The problem involves two stationary point particles with equal and opposite charges placed at a fixed distance from each other. We are tasked with identifying the geometric shape on which the points of zero electric potential lie.
To solve this, let's first understand the concept of electric potential:
1. **Electric Potential Due to a Point Charge:** The electric potential \(V\) at a distance \(r\) from a point charge \(Q\) is given by the formula:
\(V = \frac{kQ}{r}\)
where \(k\) is Coulomb's constant.
2. **Superposition Principle:** When there are multiple charges, the total electric potential at a point is the algebraic sum of the potentials due to each charge separately.
For two charges \(+Q\) and \(-Q\), the potentials add up algebraically:
\(V_{\text{total}} = V_{+Q} + V_{-Q} = \frac{k(+Q)}{r_1} + \frac{k(-Q)}{r_2}\)
3. **Zero Electric Potential:** At the locations where these two potentials cancel each other out, the total electric potential is zero:
\(\frac{k(+Q)}{r_1} = \frac{k(-Q)}{r_2}\)
Rearranging gives:
\(\frac{1}{r_1} = \frac{1}{r_2}\)
4. **Geometrical Implication:** The condition \(r_1 = r_2\) describes the locus of points equidistant from both charges. This geometrical locus is a plane perpendicular to the line joining the two charges and passing through the midpoint.
Therefore, the correct answer is that the points of zero potential lie on a plane.
In a coaxial cable, the radius of the inner conductor is 2 mm and that of the outer one is 5 mm. The inner conductor is at a potential of 10 V, while the outer conductor is grounded. The value of the potential at a distance of 3.5 mm from the axis is: 
A sphere of radius \( R \) has a uniform charge density \( \rho \). A sphere of smaller radius \( \frac{R}{2} \) is cut out from the original sphere, as shown in the figure. The center of the cut-out sphere lies at \( z = \frac{R}{2} \). After the smaller sphere has been cut out, the magnitude of the electric field at \( z = -\frac{R}{2} \) is \( \frac{\rho R}{n \epsilon_0} \). The value of the integer \( n \) is: 