.edu /~metin Page 1. utdallas. Science of Energy. Traditional Forms of Energy Potential Energy Kinetic Energy Thermal Energy

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1 Science of Energy Page 1 Traditional Forms of Energy Potential Energy Kinetic Energy Thermal Energy

2 Forms of Energy Page Fundamental law of Science: Energy and mass are conserved. You cannot waste energy even if you want. Wasting energy is actually transforming energy to an undesirable form Forms of Energy: 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 Dropping a pen: Potential Kinetic Rubbing hands: Kinetic Thermal Boiling water rises: Thermal Potential

3 Potential Energy Page 3 Gravitational force = Weight Weight = Mass m x Gravitational acceleration constant g Mass in kilogram (kg); Acceleration constant in metres per square second (m/s ) Then gravitational force in Newton = kg x m/s 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. 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?» It is 80(9.8)10 kg square meters per square second or simply 7,840 joule. Comparison between metric and imperial units Mass Gravitational Acceleration g Force Metric Kilogram kg 9.81 Metre per (second * second) m/s Newton N Imperial Slug s 1.00 Feet per (second * second) ft/s Pound force lbf Imperial Pound mass lbm 3.0 Feet per (second * second) ft/s Pound force lbf 1 kg = s =.0 lbm kg = 1 s = 3. lbm kg= slug = 1 lbm. Roughly Slug:Kilogram:Pound force ratios are 3 : : 1. If you never hear of slug & wonder where it is used, check crossword puzzles.

4 Kinetic Energy Page 4 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/ We must have h=distance=average speed * Time=(gT/)T Furthermore, potential energy becomes kinetic energy at the bottom Potential Energy = mmmmm = mmmm gggg TT = 1 mm gggg = Kinetic energy Kinetic energy is proportional to the mass and square of the speed from above. In general, Kinetic energy= 1 mmvv, where vv is the speed Ex: An accounting book weighs 1 kg and is dropped from 1 meter, what is its kinetic energy at the bottom of the drop? 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? The kinetic energy of each incoming air particle is mmvv ii at the entry and mmvv ii mmvv ii = 1 3 = 1 mm vv ii mmvv 4 oo at the exit for vv oo = vv ii. Hence, air particles lose half of their speed after transferring their energy to the propeller.

5 Thermal Energy Page 5 Thermal Energy Thermal Energy = Mass m x Heat capacity constant C x Temperature K 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 73 celsius Heat capacity constant is the ability of a substance to absorb thermal energy For water 400 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? The energy needed for water is 0.33(400)50=69,300 joules. For storage and transportation of energy, the industry needs substances with high heat capacity such as water and molten salts. 1 calorie = 4. joule and increases the temperature of 1 gram water by 1 Celsius Ex: A cafeteria sells 1000 calorie salads. How many meters a person with 80 kg mass can climb with this amount of energy? 1000 calorie = 400 joule, which is the potential energy of 80 kg person at 5.35 meters = 4,00/(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!

6 Summary Page 6 Potential Energy Kinetic Energy Thermal Energy

7 Which Empire had Imperial Units? A lot of them! British Empire British used pound as the unit of mass. Not only one, but many pounds in use:» London Pound» Merchant Pound» Tower Pound» Troy Pound» Avoir-du-pois (goods-of-weight in French) Pound But these are all shortened as lb, why? French Empire» Livre esterin» Livre de Paris» Livre metrique 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. Handle with hook for suspension Cup A chain with head of youthful Mercury Bronze Balance, Naples, Museo Archeologico Nazionale. Libra lb Litra Litre Livre Pondo Pund Funt Pond Pfund Pound Page 7