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Homework Help: Could use some help on this e&m question

  1. Sep 4, 2005 #1
    Hi, I've been trying to figure out this problem for a while now and I could use some help. The problem reads "An electron and proton attract each other with a [tex]1/r^2[/tex] electric force, just like the gravitational force. Suppose that an electron moves in a circular orbit about a proton. If the period of motion is 24 hours, what is the radius of the orbit?"

    My train of thought so far has been that I would need to equate the [tex]1/r^2[/tex] to the equation for centripetal force [tex]F=mv^2/r[/tex], and solve that for r. I also know that the period comes into play, so I'm guessing that [tex]T=2\Pi r/v[/tex] would also be used somehow. However, I can't figure out how I would solve for the velocity just by plugging in the period to that equation, since they don't give us the value for the radius, and I need the value of the velocity to solve for the radius in the force equation. Am I on the right track or totally off?
    Last edited: Sep 4, 2005
  2. jcsd
  3. Sep 4, 2005 #2


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    Homework Helper
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    The electron and proton attract each other with a force that is propotional to [tex]1/r^2[/tex]. The actual force of attraction is given by coloumb's law.

    You are on the right track by equating [tex] mv^2/r [/tex] and the force of attraction between the proton and electron.

    Also, you do use [tex] T=2 \pi/w [/tex]. Use the fact that for a revolution of [tex] 2 \pi [/tex] radians, it takes 24*3600 seconds. So what is the angular velocity in radians per second? You should get the answer from this.
    Last edited: Sep 4, 2005
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