Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Please hlep me with this ELECTRICAL FORCE QUESTION.

  1. Jan 25, 2005 #1
    5 An -=- particle (helium nucleus, 2 protons, and 2 nuetrons) is place between two stationary, charged objects as shown. It is 0.0030 meters to the right of the object carrying a +1.0 X 10^-12C charge and 0.0020 meters to the left of the object carrying a -2.0 X 10 ^-12 C charge.

    a What is the net magnitude and direction of the electrical force acting on the particle?

    I put 1.12 X 10^-15 N in the same direction and got the question WRONG.

    b. At what rate and in what direction will the particle accelerate?

    I put 3.4 X 10^11 m/s/s and also got it WRONG.

    Any help would be greatly appreciated.

    Im pretty sure I got the second one wrong since I got the 1st one wrong, and I have a feeling I got the 1st one wrong because I either added or subtraced wrongly, but Im confused about how I'm supposed to know the signs etc.
  2. jcsd
  3. Jan 25, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Yes,the answer to the second part/question relies on te one to the first...
    U'd have to add the 2 vectors which represent the electric forces which act on the [itex]] \alpha [/itex] particle...Can u do that??U need to chose an axis which goes through the 3 particles and a positive sense on it.Then u'd have to project the vector equation on this axis...

  4. Jan 25, 2005 #3
    How did you get
    and what do you mean by
    Which direction?
  5. Jan 25, 2005 #4
    I found the force between the particle and the 2 objects and added them together. I found the force b/w the particle and the first object (leftmost) at 3.2^-16 and then found the force between it and the last object at -1.44X10^-15. Then I added them together. And as for the direction part, I probably just put that down because I had no real idea what it was talking about.
  6. Jan 25, 2005 #5
    You forgot to consider that the direction of each force is related to the signs of the charges.

    (Hint: what is the sign of the alpha particle's charge?)
    Last edited: Jan 25, 2005
  7. Jan 25, 2005 #6
    The alpha particle is postive so the force between it and the first object would be positvie too right? and the force between it and the second object would be negative? And I would have to add these together?
  8. Jan 25, 2005 #7
    Which force is "positive" and which is "negative" depends on how you define your coordinates.

    So the way I would state it is:
    The charge of the alpha particle is positive so there is a repulsive force between it and the stationary positive charge, and an attractive force between it and the stationary negative charge. So both forces act in the direction towards the negative charge, and therefore, yes, you have to add them together.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook