source charge

Meaning: A source charge is the charge that sets up the electric field and potential around it. A test charge is a small charge placed in that field to probe it, assumed too small to disturb the source. The field points away from a positive source and toward a negative one. For a point charge, or outside a spherically symmetric distribution, the field falls as 1/r² and the potential as 1/r.

Equation: E = kQ/r², V = kQ/r, F = qE, U = qV. E is field in N/C, V is potential in volts (J/C), k = 9.0 × 10⁹ N·m²/C², Q is the source charge in C, r is distance in m, q is the test charge in C, F is force in N, U is potential energy in J.

On the MCAT: Passages give one or more fixed charges and ask for the field or potential at a point, or the force on a test charge. Doubling r quarters E and halves V. With several sources, add fields as vectors and potentials as signed scalars.

Common confusion: Students put the test charge into E or V. Field and potential at a point depend only on the source charges and the distance. The test charge sets only the force and potential energy it experiences, so changing q changes F and U, not E or V.