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Homework Help: Electric Potential Difference Please HELP

  1. Jan 17, 2008 #1
    Electric Potential Difference...Please HELP!!!

    1. The problem statement, all variables and given/known data
    What potential difference is needed to accelerate a He+ ion (charge +e, mass 4u) from rest to a speed of 2690000 m/s?

    2. Relevant equations

    3. The attempt at a solution
    Change in V = Change in K
    Change in V = 1/2 mv^2
    But what dose mass = 4u mean? What is u???
  2. jcsd
  3. Jan 17, 2008 #2
  4. Jan 17, 2008 #3
    Oh ok...
    so is the rest of the problem set up correctly?
    So I do...
    1/2 * (4 * 1.66x10^-27) * (2690000^2) = 2.4x10^-14 V
  5. Jan 17, 2008 #4
    actually.. i think you have something fundamentally wrong with:

    change in V = change in K

    this is because, the equation is dimensionally incorrect. 'K' or it's change, has the units of energy, whereas 'V' has the units of 'Energy/Charge'.

    your solution maybe right.. or i don't know what.. just explain to me why you took the above mentioned relation.

    You need to think of this problem in terms of the Electric field? For a given potential difference, what is the field present? Or to think in the direction of the problem, what is the electric field required so that the necessary velocity is achieved?
  6. Jan 17, 2008 #5
    I think maybe it should be...
    Change in K = q*Change in V
  7. Jan 17, 2008 #6
    and yes.. you're right. Now, just substitute the values of 'm', 'v', 'q' and find what 'V' comes out to be.

    also.. don't use the terminology 'change in V'. 'V' in itself refers to the potential difference.

    Change in 'K', basically means the total Work done [following by 'Work-Energy theorem']. And 'V' is defined as the amount of work done per unit charge. So what you effectively did was first get the amount of work required to accelerate the given particle and then equate it with the amount of potential that could provide such energy [or in other words, do this work].
  8. Jan 17, 2008 #7
    Change in K = q*Change in V
    (1/2 (6.6423x10^-24) * 26900000^2) / (1.6x10^-19) = V = 1.5x10^10 V
    Is that right then?
  9. Jan 17, 2008 #8
    Oh wait to many 0's is velocity...answer I got is 1.5x10^8 V
  10. Jan 17, 2008 #9
    OMG....wrong number for mass lol...

    (1/2 (6.6423x10^-27) * 2690000^2) / (1.6x10^-19) = V = 1.5x10^5 V
    Is that right then?
  11. Jan 17, 2008 #10
    oh nooo its wrong :( I only have 1 try left...what did I do wrong?
  12. Jan 17, 2008 #11
    well.. seems right to me..

    also.. try not to double post. Use the [edit] button in the bottom right corner to edit ur latest reply in case u have to add something. Make a new reply only when somebody has already commented on something or ur replies are far apart. Your last 3 replies were less than 5 mins. apart. Just keeps the forums clean. No offense.


    i can't see anything wrong with the above. Are you sure it's a [itex]He^+[/itex] ion and not a [itex]He^{+2}[/itex] nucleus?
    Last edited: Jan 17, 2008
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