RC circuit
Also called Resistor-capacitor circuit
An RC circuit contains at least one resistor and one capacitor, so its currents and voltages change over time as the capacitor charges or discharges. An uncharged capacitor acts like a wire the instant the switch closes and like a break in the circuit once fully charged.
An RC circuit puts a resistor and a capacitor in the same loop, which gives the circuit a clock: the capacitor cannot change its charge instantly and the resistor limits how fast charge arrives.
Those two states are what AP Physics 2 asks you to compute.
- The instant the switch closes. An uncharged capacitor acts like a wire: with there is no potential difference across it to push back. Redraw with the capacitor replaced by a plain wire and solve the resistor circuit left behind. The current is largest here.
- After a long time. A fully charged capacitor sits at a maximum potential difference at which there is zero current in its branch. Redraw with that branch deleted as an open circuit, solve, then read the capacitor's potential difference off the circuit and get .
Swapping those two is the error to guard against; both are stated outright in the CED.
The boundary statement fixes how much mathematics the exam wants. Descriptions of charging and discharging RC circuits in AP Physics 2 are limited to qualitative descriptions and representations; students should be able to mathematically describe the initial and final states, but are not expected to model these behaviors with respect to time. No exponential formula is required and none is printed on the sheet. Between the two states the pace is set by the time constant, and every curve approaches its asymptote without reaching it.