Final answer:
The magnitude of the electric field at the location of charge qa can be calculated using Coulomb's law, by summing up the contributions from surrounding charges and taking vector components into consideration. The net electric field is the vector sum of electric fields due to individual point charges. For force calculation, the equation F = qE is used.
Step-by-step explanation:
To calculate the magnitude of the electric field at the location of charge qa, we need to consider the effects of the other charges around it (i.e., qb, qc, and potentially qd). The electric field (E) created by a point charge at a distance (r) is given by Coulomb's law, which states that E = k * |q| / r2, where k is Coulomb's constant (8.988 x 109 N m2/C2), |q| is the magnitude of the charge, and r is the distance from the charge to the point where the electric field is being measured.
For charges arranged in a geometric shape like a square, we need to vectorially add the electric fields due to each charge to find the net electric field at the point of interest. If charges are equidistant from the point where we want to find the electric field, symmetry can be used to simplify calculations. For questions involving the force (F) on a charge in an electric field, we use the equation F = qE, where q is the charge on which the force is being calculated.