An open circuit is a circuit with a break in the conducting path (an open switch, a burned-out element, or a fully charged capacitor), so no complete loop exists and the current is zero everywhere in that branch, regardless of the emf in the circuit.
An open circuit is any circuit where the conducting path is incomplete. Charge needs a continuous loop from one battery terminal back to the other. Break that loop anywhere, with an open switch, a snipped wire, or a burned-out resistor, and current stops in that entire branch. Not "less current." Zero current.
The physics behind it is simple but easy to forget under exam pressure. Ohm's law (I = V/R) still works, but an open circuit acts like a gap with effectively infinite resistance. Plug R → ∞ into I = ε/R and you get I → 0. The battery still has its emf, and there can still be a potential difference across the gap (in fact, the full emf often appears across an open switch), but no charge flows through it. That distinction, voltage across versus current through, is exactly what AP questions probe.
Open circuits live in Topic 11.2: Electric Circuits in AP Physics C: E&M, and the idea echoes through every circuit topic after it. You need it to reason about switches in circuit schematics, to predict what happens when a component fails in series versus parallel, and to understand the end state of RC charging, where a fully charged capacitor behaves exactly like an open circuit. It's also the conceptual opposite of a short circuit, and the exam loves making you tell those two failure modes apart. If you can instantly say "open means I = 0, but voltage can still exist across the break," you've got a tool that works in Kirchhoff's loop problems, equivalent resistance problems, and RC transient analysis alike.
Keep studying AP® Physics C: E&M Unit 11
Short circuit (Unit 11)
These are opposite failure modes. An open circuit is a break in the path, so resistance is effectively infinite and current is zero. A short circuit is an unintended low-resistance path, so resistance drops toward zero and current spikes. Same equation, I = V/R, pushed to opposite extremes.
Switch (Unit 11)
A switch is just a deliberate, reversible open circuit. Open switch means no current in that branch; closed switch means the branch behaves normally. Many circuit problems hinge on re-analyzing the circuit the instant a switch opens or closes.
Capacitors and RC circuits (Unit 11)
A fully charged capacitor in a DC circuit acts like an open circuit. Once the capacitor's voltage matches the source voltage across it, current through that branch drops to zero. So "steady state with a capacitor" and "open circuit" are the same analysis move.
Parallel combination (Unit 11)
Where the break happens determines the damage. One element going open in a series loop kills current everywhere. One element going open in a parallel combination only kills its own branch, and the other branches keep running at the same voltage with their same currents.
Open circuits show up in conceptual MCQs and as a setup step in FRQs, almost always testing whether you apply circuit rules to a complete loop only. Classic stems include: a student calculates I = V/R with the switch open (the error is treating an open circuit as a complete one, since the real current is zero), a resistor in a parallel set burns out (total current decreases because total resistance increases, but the surviving branches are unchanged), a series bulb burns out (current drops to zero everywhere, the working bulbs have zero potential difference, and the full emf appears across the break), and a fully charged capacitor in an RC circuit (current is zero, so the circuit behaves as if that branch is open). The move the exam rewards is always the same one. First identify whether a complete conducting loop exists, then apply Ohm's law and Kirchhoff's rules only to loops that actually carry current.
Both are circuit "failures," but they're physical opposites. An open circuit is a gap in the path, so R → ∞ and I = 0; the battery is safe but nothing works. A short circuit is an accidental near-zero-resistance path, so I spikes toward dangerous values; things work too well and then overheat. Quick check on the exam: if the question says broken, burned out, or switch open, current goes to zero. If it says wire connected directly across or resistance bypassed, current shoots up.
An open circuit has a break in the conducting path, so the current is zero everywhere in that loop, not just at the break.
An open circuit behaves like infinite resistance, while a short circuit behaves like zero resistance; they are opposite failure modes.
Voltage can still exist across an open gap even though no current flows through it; an open switch in a simple loop has the full emf across it.
If one element in a series circuit goes open, every element in that loop stops carrying current, and the working elements have zero potential difference across them.
If one branch in a parallel combination goes open, only that branch dies; the other branches keep the same voltage and current, and the total current from the battery decreases.
A fully charged capacitor in a DC circuit acts as an open circuit, which is the key to analyzing steady-state RC circuits.
An open circuit is a circuit with an incomplete conducting path, caused by an open switch, a cut wire, or a burned-out component. Because no complete loop exists, the current is zero in that branch no matter how large the battery's emf is.
Yes. Current is zero, but potential difference can still exist across the gap. In a simple loop with one battery, the entire emf appears across the open switch or break, which is a favorite conceptual MCQ.
An open circuit is a break in the path (R → ∞, I = 0), while a short circuit is an unintended low-resistance path (R → 0, current spikes). They're opposite extremes of I = V/R.
Yes. Once a capacitor is fully charged in a DC circuit, the current through its branch is zero, so you analyze the steady-state circuit as if that branch were open. This is the standard trick for RC steady-state problems.
In series, a burned-out bulb opens the only loop, so all current stops and the other bulbs go dark with zero voltage across them. In parallel, only that bulb's branch dies; the remaining branches keep their voltage and current, and the battery's total current decreases.
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