Final answer:
The electric potential at the midpoint P of a rod with charges q and -q at its ends will be zero, as the contributions from both charges cancel each other out.
Step-by-step explanation:
When two charges of equal magnitude and opposite sign, q and -q, are placed at the ends of a rod, the electric potential at the midpoint P will be zero. This is because potential is a scalar quantity and the contributions from the two charges will cancel out each other due to their equal magnitude and opposite nature. The electric potential due to a single point charge is given by V = kq/r, where k is Coulomb's constant, q is the charge, and r is the distance from the charge. In the case of the midpoint P, the distances from q and -q are the same, hence the potentials due to each charge are equal in magnitude and opposite in sign, leading to a net electric potential of zero.