Energy and Power

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Energy and Power

• ENGR 10 Intro to Engineering
• College of Engineering
• San Jose State University

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What is energy?

• Meriam-Webster ? Energy is a fundamental entity
  of nature that is transferred between parts of a
  system in the production of physical change
  within the system and usually regarded as the
  capacity for doing work
• Simplified Energy is what it takes to change the
  physical state of an object.
• Energy may be added or released during such a
  change. The transfer of energy is called work.

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• Examples

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A mechanical system example
The work of changing the state of a mass of m
(kg) from standing still to moving at a speed of
v (m/sec) requires
?

• Note Energys unit is joule
• 1 joule 1 kgm2/sec2

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And
The work of lifting a mass of m (kg) a total of h
(m) requires where g 9.807 m/sec2, the
gravitational constant.
?
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(No Transcript)
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Other Energy Units
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Where does energy come from and where does it go?

• Theoretically, the question where does energy
  come from? is the same question where does the
  universe come from?
• In a more practical term, energy is stored in
  various forms around us.

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Where does Energy go?

• Energy used in the physical state
  transformation process is contained in the new
  physical state of the objects.
• Example
• It takes 5x105J of energy to move a Honda Accord
  from 0 to 60 mph. This amount of energy is
  contained in this moving car. This energy is
  called Kinetic Energy.
• Example
• The energy that was used in lifting a weight is
  stored in the new state (a higher elevation) of
  the weight. This energy is called Potential
  Energy.

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• Example
• The energy that was used in heating up a room
  (from burning wood, e.g.) is stored in the warm
  air in the room.
• Example
• The energy that was used for bringing you from
  home to school this morning is contained in your
  car parked in the garage right now. Correct?

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Can energy be destroyed? No!

• Conservation of Energy
• Energy never vanishes. Energy only changes into
  different forms.
• Efficiency of an energy conversion is the ratio
  between the part of the energy that caused the
  desired effect and the total energy used, i.e.,
  in this example,

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If energy never disappears, why do we have an
energy crisis?

• Some forms of energy are difficult or even
  impossible to transform efficiently to a
  re-useable form. The energy in the exhaust gas
  from a car engine or heating of the brake are
  such a form of energy. When energy is converted
  into such a form, it is essentially lost, from a
  practical point of view.

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How difficult is it to convert energy from one
form to another (a more useful) form?

• Very Easy
• Burning wood, coal, fossil fuel, potential
  energy stored in the water in a reservoir,
  nuclear reaction, energy stored in a battery,
  etc.
• Not difficult but higher cost
• Solar, wind (kinetic energy of the air mass),
  ocean current (kinetic energy of water).
• Impossible to transform efficiently to a useful
  form
• Energy in the exhaust gas from a car engine.
• Energy in the warm ocean water in a tropical
  region.

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• Energy Conversion Machines
• Some energy conversion can be done by a natural
  process such as burning wood for heat.
• To better suit our needs, we build machines to
  facilitate, manage, and control the energy
  conversion.

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• For example, in a cars engine
• The spark plugs initiate the combustion, which
  releases energy from the gasoline.
• The cylinders and pistons transform the expansion
  force of the combusted fuel/air to mechanical
  rotational force (torque).
• The cooling systems take the heat energy (a
  byproduct) away from the engine.

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Energy is transformed intoseveral forms
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Energy is converted into several forms
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Key Concepts

• Energy is added or released to change the
  physical state of an object.
• Energy cannot be created or destroyed.
• Efficiency is the ratio of the part of output
  energy that is beneficial to us and the total
  used energy
• We build machines to manage energy conversion.
• While energy cannot be destroyed, once it is
  transformed into a certain form (heat, often the
  case), it is basically difficult to use (some say
  lost but we cannot actually lose energy).

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What is power?
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It takes time to convert energy from one form to
another!

• Power is a measure of how fast energy is
  converted.
• A unit of power is the WATT.
• 1 watt 1 joule/second

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Example
1500 J of electrical energy per second
1500 J heat energy per second

• This heater converts 1500 J of electrical energy
  into heat energy per second.
• This electric heaters power rating is 1500 J/s
  or 1500 watts.

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1500 gallon per minute
This pump is rated 1500 gpm
1500 gallon per minute
Analogy
Gallon is like ??? Gallons per minute is like ????
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Other Units of Power

• horsepower (HP) 1HP 746 watts
• (Used mostly for mechanical systems)
• kilowatt (KW) 1kW 1000 watts
• BTU per Hour (BTUH) 1BTU/h 0.3 watts
• (Used mostly for thermal systems)

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Back to Energy for a moment

• kilowatt-hour (kWh) is a unit of energy.
• Since 1 W1 joule/second
• 1 kWh1000 (J/s) 3600 (s) 3.6106 joules
• Your monthly energy consumption is measured in
  kWh. You pay about 25 cents for 1 kWh of energy
  these days.

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PGE Power costs to increase in '08David R.
Baker, Chronicle Staff Writer9/1/07

• According to PGE's estimates, homeowners using
  560 kilowatt-hours per month would see their
  electricity bills rise by 28 cents, to 74.54.
  Those using 1,000 kilowatt-hours per month would
  spend 2.70 more on their monthly electric bills,
  for a total of 199.83.

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Energy vs. Power

• The information about a certain amount of energy
  (J) does not involve a sense of time.
• (Analogy 100 gallon tank )
• Power (J/sW), on the other hand, tells how fast
  the energy being converted.
• (Analogy 2 gallon/sec pump)

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Another Analogy

• It makes sense to say
• The tank contains 500 gallons of water.
• The pump is pumping 10 gallons per minute.
• It does not make sense to say
• The tank contains 10 gallon per minute of water.
• The pump is pumping 500 gallons.

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(Q1) Which of the following heaters can heat up a
gallon of water to 90oC?(A) 5 W heater(B) 90 W
heater(C) 100 W heater(D) All of the above
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(Q2) Which of the following heaters can heat up a
gallon of water from 10oC to 90oC faster? (A) 5
W heater(B) 90 W heater(C) 100 W heater(D)
All heaters above will take the same amount of
time.
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(Q3) Which equipment will cost you more to run?
(A) a 5 W piece of equipment(B) a 50 W piece
of equipment(C) a 100 W piece of equipment(D)
Insufficient information.
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(Q4) John and Peter each has 1000 in his
piggybank. They decided to use the money to buy
music CDs. Each CD cost exactly 5. John took
20 out of the piggybank each week and Peter 50.
Who will have the most CDs?(A) John(B)
Peter(C) Same(D) Insufficient information.
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(Q5) Both your and your neighbor's basement are
filled with water from a storm. You are given a
5 HP gasoline power pump and your neighbor a 10
HP pump. Each of you is given only 1 gallon of
gasoline. If both pumps have the same
efficiency, which one can pump more water out of
the basement ? (A) You(B) Your neighbor(C)
Same(D) Insufficient information.