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Homework Help: Why is the answer to this electricity question negative?

  1. Dec 14, 2011 #1
    A proton (m = 1.7 * 10–27 kg, q = +1.6 * 10–19 C) starts from rest at point A and has a speed of 40 km/s at point B. Only electric forces act on it during this motion. Determine the electric potential difference VB-VA.

    Here is what I did:
    [tex]V = \frac{U}{q}[/tex]
    [tex]U = qV = \frac{1}{2}mv^2[/tex]
    [tex]V_B - V_A = \frac{m(v_B^2-v_A^2)}{2q}[/tex]

    Gives me 8.5V

    My prof's answer says it is -8.5V. Why is it negative?
    Last edited: Dec 14, 2011
  2. jcsd
  3. Dec 14, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Because B is at a lower potential than A. The electric field points from higher to lower potential.

    That equation should be ΔKE = -qΔV.
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