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Traditional Forms of Energy
 Potential Energy
 Kinetic Energy
 Thermal Energy
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Science of Energy
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Fundamental law of Science: Energy and mass are conserved.
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Forms of Energy:
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You cannot waste energy even if you want.
Wasting energy is actually transforming energy in an undesirable form
Traditional Energy: Potential, Kinetic and Thermal
Fuel-Based Energy: Chemical and Nuclear
Electromagnetic Energy: Electric and Magnetic
Energy generation is actually energy transformation from one form to another
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Dropping a pen: Potential → Kinetic
Rubbing hands: Kinetic → Thermal
Boiling water rises: Thermal → Potential
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Forms of Energy
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Potential Energy
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Gravitational force = Weight
Weight = Mass m x Gravitational acceleration constant g
– Mass in kilogram (kg); Acceleration constant in metres per square second (m/s2 )
– Then gravitational force in Newton = kg x m/s2
– Ex: What is the weight (gravitational force applying to) of a person with 80 kilogram mass?
» The weight is 784=80(9.8) kilogram meters per square second or 784 newtons.
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Potential Energy
Potential Energy = Gravitational force mg x Height h
– Gravitational force in Newton & Height in meters ⇒ Potential energy in Joule = Newton x metre
– Ex: What is the potential energy gained by a person with 80 kg mass walking stairs up for 10
metres? Would your answer change if the person takes an elevator?
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Comparison between metric and imperial units
Mass
Gravitational Acceleration g
Force
Metric
Kilogram kg
9.81 Metre per (second * second) m/s2
Newton N
Imperial
Slug s
1.00 Feet per (second * second) ft/s2
Pound force lbf
Imperial Pound mass lbm 32.20 Feet per (second * second) ft/s2
Pound force lbf
1 kg = 0.068 s = 2.20 lbm. 14.65 kg = 1 s = 32.2 lbm. 0.45 kg= 0.031 slug = 1 lbm.
Roughly Slug:Kilogram:Pound force ratios are 32 : 2 : 1.
If you never hear of slug
& wonder where it is used,
check crossword puzzles.
» It is 80(9.8)10 kg square meters per square second or simply 78,400 joule.
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Drop an object of mass m from a height of h, which takes time T
Initial speed 0 and final speed at the bottom gT, average speed gT/2
We must have h=Distance=Average speed * Time=(gT/2)T
Furthermore, potential energy becomes kinetic energy at the bottom
Potential Energy = 𝑚𝑔ℎ = 𝑚𝑔
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𝑔𝑇
2
1
2
𝑇 = 𝑚 𝑔𝑇
2
= Kinetic energy
Kinetic energy is proportional to the mass and square of the speed from above. In general,
1
2
Kinetic energy= 𝑚𝑣 2 , where 𝑣 is the speed
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Ex: An accounting book weighs 1 kg and is dropped from 1 meter, what is its kinetic energy at the
bottom of the drop?
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By using the conservation of energy, we can say the kinetic energy gained is equal to the potential energy
lost, which is mgh=1(9.8)1=9.8 joule.
Ex: Consider a horizontal cylinder which is subject to air flow from left to right. There is a
propeller at the right-hand side of the cylinder. The air flow passes 75% of its kinetic energy to
the propeller to rotate it. Assuming that the energy conversion is perfect (no energy is lost when
air flow's kinetic energy is passed to the propeller), how much does the air flow slows down after
passing through the propeller?
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The kinetic energy of each incoming air particle is
𝑚𝑣𝑖2
2
−
3 𝑚𝑣𝑖2
4 2
1
2
= 𝑚
𝑣𝑖 2
2
1
2
𝑚𝑣𝑖2
2
at the entry and
𝑣
2
= 𝑚𝑣𝑜2 at the exit for 𝑣𝑜 = 𝑖 .
Hence, air particles lose half of their speed after transferring their energy to the propeller.
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Kinetic Energy
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Thermal Energy
Thermal Energy = Mass m x Heat capacity constant C x Temperature K
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Mass is in kilogram
Heat capacity constant is in joule per (kilogram x celsius) or joule per (kilogram x kelvin)
Sometimes temperature is measured in kelvin; 0 kelvin is −273 celsius
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Thermal Energy
Heat capacity constant is the ability of a substance to absorb thermal energy
For water 4200 joule per (kilogram x celsius) > For sand 835 joule per (kilogram x celsius)
Ex: To increase the temperature of 0.33 kilogram (330 millilitre, can soda size) of water by 50 celsius,
how much thermal energy is needed?
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The energy needed for water is 0.33(4200)50=69,300 joules.
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For storage and transportation of energy, the industry needs substances with high heat capacity such as
water and molten salts.
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1 calorie = 4.2 joule increases the temperature of 1 gram water by 1 Celsius
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Ex: A cafeteria sells 1000 calorie salads. How many meters a person with 80 kg mass can climb with
this amount of energy?
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1000 calorie = 4200 joule, which is the potential energy of 80 kg person at 5.35 meters = 4,200/(80 * 9.8).
To burn a salad, is it sufficient to climb two floors of a building?
No! Dieticians use calorie to mean kilocalories. The salad actually has 1,000,000 calories which can be burnt by
climbing a mountain of 5350 meters!
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Potential Energy
Kinetic Energy
Thermal Energy
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Summary
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British Empire
– British used pound as the unit of mass.
– Not only one, but many pounds in use:
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London Pound
Merchant Pound
Tower Pound
Troy Pound
Avoir-du-pois (goods-of-weight in French) Pound
Cup
– But these are all shortened as lb, why?
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A chain with head of
youthful Mercury
French Empire
» Livre esterin
» Livre de Paris
» Livre metrique
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Handle with hook
for suspension
Bronze Balance, Naples,
Museo Archeologico Nazionale.
Pfund in Prussian Empire
Pond in the Netherlands
Funt in Russian Empire
(Skal)Pund in Swedish Empire
Litra in Byzantine Empire → Litre for volume now
Libra Pondo (weights of weighing) in Roman Empire.
Libra is shortened as lb. Ponderis: To weigh in Latin.
Linguistic Insight: Words “Pound” & “Ponder” are relatives.
Libra Pondo
lb
Pund
Litra
Funt
Litre
Pond
Livre
Pfund
Pound
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Which Empire had Imperial Units?
A lot of them!
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