---
title: "Fixed Resistor | Principles of Physics II"
description: "Fixed resistor in Principles of Physics II: a component with constant resistance that limits current, sets voltages, and helps you read circuit behavior."
canonical: "https://fiveable.me/principles-physics-ii/key-terms/fixed-resistor"
type: "key-term"
subject: "Principles of Physics II"
unit: "Unit 4"
---

# Fixed Resistor | Principles of Physics II

## Definition

A fixed resistor is a circuit component with one constant resistance value, measured in ohms (Ω). In Principles of Physics II, it is used to control current, create voltage drops, and model simple circuit behavior.

## What It Is

A fixed resistor is a circuit element in Principles of Physics II that provides a set resistance value and keeps that value essentially constant during normal operation. If a circuit calls for 220 Ω, 1 kΩ, or 10 kΩ, the resistor is built to stay near that number rather than being adjusted by a knob or slider.

What makes it useful in physics is that it creates a predictable relationship between voltage, current, and power. Through Ohm's law, V = IR, a fixed resistor gives you a clean way to predict how much current will flow when a voltage is applied. That predictability is why resistors show up in almost every basic circuit diagram you analyze.

A fixed resistor does not stop current completely. Instead, it limits current by converting some electrical energy into thermal energy. That is why a resistor can warm up when current passes through it, and why each resistor has a power rating in watts. If you exceed that rating, the resistor can overheat and its behavior can drift or fail.

In this course, fixed resistors often appear in series circuits, where they add to the total resistance, and in parallel circuits, where they provide alternative paths for current. They are also used in voltage divider setups, where two or more resistors split a source voltage into smaller, predictable voltages. That is a common lab and homework pattern because it connects the algebra to a real circuit.

The material and construction matter too. Carbon film, metal film, and wire-wound resistors can behave slightly differently in precision, noise, and power handling. For most intro physics problems, though, you treat a fixed resistor as an ideal linear resistor unless the question gives you evidence otherwise. The main job is simple: a known resistance lets you trace what the circuit will do before you ever build or measure it.

## Why It Matters

Fixed resistors are one of the main tools you use to make circuit analysis manageable in Principles of Physics II. Once you know the resistance value, you can find current with Ohm's law, compute voltage drops, and compare what happens in series versus parallel arrangements without guessing.

They also connect the math to real devices. A resistor in a circuit board might protect an LED, set the bias in a sensor circuit, or act as part of a voltage divider that feeds a smaller voltage to another component. In class problems, that same idea shows up when you are asked to choose a resistor value, calculate power dissipation, or decide whether a circuit branch carries more or less current than another branch.

Fixed resistors also make it easier to spot the difference between an ideal model and a real component. If the resistance is treated as constant, your calculations stay linear. If the problem hints at heating, tolerance, or frequency dependence, you know the real-world behavior is starting to matter.

Because so many circuit questions reduce to resistance, a fixed resistor is often the first place to start when you are mapping what a circuit will do.

## Connections

### Ohm's Law

Ohm's law is the equation that links a fixed resistor to measurable circuit behavior: V = IR. Once you know the resistor value, you can solve for current or voltage drops directly. Most resistor questions in this course are really Ohm's law questions in disguise, especially when the problem gives you a battery and asks what happens in one branch or across one component.

### Series Circuit

In a series circuit, fixed resistors add together to make the total resistance larger. That lowers the current everywhere in the loop because the same current passes through each resistor. This is where you see resistors used as current limiters, since adding resistance in series is a straightforward way to control flow.

### Parallel Circuit

In a parallel circuit, each fixed resistor gives current another path to take. The equivalent resistance drops below the smallest individual branch resistance, so the source current usually increases compared with a single path. This setup is common when you need the same voltage across multiple branches but different branch currents.

### [current divider](/principles-physics-ii/key-terms/current-divider)

A current divider tells you how current splits between branches that contain resistors. The branch with smaller resistance takes more current, while the branch with larger resistance takes less. Fixed resistors make this calculation possible because their values stay known and stable, which is exactly what the current-divider formulas depend on.

## On the AP Exam

A quiz or problem set will usually ask you to identify the resistor value from a color code, use it in V = IR, or compare currents and voltage drops in a circuit diagram. You may also need to decide whether the resistor is in series or parallel and then find the equivalent resistance before solving. In lab questions, you might measure a resistor with a multimeter and compare the measured value to the labeled value to see how close it is to the tolerance. If a question includes power, use P = I^2R or P = V^2/R to check whether the resistor can safely handle the energy it dissipates.

## fixed resistor vs Linear Resistor

A fixed resistor is a physical component with a set resistance value, while a linear resistor is the ideal behavior you use in circuit models. Most fixed resistors are treated as linear in intro physics, meaning their V-I relationship is proportional. The difference matters when a problem asks about the actual part versus the simplifying model you use to analyze it.

## Key Takeaways

- A fixed resistor is a circuit component with a constant resistance value, measured in ohms.
- In Principles of Physics II, you use fixed resistors to control current, create voltage drops, and build predictable circuit models.
- Ohm's law, V = IR, is the main equation that connects a fixed resistor to current and voltage.
- Fixed resistors matter in both series and parallel circuits because they change the total resistance and how current splits.
- A resistor's power rating tells you how much energy it can turn into heat before it becomes unsafe or unreliable.

## FAQs

### What is a fixed resistor in Principles of Physics II?

A fixed resistor is a component that has one set resistance value and is used to limit current or drop voltage in a circuit. In Physics II, it is usually treated as a linear element, so you can use Ohm's law to calculate what it does in a circuit.

### How is a fixed resistor different from a variable resistor?

A fixed resistor stays at the same resistance value during normal use, while a variable resistor can be adjusted. That makes the fixed version better for predictable circuit analysis, since the resistance does not change when you turn a dial or slide a contact.

### Why do fixed resistors get hot?

They get hot because electrical energy is converted into thermal energy as current passes through them. The amount of heating depends on the current, voltage, and resistance, which is why the power rating matters in circuit problems and labs.

### How do fixed resistors show up in circuit problems?

You will often see them in series or parallel circuit diagrams, voltage divider setups, and current-limiting situations. A common task is to find the equivalent resistance, then use Ohm's law or the current divider rule to solve for current or voltage.

## Related Study Guides

- [4.3 Resistance](/principles-physics-ii/unit-4/resistance/study-guide/HrcfXHJ7THkryv1W)

## About This Document

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