The sign of the derivative: dQ/dt = ±i

Before writing the differential equations of LC and RLC circuits, it is necessary to clarify when $dQ/dt = +i$ and when $dQ/dt = -i$. The answer goes through the continuity equation for charge and two conventional choices: which plate we enclose and which direction we orient the current.

• Closed surface and charge balance
• The two cases, with figures
• Why it is the same i throughout the circuit