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Laws of Energy

The rate of energy conversion or transmission (i.e. power) is related to the physical quantities such as force, speed, voltage, current, etc.For mechanical system, rate of energy transfer (i.e., power) to an object is the product of the force (F in Newton) and the speed (S in meter/sec) of the point where the force is applied.Q1. A person pushes an out-of-gas car with a force of 100 Newton (about 22.5 lb of force) to maintain a speed of 0.2 m/s. It took him 10 minutes to get to the nearest gas station. How much energy did this person use to do this work?. | Laws of Energy Engineering 10 San Jose State University (c) P.Hsu 2009 The rate of energy conversion or transmission (i.e. power) is related to the physical quantities such as force, speed, voltage, current, etc. Power in terms of physical quantities (c) P.Hsu 2009 For mechanical system, rate of energy transfer (i.e., power) to an object is the product of the force (F in Newton) and the speed (S in meter/sec) of the point where the force is applied. Power = F x S Force (Newton) Speed (m/s) (c) P.Hsu 2009 Q1. A person pushes an out-of-gas car with a force of 100 Newton (about 22.5 lb of force) to maintain a speed of 0.2 m/s. It took him 10 minutes to get to the nearest gas station. How much energy did this person use to do this work? (Hint: Power = Force x Speed) 20 J 600 J 1200 J 2400 J 12000 J Energy = Power x Time (c) P.Hsu 2009 If the system is 100% efficient, Power = 3*F*S = V*I (c) P.Hsu 2009 Assuming solar panel’s efficiency is 15% and the motor efficiency is 80%, the combined efficiency is about 12%. F*S = 0.15 * 0.8 * P = 0.12 * P (c) P.Hsu 2007 A book lying on a table exerts a force (F) on the table top. There is no energy transfer since nothing moves (S=0). Power = Force x 0 = 0 From Newton’s first law, force is not required to maintain a constant speed There is no energy transfer in this case because Power = 0 x Speed = 0 Consider two special cases: If force and speed are constant, power is constant. In this case, the amount of work (or the amount of energy converted) over a period of T seconds is Work (J) = Power (J/s or W) x T (s) = F (N) × S (m/s) × T (s) = F(N) x D (m) (where D is the travel distance) (c) P.Hsu 2009 F F D (c) P.Hsu 2009 A person pushes an out-of-gas car with a force of 100 Newton (about 22.5 lb of force) to maintain a constant speed. The nearest gas station is 120 meters away. How much Work does this person has to do to push the car to the gas station? Work = Force x Distance = 100 (N) x 120 (m) = 12000 (J) F F D (c) P.Hsu | Laws of Energy Engineering 10 San Jose State University (c) P.Hsu 2009 The rate of energy conversion or transmission (i.e. power) is related to the physical quantities such as force, speed, voltage, current, etc. Power in terms of physical quantities (c) P.Hsu 2009 For mechanical system, rate of energy transfer (i.e., power) to an object is the product of the force (F in Newton) and the speed (S in meter/sec) of the point where the force is applied. Power = F x S Force (Newton) Speed (m/s) (c) P.Hsu 2009 Q1. A person pushes an out-of-gas car with a force of 100 Newton (about 22.5 lb of force) to maintain a speed of 0.2 m/s. It took him 10 minutes to get to the nearest gas station. How much energy did this person use to do this work? (Hint: Power = Force x Speed) 20 J 600 J 1200 J 2400 J 12000 J Energy = Power x Time (c) P.Hsu 2009 If the system is 100% efficient, Power = 3*F*S = V*I (c) P.Hsu 2009 Assuming solar panel’s efficiency is 15% and the motor efficiency is 80%, the