---
title: "Hall Effect — AP Physics 2 Definition & Exam Guide"
description: "The Hall effect is the voltage that appears across a current-carrying conductor in a magnetic field. Learn how it proves charge carriers feel F = qvB on the AP exam."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/hall-effect"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 12"
---

# Hall Effect — AP Physics 2 Definition & Exam Guide

## Definition

The Hall effect is the creation of a potential difference across a current-carrying conductor when a magnetic field has a component perpendicular to the charge motion; the field pushes moving charges to one side until the resulting electric force balances the magnetic force (qE = qvB).

## What It Is

Run a [current](/ap-physics-2-revised/unit-11/1-electric-current/study-guide/QaFR8etPqRmh5pdg "fv-autolink") through a flat metal strip, then place that strip in a magnetic field perpendicular to the current. The moving charge carriers feel a [magnetic force](/ap-physics-2-revised/key-terms/magnetic-force "fv-autolink") F = qvB sin θ that shoves them sideways, toward one edge of the strip. Charges pile up on that edge, leaving the opposite edge with the opposite sign of charge. That separation creates an electric field inside the strip, and a measurable voltage (the Hall voltage) across its width.

The pileup doesn't continue forever. As more charge accumulates, the internal electric field grows until the [electric force](/ap-physics-2-revised/unit-10/3-electric-fields/study-guide/I5lSNgudkyVNrR1L "fv-autolink") on each carrier exactly cancels the magnetic force. At equilibrium, qE = qvB, which is the same balance idea behind a velocity selector. The Hall effect is essentially a velocity selector that builds its own electric field. Here's the part that made it historically famous: the *side* the voltage appears on depends on the *sign* of the charge carriers. Positive carriers (holes) and negative carriers (electrons) moving as the same conventional current get pushed to the same edge, so the polarity of the Hall voltage flips. This is how physicists proved that current in metals is carried by negative electrons.

## Why It Matters

The Hall effect lives in **Topic 12.2 (Magnetism and Moving Charges)** in **[Unit 12](/ap-physics-2-revised/unit-12 "fv-autolink"): Magnetism and Electromagnetism**. It directly supports learning objective **12.2.B**, describing the force a magnetic field exerts on moving charged objects, with the magnitude given by [F_B = qvB sin θ](/ap-physics-2-revised/key-terms/f-b-qvb-sin "fv-autolink") and the direction found with the right-hand rule (12.2.B.2.i and 12.2.B.2.ii). It also leans on **12.2.A**, since you're reasoning about moving charges interacting with fields.

It matters on the exam because it's one of the cleanest scenarios for testing whether you can apply F = qvB sin θ and the right-hand rule to charges inside a wire, not just free particles in space. It also tests proportional reasoning (how does Hall [voltage](/ap-physics-2-revised/key-terms/voltage "fv-autolink") change if thickness doubles or carrier density drops?) and force-balance thinking (setting qE = qvB). Bonus relevance: Hall probes are the real-world tool for measuring magnetic field strength, so lab-flavored questions love this setup.

## Connections

### [F_B = qvB sin θ (Unit 12)](/ap-physics-2-revised/key-terms/f-b-qvb-sin)

This equation is the engine of the whole effect. The sideways push on [charge carriers](/ap-physics-2-revised/unit-10/1-electric-charge-and-electric-force/study-guide/E6OYkOGeroCXwgw1 "fv-autolink") in the strip is just the magnetic force on a moving charge, and the sin θ dependence explains why a tilted Hall probe reads a smaller voltage.

### [Velocity selector (Unit 12)](/ap-physics-2-revised/key-terms/velocity-selector)

Both setups end in the same equilibrium, qE = qvB. The difference is that a [velocity selector](/ap-physics-2-revised/key-terms/velocity-selector "fv-autolink") uses an externally applied electric field, while the Hall effect generates its own electric field from the charges that pile up on the strip's edges.

### [Charged particle trajectory (Unit 12)](/ap-physics-2-revised/key-terms/charged-particle-trajectory)

Before equilibrium is reached, carriers in the strip curve sideways just like a free charge curving in a magnetic field. The Hall effect is what happens when that curving motion runs into the wall of the conductor.

