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Questions on Electrostatics/Electricity/Stuff?

  1. Mar 22, 2006 #1
    okay i just found this forum and i think now would be a good time to utilize it...

    okay first q....

    1) Two lightbulbs, one rated 30 W at 120 V and another rated 40 W at 120 V, are arranged in two different circuits, if connected in series to a 120 V source, what is the resistance and current of each lightbulb?

    well i thought since they are in series, the current had to be the same throughout, the Voltages had to add up to total V of the circuit, except with the different wattages, I have little idea where to start because I cant use P=V^2/R because Voltages of each arent teh same and I cant use P=(I^2)R because I dont think I'm given enough info on how to calc. the current of each...can someone at least get me started? or see if there are any misconceptions?

    2)Determine the potential difference through which an electron must be accelerated in the electron gun in order to have a speed of 6E7 M/S
    qV = 1/2mv^2 correct? i just wanted confirmation...

    and suppose there were two plates with a potential difference of 200, separated by .012 M and had a length of .04 M, how would I calculate the time required to move through the plates and the electron's vertical displacement?...someone please just throw me a bone here?

    Thanks muchooo to whoever can help
  2. jcsd
  3. Mar 22, 2006 #2


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    A lightbulb is essentially a resistor. If you know the power dissipated at a certain potential difference, you can use that to find the resistance of the lightbulb.

    You first find the resistances of each lightbulb using their power rating at 120 volts. Then you proceed with the two resistances in series connected to a potential difference of 120 volts (notice that the power in each lightbulb woul dno longer be 30 and 40 W, if you would calculate them now)

    yes, neglecting relativistic effects.

    First, you find the electric field between the plates. It has a magnitude given by the pitential difference divided by the separation between the plates.

    Then, you calculate the force using F = q E. That allows you to find the acceleration (perpendicular to the plates) a = F/m. (we can neglect gravity here)

    Now it becomes a problem of constant acceleration. If the electron is initially moving parallel to the plates (let's say along x), it will curve following a parabola. The velocity along x will not change. The time to get across the plates is the length of the plates divided by the initial velocity (assuming that the electron won't hit a plate!). Knowing this time you can use find the deflection along y since you know the acceleration along y.

  4. Mar 22, 2006 #3
    thank you very much, electrostatics really isnt my strong suit....meaning there will probably more to come soon...
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