Verified answer

Are we to assume that the point charge is situated exactly half way between the plates? If so, then

for a) if both plates are positive or negative,the forces on the charge will cancel to zero and the charge will remain stationary.

For b) the charge will be repelled by the positive plate and attracted to the negative plate.

The supply voltage shouldn't change, if you're talking about an ideal voltage source. Could you be more specific regarding the circuit you're talking about? The current from the battery will change because depending on whether the circuit elements are in series or parallel, the overall resistance of the circuit can change.

very confusing stuff. look onto google or bing. just that can assist!