---
title: "Amplitude (A) — AP Physics 2 Definition & Exam Guide"
description: "Amplitude is a wave's maximum displacement from equilibrium. It sets a wave's energy and intensity, and it adds (or cancels) in interference on AP Physics 2."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/amplitude-a"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit uijO7xntwckmWJch"
---

# Amplitude (A) — AP Physics 2 Definition & Exam Guide

## Definition

Amplitude (A) is the maximum displacement of a wave from its equilibrium position, measured from the rest line to a crest or trough. On AP Physics 2, amplitude determines a wave's energy and intensity (energy scales with A²), and amplitudes combine through superposition in interference problems.

## What It Is

[Amplitude](/ap-physics-2-revised/key-terms/amplitude "fv-autolink") is how far a [wave](/ap-physics-2-revised/unit-14/1-properties-of-wave-pulses-and-waves/study-guide/Ql0FLnrI6dIHcNlL "fv-autolink") swings away from its resting (equilibrium) position at its biggest swing. Picture a flat rope. Shake it, and the highest point of each hump above the flat line is one amplitude. You measure from equilibrium to a crest (or down to a trough), not from crest to trough. That crest-to-trough distance is 2A, and mixing those up is a classic point-loser.

What makes amplitude matter in [AP Physics 2](/ap-physics-2-revised "fv-autolink") is what it controls. Amplitude is the wave's "how strong" knob, while frequency and wavelength are the "how fast it oscillates" knobs. For sound, bigger amplitude means louder. For light, bigger amplitude means brighter (higher intensity). The energy a wave carries scales with the square of the amplitude, so doubling A quadruples the energy delivered. Amplitude is also the quantity that adds in superposition. When two waves overlap, their displacements combine, so constructive interference can stack amplitudes up and destructive interference can cancel them to zero.

## Why It Matters

Amplitude lives at the heart of the wave material in AP Physics 2, especially [Waves, Sound, and Physical Optics](/ap-physics-2-revised/unit-14 "fv-autolink") (Unit 14). When you analyze interference, diffraction patterns, or beats, you're really tracking how amplitudes add point by point through [superposition](/ap-physics-2-revised/key-terms/superposition "fv-autolink"). Bright fringes in a double-slit pattern are spots where amplitudes reinforce, and dark fringes are spots where they cancel. Amplitude also shows up in Modern Physics (Unit 15) as a contrast point. In the classical wave model, more amplitude means more energy, but the photoelectric effect shows that light's photon energy depends on frequency, not amplitude. Cranking up amplitude (intensity) just sends more photons. That distinction between the wave model and the photon model is exactly the kind of conceptual reasoning the exam rewards.

## Connections

### Crest and Trough (Unit 14)

The crest is the highest point of a wave and the trough is the lowest. Amplitude is the distance from [equilibrium](/ap-physics-2-revised/unit-14/2-periodic-waves/study-guide/P6HaooQVoZXrucys "fv-autolink") to either one, so crest-to-trough is twice the amplitude. If a graph shows a wave going from +0.4 m to -0.4 m, A is 0.4 m, not 0.8 m.

### Energy Transfer (Unit 14)

Waves move energy without moving matter, and amplitude is the dial that sets how much. Energy scales with A squared, so a wave with triple the amplitude carries nine times the energy. This is why amplitude, not [frequency](/ap-physics-2-revised/key-terms/frequency "fv-autolink"), answers "which wave delivers more energy" questions about mechanical waves.

### [Electromagnetic radiation (Units 14-15)](/ap-physics-2-revised/key-terms/electromagnetic-radiation)

For light treated as a wave, amplitude sets [intensity](/ap-physics-2-revised/key-terms/intensity "fv-autolink") (brightness). But in the photon model, each photon's energy comes from frequency alone. The photoelectric effect makes this concrete. Brighter light of too-low frequency ejects zero electrons, no matter how big the amplitude gets.

## On the AP Exam

No released FRQ hinges on the word "amplitude" by itself, but the concept is baked into how wave questions are scored. Multiple-choice stems love graph reading. You'll see a snapshot of a wave and need to pull amplitude off the vertical axis correctly (equilibrium to peak, not peak to peak) and keep it separate from wavelength on the horizontal axis. Superposition questions ask you to add displacements of overlapping waves to find the resultant amplitude, including full cancellation when equal waves meet out of phase. In Modern Physics contexts, expect a question testing whether you know that increasing a light source's amplitude/intensity increases the number of photoelectrons but not their maximum kinetic energy. Free-response answers earn points when you justify claims with the A² energy relationship or with phase-based amplitude addition rather than vague statements like "the wave is stronger."

## Amplitude (A) vs Frequency

Amplitude is how big the oscillation is; frequency is how often it repeats per second. For sound, amplitude controls loudness and frequency controls pitch. They're independent. You can have a loud low note or a quiet high note. The trap shows up in the photoelectric effect, where the energy of each photon depends only on frequency. Raising the amplitude of the light raises its intensity (more photons per second) but never raises any single photon's energy.

## Key Takeaways

- Amplitude is measured from the equilibrium position to a crest or trough, so the crest-to-trough distance on a graph equals 2A.
- The energy a wave carries scales with amplitude squared, so doubling the amplitude quadruples the energy.
- Amplitude controls loudness for sound and brightness (intensity) for light, while frequency controls pitch and color.
- In superposition, displacements add, so two identical waves in phase produce double the amplitude and two waves perfectly out of phase cancel completely.
- In the photoelectric effect, increasing amplitude means more photons hit the surface, but each photon's energy is set by frequency, not amplitude.

## FAQs

### What is amplitude in AP Physics 2?

Amplitude (A) is the maximum displacement of a wave from its equilibrium position, measured from the rest line to a crest or trough. It determines the wave's energy and intensity, with energy proportional to A².

### Is amplitude measured from crest to trough?

No. Amplitude is measured from the equilibrium line to a crest (or trough). The crest-to-trough distance is twice the amplitude, so a wave oscillating between +0.3 m and -0.3 m has A = 0.3 m, not 0.6 m.

### How is amplitude different from wavelength?

Amplitude is the vertical size of the oscillation (how far from equilibrium), while wavelength is the horizontal distance between repeating points like crest to crest. On a wave graph, amplitude lives on the displacement axis and wavelength lives on the position axis.

### Does a bigger amplitude mean more energy?

For mechanical waves and the classical wave picture of light, yes, and the relationship is quadratic. Energy scales with A², so tripling the amplitude gives nine times the energy. The exception is the photon model, where each photon's energy depends on frequency.

### Why doesn't increasing amplitude eject electrons in the photoelectric effect?

Because photon energy depends only on frequency (E = hf), not amplitude. Increasing amplitude just sends more photons per second, so if each photon's frequency is below the threshold, no electrons are ejected no matter how intense the light is.

## Related Study Guides

- [Unit 6 Overview: Geometric and Physical Optics](/ap-physics-2-revised/unit-uijO7xntwckmWJch/review/study-guide/uV8klagEJVQYCXfE0a9Z)

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