Electric potential problem with two plates and different points (i'll elaborate)

[nadeem0322]

Homework Statement

There are two parallel plates. The plate on the left has an electric potential of +60 Volts. The right plate has an electric potential of 0 Volts. The plates are 8.0mm apart. Between the two plates lies a point P 2.0mm from the left plate with a charge -3.5pC. Point R lies a mass right in the middle. Point S lies 2.0mm from the right plate.

A)
i) What is the electric potential at point R?
ii) What is the electric potential at point S?

Point R lies a mass of 4*10^-19 kg with a charge -2pC.
B) What is the speed of the mass at R when it reaches point S if it has an initial velocity of 3.0*10^4 m/s.

Homework Equations

V= kq/r
u=kqq/r

The Attempt at a Solution

I solved this problem for Ai) using kq/r where r is the distance from R and P and q is charge at P. I then added the voltage of 60 volts to that number to get the total potential. I believe this is where I went wrong but I don't see how the 60 voltage plate could not affect the potential at the point. Any help would be great. Thanks

 

[rude man]

If the potential of the RH plate is zero then the potential at a point x away from the RH plate towards the LH plate is 60V*x/d, x = distance from RH plate to R or S, ignoring the extra charge at P. Now the potential due to the extra charge at P is just the extra work you have to apply to a unit positive test charge to bring it from the RH plate to R or S.

The usual definition of potential as being the work on a unit test charge brought from infinity to the observation point does not apply here. That's because the point of zero potential is the RH plate and not infinity.