Efficiency

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Efficiency

• Measure of how much work is put to good use

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Efficiency of Machines

• Law of C of E says that energy in must equal
  energy out
• However, often a lot of energy is lost
• Heat, friction, sound, etc.
• Efficiency equals
• (Useful energy out) 100
• (Energy in)
• Higher the percentage.the more efficient the
  machine is

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Efficiency (cont.)

• Companies seek to find most efficient machines to
  manufacture, transport, and develop
• Ex. Gas powered cars are not very efficient,
  about 10 - 25 efficient
• Electric (hybrid) cars--much more efficient
• Get up to 3 times the mileage of some gas cars
• http//auto.howstuffworks.com/hybrid-car4.htm
• Government standards

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100 Efficient
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Problem

• http//www.bbc.co.uk/schools/gcsebitesize/physics/
  energy/energyefficiencyrev3.shtml

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Question 1

• A power plant burns 75kg of coal every second.
  Each kg of coal contains 27 MJ (27 million
  joules) of chemical energy.
• What is the energy output of the power station
  every sec?
• The Solution
• 75 x 27 million J per sec
• 2025 million J per sec
• 2025 million J/s or
• (2025 megaWatts)

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Question 2

• The electrical power output of the power plant is
  800MW (800 million watts). But Question 1 stated
  that the chemical energy output of the station
  was 2025 MW..So, What has happened to the rest
  of the energy?
• The Answer
• Most of the rest of the energy is wasted as heat
  - up the chimney of the power station, in the
  cooling towers, and because of friction in the
  machinery.

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Question 3

• Calculate the efficiency of the power plant as a
  percentage.
• The Solution
• Efficiency useful power output/total power
  input
• 800,000,000 W/2025,000,000 W
• 0.395 x 100 to create a percentage
• 39.5

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Simple Machines

• wedge, pulley, lever, ramp, screw, wheel and axle
• Multiply force but applying small force over
  greater distance
• Amount of work done is not increased by a machine
• By law of conservation of energy it is impossible
  to multiply energy

A screw applies a small force over the long
distance across the face of all of its threads at
once to accomplish the same work as a large force
over a small distance
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Mechanical Advantage

• Usefulness of machines is due to multiplication
  of force, not of energy
• Often limited by how much force we can apply, so
  we apply a small force over a large distance
• One way to measure how useful a simple machine is
  is by measuring its Ideal Mechanical Advantage
• MA Force output / force input

By applying a 10 N force and moving this end 50
cm..
Soo.MA 10/1 10
You were able to apply a 100 N force on this end
and move the heavy rock 5 cm
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Incline Plane

• MA is made up by comparing the Parallel force by
  the force of gravity, on any incline the
  parallel force will be much less than the weight
  of the object, this is why it is easier to walk
  long distance on low incline than a short
  distance and a steep incline or climbing straight
  up

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Schober Brothers embark on a hike in Yosemite
Ntnl. Park
This is easy!!
Idiots!
Day 1, short hikelittle wet. feelin good.
..And learn a harsh lesson about Incline Planes
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Day 2 Hike -- 2600 ft of vertical climb to top
of Yosemite Falls
2600 vertical feet
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Hike on Day 2-- In pain, not even close to the
top
15
Me slumped on a rock about to vomit. Will
cant feel his legs Andy takes picture and laughs
Little higher up the mountain.
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Get to the top ..Exhausted. Pass out on a rock
while squirrel eats our granola
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The Point
Activity Difficulty Who can do it Why??
Hike on flat ground Easy Anyone with 2 legs Horizontal motion, do not need to work against gravity
Hike up mountain via long switchbacks and low grade paths Harder Anyone with 2 legs and are at least a little bit in shape (Day 1 Hike) Working against gravity to gain PE but spread out over a long distance. Longer distance means shorter Force. Also spread out over long amt. of time
Hike up mtn. via steep steps with high incline Very Difficult Andy, 8 year old girls, not Will Jake ? (Day 2 Hike) Working against gravity to gain PE over a shorter distance requires larger force applied in short time intervals
Direct climb up mountain face (no path) Extremely Difficult Only highly trained rock climbers (would never even thinking of trying it) Essentially lifting your entire weight straight up with every step. Requires incredible power, strength and endurance.
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The Point. (cont.)

• The steeper the incline plane the higher
  portion of your weight you are going to have to
  lift with every step.
• Low incline, medium incline, direct climb all
  require the same amount of work because all
  produce same increase in PE
• Difference is in how that work is performed.

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Pulleys

• Like levers, ramps, and screws. Sacrifices
  displacement to achieve a greater force
• By pulling a greater displacement you have to
  apply less force
• MA is shown by how many ropes are supporting the
  load in this case there are two
• http//en.wikipedia.org/wiki/Pulley

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Another Pulley

• MA 4
• 4 ropes supporting load
• Force applied is 4 times less than 100 N
• So rope must be pulled with 25 N of force with a
  distance 4 times greater than the upward distance
  the load moves

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Levers

• Pull greater distance on long end but achieve
  greater force over a small distance on the short
  end