---
title: "Series Resistors — AP Physics 2 Definition & Exam Guide"
description: "Series resistors share the same current, and their resistances add to one equivalent resistance. Master this for AP Physics 2 Unit 11 circuit analysis."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/series-resistors"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 11"
---

# Series Resistors — AP Physics 2 Definition & Exam Guide

## Definition

Series resistors are resistors connected end-to-end along a single path, so the same current flows through each one; their individual resistances add directly (Req = R1 + R2 + ...) and the battery's potential difference splits across them in proportion to each resistance.

## What It Is

Series resistors sit one after another along the same wire, forming a single path for [charge](/ap-physics-2-revised/unit-10/1-electric-charge-and-electric-force/study-guide/E6OYkOGeroCXwgw1 "fv-autolink"). Because there's only one path, every [resistor](/ap-physics-2-revised/key-terms/resistor "fv-autolink") in the chain carries the exact same current. That's the defining feature. What gets divided up is the potential difference, not the current. Each resistor takes a share of the voltage proportional to its resistance, and those voltage drops add up to the total potential difference across the whole chain.

The math follows directly from that picture. Since the current I is the same everywhere and each drop is IR, the total drop is I(R1 + R2 + ...). So the chain behaves like one equivalent resistor with Req = R1 + R2 + R3 and so on. Adding a resistor in series always makes the total resistance bigger, which means less current from the battery (for a fixed battery voltage). One more consequence worth burning into your brain is that a series chain is fragile. Break any single element and the whole loop becomes an [open circuit](/ap-physics-2-revised/key-terms/open-circuit "fv-autolink"), and current everywhere drops to zero.

## Why It Matters

Series resistors live in [Topic 11.2](/ap-physics-2-revised/unit-11/2-simple-circuits/study-guide/LROjr9EJ6hjfPDMC "fv-autolink") (Simple Circuits) in [Unit 11](/ap-physics-2-revised/unit-11 "fv-autolink"): Electric Circuits, supporting learning objective 11.2.A, which asks you to describe the behavior of a circuit. The CED treats a circuit as a set of closed electrical loops containing elements like batteries, resistors, switches, and meters, and series combinations are the simplest loop structure you'll analyze. Almost every circuit problem in Unit 11 starts with collapsing series and parallel combinations into one equivalent resistance, so if the series rule (same current, voltages add, resistances add) isn't automatic, the harder mixed circuits fall apart fast. It's also the foundation for reasoning about ammeters, which are wired in series precisely because series elements share the same current.

## Connections

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

Series and parallel are mirror images. In series, current is shared and [voltage](/ap-physics-2-revised/key-terms/voltage "fv-autolink") divides. In parallel, voltage is shared and current divides. Most AP circuit problems mix both, so you simplify the circuit in stages, collapsing each series chain and each parallel branch into equivalent resistors.

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

A series chain has exactly one path, so a single break anywhere creates an open circuit and kills the [current](/ap-physics-2-revised/unit-11/1-electric-current/study-guide/QaFR8etPqRmh5pdg "fv-autolink") through every element. This is why one burned-out bulb in an old string of series holiday lights darkened the whole strand.

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

If a wire with no resistance is connected across one resistor in a series chain, charge takes the free path and bypasses that resistor. The total resistance drops, the battery current jumps, and the shorted resistor gets zero [potential difference](/ap-physics-2-revised/key-terms/electric-potential-difference "fv-autolink"). Spotting a shorted-out series resistor is a classic circuit-reasoning move.

## On the AP Exam

Series resistors show up in multiple-choice questions that test whether you can predict how a change in one element ripples through the whole circuit. A typical stem gives you a battery with two resistors in series, sometimes with another resistor in parallel with that series pair, and asks what happens to the battery current if one series resistance is doubled. The move is always the same. Recompute the equivalent resistance, then apply I = V/Req. You'll also see claim-evaluation questions where a student makes a statement about current or voltage in a series chain and you have to judge the claim and justify your verdict with the series rules (same current everywhere, voltage drops add to the battery voltage). On free-response, series reasoning is baked into nearly every circuit-analysis question even when the words "series resistors" never appear, so practice stating explicitly that elements in series carry equal current.

## series resistors vs parallel resistors

Series resistors share one current and split the voltage; parallel resistors share one voltage and split the current. The equivalent resistance rules are opposite too. Series resistances add directly and the total always goes up, while parallel combinations always produce an equivalent resistance smaller than the smallest branch. A fast visual check helps. If charge has no choice of path, the elements are in series. If charge can pick between branches, they're in parallel.

## Key Takeaways

- Resistors in series carry the same current because there is only one path for charge to follow.
- The equivalent resistance of series resistors is the simple sum, Req = R1 + R2 + R3, so adding a series resistor always increases total resistance and decreases battery current.
- The battery's potential difference divides among series resistors in proportion to each resistance, and the individual drops add up to the total.
- Breaking any single element in a series chain creates an open circuit, stopping current everywhere in that loop.
- Doubling one series resistance raises the equivalent resistance, which lowers the current through the battery for a fixed battery voltage.
- Ammeters are connected in series with the element being measured because series elements share the same current.

## FAQs

### What are series resistors in AP Physics 2?

Series resistors are resistors connected end-to-end on a single path, so the same current flows through each one. Their resistances add directly, Req = R1 + R2 + ..., and the voltage divides among them in proportion to resistance. This falls under Topic 11.2 (Simple Circuits) in Unit 11.

### Is the voltage the same across resistors in series?

No. Current is the same through series resistors, but voltage is not (unless the resistances happen to be equal). Each resistor drops a voltage of IR, so the bigger resistor takes the bigger share, and all the drops sum to the battery's potential difference.

### How are series resistors different from parallel resistors?

Series resistors share one current and split the voltage; parallel resistors share one voltage and split the current. Series resistances add directly, so Req grows, while a parallel combination always has a smaller Req than its smallest branch.

### What happens to the current if you double one resistor in a series circuit?

The equivalent resistance increases, so the current through the battery decreases (I = V/Req with V fixed). The exam loves this setup, sometimes with a parallel branch added, where doubling a series resistance shifts how current splits between paths.

### What happens if one resistor in a series circuit breaks?

The entire loop becomes an open circuit and current drops to zero everywhere in that path. With only one path for charge, there's no way around the break, which is exactly why series wiring is fragile.

## Related Study Guides

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

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