Final answer:
The potential difference across an inductor when the current is 15 mA cannot be determined without additional information on the inductance and the time rate of change of the current.
Step-by-step explanation:
The student is asking about the potential difference across an inductor in a physics context, specifically when the current reaches 15 mA. Potential difference, also known as voltage, across an inductor can be found using Faraday's Law of Induction, which states that the induced electromotive force (emf) in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit. If we denote the inductance by L, and the change in current by ΔI over a change in time Δt, the induced emf (Ε) can be calculated using the formula Ε = -L (ΔI/Δt). However, the question does not provide values for the inductance (L) or the time rate of change of the current (ΔI/Δt), so it is not possible to calculate the exact potential difference.