What is Energy?

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What is Energy?

• Physics Definition The ability to do work
• Work Force applied over a distance (W fd)
• Force From Newton, force is the product of a
  mass and its acceleration (Fma) also known as
  Newtons second law.
• But this applies mostly to mechanics, the study
  of the physics behind an objects motion

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What is Energy?

• Thermodynamics the study of the conversion of
  heat energy into other forms of energy.
• In themodynamics, work is defined as the quantity
  of energy transferred from one system to another
  without a change in its amount of order (called
  entropy)

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Units of energy

• Joules
• The work done by a force of one newton traveling
  through a distance of one meter
• The work required to move an electric charge of
  one coulomb through an electrical potential
  difference of one volt or one coulomb volt, with
  the symbol CV
• The work done to produce power of one watt
  continuously for one second or one watt second
  (compare kilowatt hour), with the symbol Ws.
  Thus a kilowatt hour is 3,600,000 joules or 3.6
  megajoules
• The kinetic energy of a 2 kg mass moving at a
  velocity of 1 m/s. The kinetic energy is linear
  in the mass but quadratic in the velocity, being
  given by E ½mv²

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Everyday examples of the Joule

• the energy required to lift a small apple one
  meter straight up.
• the energy released when that same apple falls
  one meter to the ground.
• the energy released as heat by a quiet person,
  every hundredth of a second.
• the energy required to heat one gram of dry, cool
  air by 1 degree Celsius.
• one hundredth of the energy a person can receive
  by drinking a drop of beer.
• the kinetic energy of an adult human moving a
  distance of about a hand-span every second.

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Power

• Power the rate at which work is performed
• Or, the rate at which energy is transmitted
• Or the amount of energy expended per unit time
• Measured in Watts
• Other units
• HP or horse power
• BTUs

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Horse power

• Arose as a result of the invention of the steam
  engine. People needed a way to compare the power
  of a steam engine to that of the horses it was
  replacing.
• Confusing unit there are too many different
  definitions!

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BTU

• BTU British Thermal Units - an energy unit
• the amount of heat required to raise the
  temperature of one pound of liquid water by one
  degree from 60 to 61Fahrenheit at a constant
  pressure of one atmosphere
• Used in the power, steam generation, heating and
  air conditioning industries and the energy
  content of fuels.
• However, BTU is often used as a unit of power,
  where BTU/hour is often abbreviated BTU.
• So you need to watch the context!

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Back to Watts..

• A human climbing a flight of stairs is doing work
  at a rate of about 200 watts.
• A typical household incandescent light bulb uses
  electrical energy at a rate of 25 to 100 watts,
  while compact fluorescent lights typically
  consume 5 to 30 watts.
• A 100 Watt light bulb consumes energy at the rate
  of 100 joules/second.
• After 1 hour, this light bulb uses 100 watt-hours
• 1 kilowatt (kw) is 1000 Watts

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Examples

• In a certain room in your house, you use a 100 W
  light bulb. This light is on for 5 hours every
  day. How much energy does it use?
• 1 W 1 J/s and there are 5h x 60min/hour x 60
  sec/min 18,000s in 5 hours so the total energy
  used is 100 j/s 18000s 1.8 x 10 6 J.
• Lets assume the same lighting level can be
  achieved using a 30 W compact florescent bulb.
  How much energy is used by the compact florescent
  bulb?

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Examples

• Total energy 30 j/s x 18000 s 5.4 x 105 j.
• So how much energy is saved every day using the
  compact florescent bulb? Take the difference
  between the energy used by the two different
  light bulbs 1.8 x 10 6 j - 5.4 x 105 j 1.3
  x106 j.
• Lets look at this in something you might be able
  to relate to better than joules---dollars!

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Example continued

• After 5 hours, our 100 W light bulb uses 500
  Watt-hours, or 0.5 Kwh. The 30 W bulb will use
  150 Watt hours or 0.15 Kwh.
• Assume electricity costs 11 cents/Kwh (average
  cost in the US in April 2008). So it costs .5 KwH
  x 11 cents/Kwh 5.5 cents every day to run the
  100 W light bulb and 0.15Kwh x 11 cents 1.65
  cents every day to run the compact florescent.

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Example continued

• So in a year, the 100 W light bulb costs you 5.5
  cents/day X 365 days/year 20.00 and the 30 W
  bulb costs costs you 1.65 cents/day x 365
  days/year 5.50.

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Types of Energykinetic and potential Energy

• Kinetic energy - energy of a moving object
  KE1/2mv2
• Potential Energy Energy stored in a system, for
  example an object of mass m, a distance h above
  the surface of the earth has a potential energy
  given by mgh. g is the acceleration due to
  gravity 9.8 m/s2

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More examples of potential energy

• Another example is a spring, compressed a
  distance x from its equilibrium point has a
  potential energy 1/2kx2, where k is the spring
  constant, a property of the spring.

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Chemical Energy

• Energy that is released via chemical reactions.
• Often times release is through combustion such as
  energy generation via coal
• Another example is a battery

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Heat Energy

• Temperature Scales
• Fahrenheit based on the height of liquid (often
  mercury or alcohol) in a glass tube.
• Celsius another scale using height of liquid in
  a tube
• Kelvin-absolute scale
• True measure of energy

• Energy associated with the random motions of the
  molecules in a medium.
• Measured by temperature