---
title: "Velocity Selector — AP Physics 2 Definition & Exam Guide"
description: "A velocity selector uses crossed E and B fields so only particles with v = E/B pass straight through. Learn how AP Physics 2 tests it in Unit 12 and mass spec problems."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/velocity-selector"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 12"
---

# Velocity Selector — AP Physics 2 Definition & Exam Guide

## Definition

A velocity selector is a region with perpendicular (crossed) electric and magnetic fields arranged so the electric force and magnetic force on a charged particle cancel. Only particles moving at exactly v = E/B travel through undeflected, because that's the speed where qE = qvB.

## What It Is

A velocity selector is a setup where a uniform [electric field](/ap-physics-2-revised/unit-10/1-electric-charge-and-electric-force/study-guide/E6OYkOGeroCXwgw1 "fv-autolink") and a uniform magnetic field point perpendicular to each other, and a charged particle shoots through perpendicular to both. The electric field pushes the charge one way with force F_E = qE. The magnetic field pushes it the other way with force [F_B = qvB sin θ](/ap-physics-2-revised/key-terms/f-b-qvb-sin "fv-autolink") (and since the velocity is perpendicular to B, sin θ = 1, so F_B = qvB). If those two forces are equal and opposite, the net force is zero and the particle sails through in a straight line.

Here's the trick that gives the device its name. The [electric force](/ap-physics-2-revised/unit-10/3-electric-fields/study-guide/I5lSNgudkyVNrR1L "fv-autolink") doesn't care how fast the particle moves, but the magnetic force does. So there's exactly one speed where the forces balance. Set qE = qvB, cancel the charge, and you get **v = E/B**. Anything faster gets bent one way (magnetic force wins), anything slower gets bent the other way (electric force wins). Notice that q dropped out of the equation. The selector filters by speed, not by charge or mass, which is exactly why mass spectrometers use one as the front door.

## Why It Matters

The velocity selector lives in Topic 12.2 (Magnetism and Moving Charges) in [Unit 12](/ap-physics-2-revised/unit-12 "fv-autolink") of [AP Physics 2](/ap-physics-2-revised "fv-autolink"). It directly supports learning objective 12.2.B, describing the force a magnetic field exerts on a moving charge, and it forces you to use the right-hand rule and F_B = qvB sin θ in the same breath as Coulomb-style electric forces from earlier units. It's also the single best example of a 'balanced forces' argument in electromagnetism. Instead of memorizing a gadget, you're applying Newton's first law to a charged particle, which is the kind of reasoning AP Physics 2 rewards everywhere.

## Connections

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

The selector condition qE = qvB is just this equation set equal to the electric force. The whole device is one application of the [magnetic force](/ap-physics-2-revised/key-terms/magnetic-force "fv-autolink") law with θ = 90°.

### Charge-to-mass ratio and the mass spectrometer (Unit 12)

A mass spectrometer puts a velocity selector first so every ion enters the next region at the same known speed v = E/B. Then a [magnetic field](/ap-physics-2-revised/unit-12/1-magnetic-fields/study-guide/8CQ1URzqZQqRb7qQ "fv-autolink") alone bends them into circles, and the radius reveals each ion's mass. Without the selector, you couldn't tell whether a wide circle meant a heavy ion or a fast one.

### Circular motion and radius of curvature (Units 1 & 12)

Inside the selector the particle moves in a straight line because forces balance. The moment it exits into a B-field-only region, the unbalanced magnetic force becomes a [centripetal force](/ap-physics-2-revised/key-terms/centripetal-force "fv-autolink") and the path curves into a circle with r = mv/(qB). Exam problems love chaining these two regions together.

### [Hall effect (Unit 12)](/ap-physics-2-revised/key-terms/hall-effect)

The Hall effect is the same physics happening inside a conductor. Moving charges get pushed sideways by a magnetic field until the charge buildup creates an electric field that balances the magnetic force. It's a velocity selector that builds its own E field.

## On the AP Exam

Velocity selector questions show up mostly as multiple choice and as the setup phase of longer quantitative problems. The classic stem hands you E and B values and asks for the speed of an undeflected particle (v = E/B), or describes a charged particle in crossed fields and asks under what condition it travels in a straight line. Be ready to (1) state the balance condition qE = qvB and explain why charge cancels, (2) use the right-hand rule to figure out which way each force points and check that they actually oppose each other (watch the sign of the charge), and (3) predict what happens to a particle that's too fast or too slow. The most common multi-step version is the mass spectrometer problem, where ions pass the selector at v = E/B and then curve in a magnetic-field-only region, so you combine the selector equation with r = mv/(qB) to find mass or radius. No released FRQ has used the term verbatim, but the underlying skill, balancing electric and magnetic forces on a moving charge, is core LO 12.2.B territory.

## Velocity selector vs Hall effect

Both involve an electric force balancing a magnetic force on moving charges, so the math looks identical. The difference is who sets up the fields. In a velocity selector, you impose external E and B fields, and only particles with v = E/B pass through. In the Hall effect, charges drifting in a conductor get pushed sideways by an external B field, pile up on one edge, and create their own internal E field that grows until it balances the magnetic force. The selector filters particles; the Hall effect measures fields or reveals the sign of charge carriers.

## Key Takeaways

- A velocity selector uses crossed (perpendicular) electric and magnetic fields so that the electric force qE and the magnetic force qvB cancel for one specific speed.
- The selected speed is v = E/B, and charge cancels out of the equation, so the selector picks particles by speed regardless of their charge or mass.
- Particles moving faster than E/B deflect toward the side where the magnetic force wins, and slower particles deflect the other way, because F_B depends on v but F_E does not.
- Use the right-hand rule to confirm the magnetic force actually opposes the electric force, and remember to flip the magnetic force direction for negative charges.
- In mass spectrometer problems, the velocity selector guarantees all ions enter the deflection region at the same speed, so the circular radius r = mv/(qB) sorts them by mass.

## FAQs

### What is a velocity selector in AP Physics 2?

It's a region with perpendicular electric and magnetic fields tuned so the electric force (qE) and magnetic force (qvB) on a charged particle cancel. Only particles moving at exactly v = E/B pass through undeflected; everything else gets bent off course.

### Does a velocity selector only work for one type of charge?

No. The condition qE = qvB has charge on both sides, so q cancels and v = E/B works for any charge or mass. Positive and negative particles both pass if they're at the right speed (the forces just swap directions for negative charges, but they still balance).

### How is a velocity selector different from a mass spectrometer?

The velocity selector is the first stage of a mass spectrometer, not the whole thing. The selector lets only ions with v = E/B through; then a magnetic-field-only region bends those ions into circles with r = mv/(qB), and the different radii separate the masses.

### What happens if a particle enters a velocity selector too fast or too slow?

If v > E/B, the magnetic force qvB exceeds the electric force qE and the particle deflects toward the magnetic-force side. If v < E/B, the electric force wins and it deflects the opposite way. Either way, it doesn't make it through the exit slit.

### How do I find the speed selected by a velocity selector?

Set the forces equal, qE = qvB, and solve for v = E/B. For example, with E = 2.0 × 10^4 N/C and B = 0.50 T, the selected speed is 4.0 × 10^4 m/s.

## 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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