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Basic Potential Difference Problem

  1. Aug 28, 2009 #1
    1. The problem statement, all variables and given/known data
    What potential difference is needed to stop an electron having an initial speed of 4.2*10^5 m/s?


    2. Relevant equations
    W = delta K
    W = q*delta V


    3. The attempt at a solution
    So I'm pretty sure that the potential difference should be negative in order to stop the electron, but I keep getting a positive answer (the correct magnitude, but incorrect sign).

    I set delta K = q*delta V.

    I know delta K will be less than zero since K(final) < K(initial) and q should also be negative since we're dealing with an electron. But this gives me a positive delta V.

    Where am I going wrong?

    Thanks in advance.
     
  2. jcsd
  3. Aug 28, 2009 #2

    rock.freak667

    User Avatar
    Homework Helper

    so you have 1/2 mv2=VQ

    for an electron Q=-1.60*10-19C
     
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