A discharging capacitor releases stored electric energy back into a circuit. As the discharge proceeds, the capacitor’s charge, potential difference, stored energy, and branch current all decrease toward zero.

What discharge means

A charged capacitor stores separated positive and negative charge on its plates. That separation creates an electric field and stores electric potential energy. If a conducting path is provided between the plates, charge can move through the external circuit, so the capacitor begins to discharge.

During discharge, the capacitor is no longer building up charge separation. Instead, the existing separation is reduced. This means the amount of charge on the plates gets smaller with time. As the charge separation weakens, the electric field between the plates also weakens, so the capacitor becomes less able to push charge through the circuit branch.

A capacitor does not release all of its stored energy in one instant in a typical circuit. The discharge is a process: the electrical quantities change continuously until the circuit reaches a final condition in which no further change occurs.

Quantities that decrease

The AP focus for this subsubtopic is the linked decrease of four quantities:

• Charge on the capacitor

• Stored electric potential energy

• Potential difference across the capacitor

• Branch current

As discharge continues, each of these becomes smaller. The branch current exists because the capacitor’s potential difference drives charge through the branch. When the potential difference becomes smaller, the current also becomes smaller. Eventually, the discharge ends.

At steady state, an ideal discharging capacitor has no remaining charge separation to drive current through the branch. The current therefore falls to zero, and the capacitor no longer transfers energy to the rest of the circuit.

Why the decreases are linked

These decreases are not independent. For a given capacitor, the capacitance stays constant, so a smaller amount of stored charge means a smaller potential difference across the capacitor. That smaller potential difference means a weaker push on charges in the branch, so the current decreases as well.

The stored energy also decreases because the capacitor’s energy depends on how much charge separation and potential difference remain. As the capacitor loses charge, the electric field between its plates weakens, and the stored energy drops.

These relationships help explain the discharge qualitatively. If $C$ is fixed, then decreasing $Q$ requires decreasing $V$. Because the stored energy depends on $V^2$, the energy falls as the capacitor discharges and reaches zero in the ideal fully discharged state.

Physical picture during discharge

At the instant discharge begins, the capacitor has its greatest charge, greatest potential difference, and greatest stored energy for that situation. Because the driving potential difference is initially largest, the branch current is also initially largest.

Textbook RC-discharge graphs showing $Q(t)$, $I(t)$, and $V(t)$ all decreasing in magnitude as the capacitor discharges through a resistor. The curves illustrate that the branch current starts at its maximum magnitude and then decays as the capacitor’s potential difference falls, approaching the steady-state condition of zero current. Source

As charge flows through the branch, the capacitor loses some of the charge separation that created its electric field. With a weaker electric field, the capacitor produces a smaller potential difference. Since the driving potential difference is smaller, the branch current decreases. This is why the current is not constant during discharge.

The discharge therefore slows down as it proceeds. Early in the process, the changes are larger because the capacitor still has a substantial potential difference. Later in the process, the remaining charge separation is small, so the current becomes very small as well.

Branch current and energy transfer

The branch current during discharge is the current in the part of the circuit through which the capacitor is releasing energy. That current is evidence that energy is being transferred from the capacitor to other circuit elements or to the surroundings.

As the current decreases, the rate of energy transfer also decreases. In many practical cases, the energy released by the capacitor is converted into thermal energy in the circuit. The key AP idea is not the detailed mechanism, but the overall trend: the capacitor’s stored energy steadily decreases until the discharge is complete.

A useful way to think about the process is that the capacitor gradually runs out of the electrical “push” that originally drove the current. When that push is gone, the current stops.

Qualitative graphs and endpoint

If you sketch the discharge qualitatively against time, the graphs of charge, potential difference, stored energy, and branch current all slope downward toward zero.

Charge/discharge curves for an RC circuit: during discharge, the capacitor’s voltage drops exponentially while the current magnitude also decays, approaching zero as time increases. The plot emphasizes the “fast at first, then slower” shape that matches how the electric “push” weakens as the capacitor discharges. Source

They are not flat lines during the discharge because the values are changing continuously.

Useful graph features to remember:

• Charge vs. time: decreases toward zero

• Potential difference vs. time: decreases toward zero

• Branch current vs. time: decreases toward zero

• Stored energy vs. time: decreases toward zero

All four graphs approach the final steady state. In an ideal circuit, that final state corresponds to a fully discharged capacitor.

Common misconceptions

• A discharging capacitor does not keep the current constant. The current decreases as the capacitor’s potential difference decreases.

• Zero current at the end of discharge does not mean charges in the wire have stopped existing. It means there is no longer a net current driven by the capacitor.

• The capacitor’s stored energy does not remain in the capacitor once discharge is complete. In the ideal fully discharged state, the stored energy is zero.

• A larger current at the start of discharge does not mean the capacitor is gaining charge. It means the capacitor is losing charge more rapidly at the beginning.