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Homework Help: Electric energy - potential difference problem!

  1. May 22, 2009 #1
    1. The problem statement, all variables and given/known data

    On planet Tehar, the free-fall acceleration is the same as that on Earth, but there is also a strong downward electric field that is uniform close to the planet's surface. A 2.00 kg ball having a charge of 5.00 µC is thrown upward at a speed of 20.1 m/s. It hits the ground after an interval of 4.10 s. What is the potential difference between the starting point and the top point of the trajectory?

    2. Relevant equations

    Work = (q)(E)(delta x)

    q=charge, E= electric field

    delta V = (delta PE)/q

    V=electric potential difference

    3. The attempt at a solution

    I pretty sure we'd have to use the conservation of energy to determine the solution and the a physics 1 kinematic equation. I know the background of the question but I just can't figure out how the electric field comes into play for the question. Can someone please help me understand this question? Thanks a lot in advance for your help.
  2. jcsd
  3. May 23, 2009 #2


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    Homework Helper

    Remember at all times on Tehar F = MA.

    Determine through regular kinematics what the acceleration is on the ball.

    F = m*g + q*E = m*a

    So ...

    q*E = m*a - m*g = m*(a - g)

    This suggests that the E field (which is uniform) is

    E = m*(a - g) / q

    Whether the field is ± is determined by whether the Electrostatic force is attracting or repelling.
  4. May 23, 2009 #3
    Got the correct answer! Thanks a lot.
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