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Watts=Newton-meters per second

  1. Apr 18, 2009 #1
    I've got to ask because I know my students will ask (they have a game where they try to stump the teacher with physics questions.)

    A Watt is equal to a newton-meter per second, which is to say the amount of work done per second. Work is the amount of force applied for a distance. Ok, so a certain light bulb has an amount of 40 watts on it. In the light bulb, what force is being applied and what distance is being travelled?
  2. jcsd
  3. Apr 18, 2009 #2

    [tex]E=\frac{\Delta V}{\Delta x}[/tex]

    [tex]\frac{F_{el}}{q}=\frac{\Delta V}{\Delta x}[/tex]

    [tex]W= \Delta V q=F_{el} \Delta x [/tex]

    So the force is the electrical force, and delta x is the distance over which the potential difference is applied (so that is the little metal piece in the light bulb if the resistance in the wire is neglected).

    PS: You should appreciate these students. At least they pay attention :)
  4. Apr 18, 2009 #3
    The light bulb is powered by a generator at a power plant. The principle of these generators is that steam is applied to rotate a turbine, and the rotating turbine creates a magnetic field that creates a current that transmits the power to our homes at the speed of light.

    Therefore the work is not being done by the light bulb, it is being done on the light bulb. As an analogy with 'Jane pushes a block' the bulb is analogous to the block while the analogue of Jane is the steam in the electric generator. Of course, Jane gets her energy from a chemical reaction (in her digestive system) and chemical energy (in the form of petroleum) is also the thing that usually powers the steam.

    If I were you I would avoid talking about the electric forces moving charged particles in the wire, because the details of this model are somewhat messy (and giving the explanation without the details would be like misinformation).
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