Two parallel plates have equal but opposite charges on their surface. the plates are separated by a finite distance. a fast moving proton enters the space between the two plates through a tiny hole in the left plate a. the electric potential energy of the proton increases as it moves toward plate b. (a) how is the speed of the proton affected as it moves from plate a to plate b

Explanation:

The plates A and B are charged by opposite charges  but which plate is positively charged and which is negatively charged is not clear.

Now a proton which is positively charged is moving from plate A to B . If it is attracted by plate B then its kinetic energy will be increased and potential energy will be decreased due to conservation of energy . In that case B will be negatively charged .

                                                                  But in the given case it is stated that

potential energy of the proton increases . That means its kinetic energy decreases . In other words its speed decreases . It points to the fact that plate B is also positively charged.

So proton will be repelled and its speed will be decreased.