# Electric energy - potential difference problem!

1. May 22, 2009

### physics213

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. May 23, 2009

### LowlyPion

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.

3. May 23, 2009

### physics213

Got the correct answer! Thanks a lot.