Both c and a could be right but I am steering more towards a.
Answer:

Explanation:
A charge located at a point will experience a zero electrostatic force if the resultant electric field on it due to any other charge(s) is zero.
is located at the origin. The net force on it will only be zero if the resultant electric field intensity due to
and
at the origin is equal to zero. Therefore we can perform this solution without necessarily needing the value of
.
Let the electric field intensity due to
be +
and that due to
be -
since the charge is negative. Hence at the origin;

From equation (1) above, we obtain the following;

From Coulomb's law the following relationship holds;

where
is the distance of
from the origin,
is the distance of
from the origin and k is the electrostatic constant.
It therefore means that from equation (2) we can write the following;

k can cancel out from both side of equation (3), so that we finally obtain the following;

Given;

Substituting these values into equation (4); we obtain the following;


Answer:
The potential between the plates will decrease.
Explanation:
An insulator is usually placed between the parallel plates and is also called a dielectric because it makes the amount of charge a capacitor can accommodate to increase at a particular potential difference.
Furthermore, the dielectric effect will make the electric field of the charged capacitor which is not connected to a source of supply to decrease.
Now, when the battery is removed, the charge Q remains constant and Capacity C will increase.
Formula for the potential difference is here;
V = Q/C
Since the numerator Q is constant and the denominator C increases, it means the potential difference V will decrease
The velocity of the ball is 12.5 m/s
Explanation:
The velocity of the ball is given by the ratio between the distance covered by the ball and the time taken:

First, we calculate the distance covered. We know that the radius of the circle is
r = 0.450 m
And the length of the circumference is

The ball makes 25.0 revolutions, so a total distance of

In a time of
t = 9.37 s
So, its velocity is

Learn more about velocity here:
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