### [Charge-to-mass ratio (Unit 12)](/ap-physics-2-revised/key-terms/charge-to-mass-ratio)

Both are classic experiments that use magnetic forces to reveal hidden properties of charge carriers. Thomson's e/m experiment measured the electron's charge-to-mass ratio; the Hall effect revealed the carriers' sign and density.

## On the AP Exam

Hall effect questions show up as multiple-choice stems built around a strip of metal or semiconductor carrying current in a magnetic field. You're expected to do four things. First, use the right-hand rule to figure out which edge the carriers pile up on, remembering to flip for negative charges. Second, set up the force balance qE = qvB to relate the Hall voltage to drift speed and field strength. Third, do proportional reasoning. For example, doubling the strip's thickness at constant current halves the drift velocity, which halves the Hall voltage, and cutting carrier density by a factor of 3 at constant current triples the drift velocity and triples the Hall voltage. Fourth, interpret the sign of the voltage. If the majority carriers are holes instead of electrons, the Hall voltage polarity reverses even though the conventional current is identical. You may also see a Hall probe used as a measurement device, where rotating the probe so its normal makes an angle with the field reduces the reading by a factor of cos θ. No released FRQ has used the term verbatim, but the underlying skill (forces on moving charges plus right-hand rule) is core FRQ territory in Unit 12.

## Hall effect vs Velocity selector

Both end with an electric force canceling a magnetic force (qE = qvB), so they look identical in equation form. The difference is where the electric field comes from. In a velocity selector, you apply an external E field with charged plates and only particles with v = E/B pass straight through. In the Hall effect, the conductor builds its own E field from charge piling up on its edges, and that self-generated field is what you measure as the Hall voltage. One filters particles; the other diagnoses the conductor.

## Key Takeaways

- The Hall effect happens because the magnetic force F = qvB sin θ pushes moving charge carriers toward one edge of a current-carrying conductor, creating a voltage across its width.
- Equilibrium is reached when the electric force from the accumulated charge balances the magnetic force, so qE = qvB, the same balance condition as a velocity selector.
- The polarity of the Hall voltage reveals the sign of the charge carriers, which is how we know current in metals is carried by negative electrons.
- Hall voltage depends inversely on carrier density and strip thickness, so a thicker strip at the same current gives a smaller voltage, and fewer carriers per volume give a larger one.
- A Hall probe measures magnetic field strength, and its reading drops by a factor of cos θ when the probe's normal is tilted away from the field direction.
- Use the right-hand rule to find which edge charges accumulate on, and flip the result for negative carriers like electrons.

## FAQs

### What is the Hall effect in AP Physics 2?

It's the voltage that appears across a current-carrying conductor when a magnetic field is perpendicular to the charge motion. The magnetic force pushes carriers to one edge until the resulting electric force balances it, with qE = qvB at equilibrium.

### Do electrons and positive charges produce the same Hall voltage?

No, the polarity flips. Electrons moving one way and holes moving the other way make the same conventional current, but the right-hand rule pushes both to the same edge, so the sign of the voltage depends on the carrier sign. This is exactly why the Hall effect proved metals conduct via electrons.

### How is the Hall effect different from a velocity selector?

Both use the balance qE = qvB, but a velocity selector applies an external electric field to filter particles by speed, while the Hall effect's electric field is created internally by charges piling up on the conductor's edges. You measure that internal field as the Hall voltage.

### Why does the Hall voltage change if you change the strip's thickness?

At constant current, a thicker strip has more cross-sectional area, so the drift velocity drops. Since the Hall voltage is proportional to drift velocity, doubling the thickness halves the Hall voltage.

### Is the Hall effect actually on the AP Physics 2 exam?

Yes, it falls under Topic 12.2 (Magnetism and Moving Charges) and learning objective 12.2.B. It shows up in multiple-choice questions about forces on moving charges, force balance, proportional reasoning with carrier density or thickness, and Hall probes measuring magnetic fields.

## Related Study Guides

- [12.2 Magnetism and Moving Charges](/ap-physics-2-revised/unit-12/2-magnetism-and-moving-charges/study-guide/EquvYgnfwi2ptpX5)

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