---
title: "Open Circuit — AP Physics 2 Definition & Exam Guide"
description: "An open circuit is a broken path where charges can't flow. Learn the AP Physics 2 definition, why a charged capacitor acts like one, and how it's tested."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/open-circuit"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 11"
---

# Open Circuit — AP Physics 2 Definition & Exam Guide

## Definition

In AP Physics 2, an open circuit is a circuit in which charges are not able to flow because the conducting path is broken or incomplete, such as when a switch is open or a wire is disconnected, so the current everywhere in that loop is zero.

## What It Is

An open circuit is a [circuit](/ap-physics-2-revised/unit-11/2-simple-circuits/study-guide/LROjr9EJ6hjfPDMC "fv-autolink") where charges cannot flow because the loop isn't complete. Think of it like a drawbridge that's up. The battery still wants to push charge around, and there can still be a [potential difference](/ap-physics-2-revised/key-terms/electric-potential-difference "fv-autolink") across the gap, but no current moves anywhere in that loop. The CED puts it bluntly in Topic 11.2: a closed circuit is one in which charges would be able to flow, and an open circuit is one in which they would not.

The most common cause is an open switch or a disconnected wire, but here's the version [AP Physics 2](/ap-physics-2-revised "fv-autolink") actually loves to test. A fully charged capacitor behaves like an open circuit. Once the capacitor reaches the battery's potential difference, no more charge flows onto its plates, and the current in that branch drops to zero, even though every wire is physically connected. So "open circuit" isn't just about visible gaps. It describes any condition where charge flow is blocked.

## Why It Matters

Open circuits live in **[Unit 11](/ap-physics-2-revised/unit-11 "fv-autolink"): Electric Circuits**, specifically **Topic 11.2 Simple Circuits**, under learning objective **11.2.A: Describe the behavior of a circuit**. The CED defines a whole vocabulary here (closed loop, closed circuit, open circuit, [short circuit](/ap-physics-2-revised/key-terms/short-circuit "fv-autolink")), and you need all four to read a schematic correctly. Knowing whether a circuit is open or closed is step zero of every circuit problem. If the circuit is open, current is zero, and every Ohm's law or power calculation built on a nonzero current is wrong. This idea also unlocks RC circuit reasoning, where a capacitor transitions from acting like a plain wire (uncharged) to acting like an open circuit (fully charged), which is exactly the before-and-after analysis the exam asks for.

## Connections

### [Short circuit (Unit 11)](/ap-physics-2-revised/key-terms/short-circuit)

These are opposite extremes. An open circuit has zero [current](/ap-physics-2-revised/unit-11/1-electric-current/study-guide/QaFR8etPqRmh5pdg "fv-autolink") because the path is broken. A short circuit lets charge flow with no change in potential difference, which usually means a huge current through a path with essentially no resistance. One is a roadblock, the other is a free highway.

### Capacitors in circuits (Unit 11)

A [fully charged](/ap-physics-2-revised/unit-11/8-resistor-capacitor-rc-circuits/study-guide/Bzx859T2I32htsAl "fv-autolink") capacitor acts like an open circuit. The plates are full, no more charge flows into that branch, and the current there is zero. This is the steady-state shortcut for RC circuit problems, and an uncharged capacitor is the opposite case (it briefly acts like a bare wire).

### [Series resistors (Unit 11)](/ap-physics-2-revised/key-terms/series-resistors)

In a [series circuit](/ap-physics-2-revised/key-terms/series-circuit "fv-autolink") there's only one path, so a single break anywhere makes the entire circuit open and kills the current in every element. That's why one dead bulb in a series string shuts off all of them, and why parallel wiring avoids the problem.

### Kirchhoff's loop rule (Unit 11)

The loop rule still works on an open circuit. The full battery voltage appears across the gap or the open switch, since with zero current there are no voltage drops across the resistors. This is how a voltmeter can read the battery's emf across an open switch.

## On the AP Exam

Open circuits show up most often as the reasoning step inside RC circuit questions rather than as a standalone definition. A classic multiple-choice setup gives you a battery, resistor, and capacitor in series and asks what happens after the capacitor is fully charged. The answer hinges on recognizing the charged capacitor acts like an open circuit, so the current is zero. The reverse version asks what happens the moment a switch closes, which means recognizing the circuit goes from open (no current) to closed (current flows, capacitor charges). You're expected to translate "open" into "I = 0 everywhere in this loop" and then reason about voltages from there, often with Kirchhoff's rules. No released FRQ has hinged on the phrase itself, but circuit-analysis FRQs routinely require you to identify which branches carry current in steady state, and that's open-circuit reasoning in disguise.

## open circuit vs short circuit

They sound similar but are physical opposites. An open circuit blocks charge flow entirely, so current is zero. A short circuit is a path where charges flow with no change in potential difference, typically a near-zero-resistance route that draws a very large current. Quick check: open means no current, short means too much current taking a shortcut around the resistance.

## Key Takeaways

- An open circuit is one in which charges would not be able to flow, because the conducting path is broken or incomplete.
- In an open circuit the current is zero everywhere in that loop, so any Ohm's law calculation assuming nonzero current is automatically wrong.
- A fully charged capacitor acts like an open circuit, which is the key to steady-state RC circuit questions.
- Open and short circuits are opposites: open means no current at all, short means charge flows with no change in potential difference and current can be huge.
- A potential difference can still exist across the gap in an open circuit, so a voltmeter across an open switch can read the full battery voltage.
- In a series circuit, one break anywhere makes the whole circuit open, since there's only one path for charge.

## FAQs

### What is an open circuit in AP Physics 2?

It's a circuit in which charges are not able to flow because the path is broken or incomplete, like a circuit with an open switch. This definition comes straight from Topic 11.2 (Simple Circuits) in the CED, under learning objective 11.2.A.

### Is the current actually zero in an open circuit?

Yes. With no complete path, no charge flows anywhere in that loop, so the current is exactly zero. The battery's emf still exists, but it appears as a potential difference across the gap instead of driving current.

### How is an open circuit different from a short circuit?

They're opposites. An open circuit has a broken path and zero current, while a short circuit lets charges flow with no change in potential difference, usually producing a very large current through a near-zero-resistance path.

### Does a fully charged capacitor act like an open circuit?

Yes, and this is one of the most tested ideas in Unit 11. Once a capacitor in a DC circuit is fully charged, no more charge flows onto its plates, so its branch carries zero current, exactly like an open circuit.

### Can there be a voltage across an open circuit if no current flows?

Yes. With zero current, resistors have no voltage drops, so by Kirchhoff's loop rule the full battery voltage appears across the open gap. That's why a voltmeter across an open switch reads the battery's emf.

## Related Study Guides

- [11.2 Simple Circuits](/ap-physics-2-revised/unit-11/2-simple-circuits/study-guide/LROjr9EJ6hjfPDMC)

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