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Homework Help: Finding the accelration of a particle in a electric field?

  1. Feb 6, 2012 #1
    i just learning this stuff and want to clarify this. ok here is my trouble

    to find the acceleration i am using the equation a=qE/m

    know if i use Mg and say its mass number is 24, and solved for electric field to be 3 x 10^4
    i then plug into equation

    a= 2x(1.6x10^-19)x(3x10^4)/( 24x(________) )

    since Mg has a positive charge do i do mass number times mass of proton? (for empty part)

    so then say if have particle with negative charge would do its mass number times mass of election?

  2. jcsd
  3. Feb 6, 2012 #2
    The force on a charged particle in an electric field is equal to the charge times the electric field strength.

    I don't know what "Mg" is supposed to represent (do you mean magnesium?) but its irrelevant. The number of protons/amount of charge/mass/whatever are all just constants. Try and visualize everything as variables.

    F = ma
    F = kq1q2 / r2

    E = kq / r2

    ma = qE, solve for whatever you want. Charge sign is irrelevant to what you're trying to find and only determines the direction of the acceleration, which you can again find independent of this equation since you know - repels -, + repels +, opposites attract.
  4. Feb 6, 2012 #3
    well what if only have one particle and given its charge (in this case is magnesium) and know the electric field is to the south of it .

    to find the acceleration on that particle we need to know m, right?

    to find that m, why would it be irrevelent, dont we need to know the mass number for Mg(magnesium)?
  5. Feb 6, 2012 #4
    Yes you would need to know m, and yes, you would need to know the mass number for Mg. But if my memory serves me right thats in g/mol. you'd need to convert and stuff.
  6. Feb 6, 2012 #5
    okay. to reclarify my question/trouble is how find that m?

    is it mass number x mass of proton, because Mg has postive charge or what is the correct method to do this
  7. Feb 6, 2012 #6
    Well are you finding the mass of 24 protons, or the mass of an atom of Mg? The molar mass of Mg is created using a weighted average of all the isotopes of Mg, whereas the mass of a proton you can find on google.

    So if you're looking for the mass of Mg, yeah I would go with 24 * the mass of a proton (at least in a physics class, not in a chem class), but I'm not sure about the specifics because I'm not taking your class/not seeing the exact problem.

    I'm sorry I think I'm as confused as you are... haha

    Mass is just a mass, honestly if you're looking for the mass of a specific... thing, I would just google it, but the physics of the question is already done, the equation is all thats important, not whether or not your teacher gets annoyed that you were off by a few hundredths of a gram...
  8. Feb 6, 2012 #7
    well any help is useful.
    i guess one more thing want to clear up is

    is there a reason would do times the mass of proton not mass of electron?
  9. Feb 6, 2012 #8
    Thats the reason I was talking about Mg vs just protons. Obviously, just protons you're taking the mass of just protons (sorry for the Layman's terms here, just trying to be specific), but if it were an Mg atom, there are electron masses and the weighted average of the isotopes (ie molar mass) to take into account.

    Also generally for these types of physics problems electron masses are negligible, since they are so small compared to a proton.

    Hope this helps.
  10. Feb 6, 2012 #9
    very helpful thanks.
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