.edu /~ Page metin 1 Science of Energy utdallas Traditional Forms of Energy Potential Energy Kinetic Energy Thermal Energy .edu /~ Page metin 2 Forms of Energy utdallas 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 → → .edu /~ Page metin 3 Potential Energy utdallas Gravitational force = Weight Weight = Mass m 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 2 – Ex: What is the weight (gravitational force applying2 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 2 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= 20.031 slug = 1 lbm. Roughly Slug:Kilogram:Pound force ratios are 32 : 2 : 1. If you never hear ofslug wonder& where it is used, check crossword puzzles. .edu /~ Page metin 4 Kinetic Energy utdallas Drop an object of mass from a height of , which takes time Initial speed 0 & final speed at the bottom , average speed /2 ℎ We must have =Distance=Average speed Time= Furthermore, potential energy becomes kinetic energy at the bottom ℎ ∗ 2 Potential Energy = = = = Kinetic energy 1 2 ℎ 2 2 Kinetic energy is proportional to the mass & square of the speed from above. In general, Kinetic energy= , where is the speed 1 Ex: An accounting book weighs2 1 kg and is dropped from 1 metre, what is its kinetic energy at the 2 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 2 at the entry and = = 2 at the exit for = . 2 2 3 1 2 1 Hence, air particles lose half of their speed after transferring2 their energy to the propeller. 2 − 4 2 2 2 2 2 .edu /~ Page metin 5 Thermal Energy utdallas Thermal Energy Thermal Energy = Mass Heat capacity constant Temperature 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 − 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? – The energy needed for water is 0.33(4200)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 = 4.2 joule & 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 = 4200 joule, which is the potential energy of 80 kg person at 5.35 metres = 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 metres! .edu /~ Page metin 6 Summary utdallas Potential Energy Kinetic Energy Thermal Energy .edu /~ Page Which Empire had Imperial Units? metin 7 A lot of them! utdallas British Empire Handle with hook – British used pound as the unit of mass. for suspension – 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 Cup – But these are all shortened as lb, why? A chain with head of French Empire youthful Mercury » Livre esterin » Livre de Paris Bronze Balance, Naples, » Livre metrique Museo Archeologico Nazionale. Pfund in Prussian Empire Libra Pondo Pond in the Netherlands Funt in Russian Empire lb Pund (Skal)Pund in Swedish Empire Litra Funt Litra in Byzantine Empire Litre for volume now Litre Pond Libra Pondo (weights of weighing) in Roman Empire. → Livre Libra is shortened as lb. Ponderis: To weigh in Latin. Pfund Linguistic Insight: Words “Pound” & “Ponder” are relatives. Pound.