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About work done and power

  1. Dec 9, 2009 #1


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    Suppose two people A and B, A walk from a distance x at constant speed B is in a car travel same distance x at same speed. If the work done by A is [tex]W_A[/tex], what about the work done by B? Does B do any work? If do, what kinda of work done by B?

    We know that power is work per second, can I say power tells how fast the energy is consumed? So for a bulb with larger power means consuming energy faster?

  2. jcsd
  3. Dec 9, 2009 #2
    Higher power means that energy is "consumed" faster.

    But, walking and driving from point A to point B...there's really not enough information there. For example, if A and B are at the same elevation, and there would be no friction, then it would require 0 work to get from one piont to the other. A satellite going millions of miles in a circle around the earth, is essentially doing 0 work.
  4. Dec 9, 2009 #3
    This is technically correct. Energy is in Joules. Time is in Seconds. Power is in Watts = Joules/second. In the English language, "work" can mean lifting many bottles of beer from the floor onto the shelves. The "potential energy" of a bottle of beer on the shelf is proportional to the mass of the bottle and the height of the shelf (and the strength of gravity). So, one guy may take his time loading the shelves and another does it quickly. Both do the same "work" but the faster guy exerts more power on averaged (averaged over the number of seconds). Work is in Joules or calories. But, since the first guy took his time, the average power (Watts) would be smaller than the 2nd guy, but the peak power needed to put the bottles up may be the same. Suppose it took both guys 3 seconds to put a 6-pack on the high shelf... if both workers took this time with the same mass on the same shelf, both would exert the same instantaneous power.

    Another use of the term "power". My car can go from 0 to 60 MPH in 8 seconds. Take the mass of my car times 60 MPH squared, you get the final (kinetic) energy. Another car, same mass, but takes 16 seconds to get to 60 MPH. Roughly speaking, the engine of my car is twice as "powerful" as his. So we often think of power as the ability to deliver. What is a "powerful punch" in boxing? The velocity of the fist is very high on impact. And the punch is done quickly. Power = energy/time whether it is power consumed by a light bulb or power delivered by a engine or the muscles of the arm.

    With our "energy crisis", some folks talk about "saving power". If it is not used, it is stored energy. Oil, gas, coal... stored energy. These can be saved by not using them. Power cannot be saved. Joules per Second seems to me that it is being used! Power consumption, however, can be minimized. Use a light bulb that is more efficient. Less oil/gas/coal used per second for the same job.

    If one has a power plant that you cannot turn off, and nobody wants the power, can *that* be saved? Kind of. Actually, we use this kind of (unwanted) power to pump water up into a reservoir in the mountains. This is actually converting the gas/oil/coal (stored energy) into potential energy of billions of beer bottles worth of water on a high shelf via a power plant and some pumps. Energy to energy. It's like a battery. If we have power that cannot be consumed right now, the best thing to do is charge a battery. We will take the energy in the battery and convert it back into power later.

    Sorry about the long post. But I think understanding the difference between energy and power is very important these days. Save energy. Reduce the amount of power needed to do some job. Insulate your house... much less power needed to keep it warm in the winter and cool in the summer. This also saves energy, since fewer power plants need to be built. We can leave the oil/gas/coal in the ground. Maybe the power of the wind will be enough to heat/cool. A well insulated house takes less Joules to maintain a comfortable space. Now I'm getting confused! But we all have to think about how we use our language and correct folks that don't see the relationship between energy, power, and our standard of living.
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