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Energy Work Power

Energy Work Power. Textbook Chp 6 ( pp 104 – 122). What is Energy?. In biology, we say “the sun is the source of energy for all life on Earth” What does this statement mean? Where does the Energy in the sun come from ??. Energy. The concept of energy is confusing for two reasons:

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Energy Work Power

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  1. Energy Work Power Textbook Chp 6 (pp 104 – 122)

  2. What is Energy? • In biology, we say “the sun is the source of energy for all life on Earth” • What does this statement mean? • Where does the Energy in the sun come from??

  3. Energy • The concept of energy is confusing for two reasons: • (i) the way we use the English word “energy” may not be the same way to use the word in Physics • (ii) even when we use the word in Physics, energy can take many different forms

  4. Energy • Definition: Energy is the capacity to do work • Qn: so….what is work? • Ans: we will get to that second half of this chapter • Unit: Joules (J) • Principle of Conservation of Energy (CoE): • Energy cannot be created or destroyed, but can be converted from one form to another. • The total energy in an isolated system is constant.

  5. Forms of Energy • There are 9 forms of energy which are in your syllabus: • 3 types of Potential Energy (Chemical, Gravitational, Elastic) • Kinetic Energy (i.e. “movement energy”) • Light, Thermal, Electrical, Nuclear, Sound Energy

  6. Potential Energy (PE) • Potential Energy is sometimes referred to as “stored” energy. (note: cannot use “stored energy” in O levels) • Energy which is doing nothing right now, but is able to do work later • A battery contains chemical PE (you will learn more in A level chemistry), so do fossil fuels and food (e.g. bio SPA) • A stretched rubber band contains elastic PE • An object lifted up contains gravitational PE

  7. Half Time • Brainiac Pendulum Demo: http://www.youtube.com/watch?v=ZbCLAZSPLNk • Bill Nye Part 1: • http://www.youtube.com/watch?v=zTXW9aRO23Y • Bill Nye Part 2: • http://www.youtube.com/watch?v=Vfcsq6ylRzg • Bill Nye Part 3:http://www.youtube.com/watch?v=XkDG02nzfSk

  8. Energy Conversion WS

  9. Did You Know: Calories • 1 calorie is 4.184 J • The recommended calorie intake is about 2000 kilo-calories per day (actual recommendation varies by size, gender and age) • How many joules of energy is that?

  10. Closed and Open Systems • Recall the definition of CoE includes the statement: The total energy in an isolated system is constant. • What is an “isolated system”? • An isolated or closed system is a system of interacting objects where no energy is added or taken from the system

  11. Case Study: Pendulum • [applet] • Assume no air resistance and no friction • Note that system is closed – no energy is added or taken from the pendulum • Total Energy is constant, even though GPE and KE are changing over time • What will happen if there is air resistance? Is the system still closed?

  12. Work Done • When a force pushes an object and the object moves in the direction of the force, it is said that work is done by that force. • Work done has units of Joules (J), same as energy • Formula: • Work = F x Distance (same direction as force)

  13. Worked Example 3 N • If a force of 3 N pushes an object for a distance of 5 m what is the work done by the 3 N force? • Ans: Work = Fd • = (3)(5) = 15.0 J (3 s.f.)

  14. GPE and KE • Formula for GPE: • GPE = mgh • Formula for KE • KE = ½ mv2

  15. Worked Example 5 m • An object of 5 kg, initially at rest, slides down the slope as shown above. Assuming no energy loss, what is the final velocity of the object? • Method: by the conservation of energy, loss in GPE = gain in KE • mgh = ½ mv2 • (5)(10)(5) = (0.5)(5)v2 • Solve for v. • Ans: v = 10.0 ms-1 (3 s.f)

  16. Remember Taman? • The trek component of Taman (just before the canopy walk) was a 300 m vertical climb (all my stairs). • If you weigh 50 kg, how much energy did you spend? • I weigh 120 kg. how much energy did I spend?

  17. A note about Equations • How do you know what units to pluck in when doing equations? • As long as all units on the right side of the equation are S.I. units, the left side of the equation will also be in S.I. units. • In fact, this convenience is precisely what S.I. units were created in the first place.

  18. Power • In Physics, power is a measure of how much work can be done in an interval of time. • If I can do a lot of work in a short amount of time, I am more powerful • Equation: Power = Work Done / Time • Units: Watts (W) • Sometimes it is difficult to calculate the Work Done directly. We may have calculate the work done indirectly using the GPE and KE equations.

  19. Worked Example 1 3 N • A 3 N force pushes an object for 5 m in 5 s. What is the power produced by the force? • Power = Work Done / time • = (force x dist) / time • = (3)(5)/5 • = 3.00 W (3 s.f.)

  20. Work Example 2 • An 10 kg object was pushed up a smooth slope in 50 s. Assuming that the final velocity of the object is 0, what is the power produced? 5 m

  21. Work Example 2 • Cannot calculate work done directly (we don’t know force, and don’t know distance) • But we know that the work done results in gain in GPE of object • Work done = gain in GPE = mgh • Power = mgh/t • = (10)(10)(5)/(50) • = 10.0 W (3 s.f.)

  22. Half Time • Energy Playa raphttp://www.youtube.com/watch?v=P2W1VQ2bAi8

  23. Useful Work / Energy / Power • In reality when we do work, only a fraction of our efforts going into completing what we achieve to do. The rest of our efforts is “wasted” overcoming friction, heat lost to the surroundings, etc. • The work which actually accomplishes our intended outcome is called the “useful” work. • Same for “useful energy” or “useful power”

  24. Work Example 1 • A 3N block pushes a block for 5 m over 10 s. If the friction force exerted on the block is 1 N, • (i) what is the total work done on the block? • (ii) what is the useful work done on the block? • (iii) what is the useful power?

  25. Work Example 2 • A 2000 kg car accelerates from 0 to 10 ms-1 in 10 seconds. • (i) what is the final KE of the car? • (ii) what is the useful power of the car? • (iii) would you expect the actual power of the car’s engine to be greater than or the same as your answer in (ii)? Why or why not?

  26. Efficiency • Efficiency measures the percentage of usefulness. • Units: Efficiency is a percentage (%) • There are three possible equations for efficiency but they are actually all the same: • (Useful Work / Total Work Done) x 100% • (Useful Energy Output / Total Energy Input) x 100% • (Useful Power Output / Total Power Input) x 100%

  27. Worked Example 3 • Refer to the box in Work Example 1 • What is the efficiency of the force pushing the box?

  28. Worked Example 4 • Refer to the car in Worked Example 2. • If the total power input to the car’s engine is 15 kW, what is the efficiency of the car’s engine?

  29. Combination Questions • A crane lifts a 1000 kg object up a height of 20 metres. • (i) If the crane took 30 s to do so, what is the useful power of the crane? • (ii) if the efficiency of the crane is 50%, what is the total input energy supplied to the crane? • (iii) if the object is accidentally dropped, determine the velocity of the object just before it hits the ground (ignore air resistance).

  30. Summary • 9 types of Energy • Law of Conservation of Energy • Conversion of Energy (qualitative) • Work Done • GPE and KE Equations • Power • Useful Work / Energy / Power • Efficiency

  31. Now: 10 min pop quiz!! MUAHAHAHAHA =P

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