practical unit called the kilowatt
  hour
• it is calculated with the same formula
• E Pt
• power is measured in kilowatts
• (1 kW 1000 W)
• time is measured in hours (h)

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Summary

• A. Power
• power is the rate at which a device converts
  energy
• measured in watts (W)
• P I V
• where P power in watts (W)
• I current in Amperes (A)
• V voltage in volts (V)

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Example Power

• Calculate the power used by an MP3 player if it
  uses a current of 5.0 A and a voltage of 3.0
  volts.
• I 5.0 A
• V 3.0 V
• P ?
•   P IV
• (5.0 A)(3.0 V)
• 15 W
•  
•  

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Summary

• B Energy
• You can use the power rating of a device and the
  time it runs to calculate the amount of energy
  used
• measured in Joules (J)
• E P t
• where P power in watts (W)
• t time in seconds (s)
• E energy in Joules (J)
•  

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Example Energy

• Calculate the energy usage if a 15 W MP3 player
  operates for 2 minutes.
• P 15 W E Pt
• T 2 min x 60 s/min (15 W)(120 s)
• 120 s 1800 J
• E ?
•  

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Assignment

• Complete Worksheet 10, only 1-5
• Well finish efficiency on Monday.
• Read textbook chapters 1,2 and 3!