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Electric filed physics problem. Help!

  1. Jul 22, 2012 #1
    Electrical field = 150 directed everywhere downward near Earth's surface.

    1. What is the net electric charge on the Earth? Earth is a spherical conductor of radius 6371. km.
    2. What is the electrostatic potential at the Earth’s surface, if the potential is taken to
    be zero at infinity?
    3. Using above info from Problem 1 Calculate the acceleration—magnitude and direction—of a proton released near the surface of the Earth. Disregard any interaction with air molecules.
    4. Calculate the charge-to-mass ratio of a particle which would hover in place if released
    near the surface of the Earth.

    Part one and two I did can you check if its right?
    1. q= Er^2 / k = -6.774*10^5 C
    2. V= kq/r = -9.55*10^8 V

    No idea about part three and four???


    Thanks!
     
  2. jcsd
  3. Jul 22, 2012 #2

    TSny

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    Hi sept26bc. In general, what causes a particle to accelerate?
     
  4. Jul 22, 2012 #3
    a force
     
  5. Jul 22, 2012 #4

    TSny

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    Is there a force on the proton? How would you calculate that force?
     
  6. Jul 22, 2012 #5
    i don't know if there is a force on it, but the electrical field of the earth, right?
    wait i think I got it would I use:
    F=ma the sub that in for E=F/q , giving a=Eq/mass of proton?
     
  7. Jul 22, 2012 #6

    TSny

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    Yes!
     
  8. Jul 22, 2012 #7
    thanks! okay but I don't understand how to find charge to mass ratio?
     
  9. Jul 22, 2012 #8

    TSny

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    Well, there's actually another force acting. And what really matters is the net force.
     
  10. Jul 22, 2012 #9
    force of gravity?
     
  11. Jul 22, 2012 #10

    TSny

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    Sure.
     
  12. Jul 22, 2012 #11
    okay I'm confused now there's something else besides the electric field of the earth?
     
  13. Jul 22, 2012 #12

    TSny

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    Yes. Earth also has a gravitational field. How do you calculate the force due to gravity?
     
  14. Jul 22, 2012 #13
    is the earth's mass involved
     
    Last edited: Jul 22, 2012
  15. Jul 22, 2012 #14

    TSny

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    No! Not the earth's mass.
     
  16. Jul 22, 2012 #15
    sorry the protons mass multiplied by the 9.81
     
  17. Jul 22, 2012 #16

    TSny

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    Good. Try calculating it and compare to the electric force.
     
  18. Jul 22, 2012 #17
    here's what I have so far but I'm not sure if its right:

    I calculated acceleration due to the earth's electric field a = Eq/m
    so the force of this is F=mass of proton x acceleration I found above
    then the other force is gravity? so F= mass of proton x 9.81
    then add the two forces to find Fnet?
     
  19. Jul 22, 2012 #18

    TSny

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    For the proton you should be able to show that the force of gravity is very much smaller than the electric force. So, you can forget about the force of gravity when calculating the acceleration of the proton.

    But, for question 4, the force of gravity is important. How can the particle in question 4 hover at rest?
     
  20. Jul 22, 2012 #19
    for it to hover at rest would the force be equal and opposite the electric field
     
  21. Jul 22, 2012 #20

    TSny

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    Well, the two forces would have to be opposite to each other so that they cancel. This will tell you something about the sign of the charge of the particle.
     
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