---
title: "Annihilation — AP Physics 2 Definition & Exam Guide"
description: "Annihilation is when a particle meets its antiparticle and both convert entirely into photon energy via E=mc². Tested in Unit 15 with PET scan examples."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/annihilation"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 15"
---

# Annihilation — AP Physics 2 Definition & Exam Guide

## Definition

Annihilation is the process in which a particle and its antiparticle (like an electron and a positron) collide and convert their entire mass into energy, released as photons, in accordance with mass-energy equivalence (E=mc²). It appears in AP Physics 2 Unit 15, Topic 15.7.

## What It Is

Annihilation happens when a particle meets its antiparticle, most famously an [electron](/ap-physics-2-revised/unit-15/2-the-bohr-model-of-atomic-structure/study-guide/gzEFr1ron8hpACPx "fv-autolink") meeting a [positron](/ap-physics-2-revised/key-terms/positron "fv-autolink"). The two don't just bounce off each other. They disappear completely, and ALL of their mass converts into energy in the form of photons. This is mass-energy equivalence in its purest form. In fission or fusion, only a tiny fraction of the mass becomes energy. In annihilation, the conversion is 100 percent.

The process still obeys the conservation laws that govern every nuclear reaction in [Topic 15.7](/ap-physics-2-revised/unit-15/7-fission-fusion-and-nuclear-decay/study-guide/nVULWHgdGjkiGhbI "fv-autolink"). Energy is conserved (you can calculate the photon energy with E=mc² using the total mass that vanished), and momentum is conserved too. That's why electron-positron annihilation typically produces two photons traveling in opposite directions instead of just one. A single photon flying off would carry momentum that came from nowhere, which physics doesn't allow.

## Why It Matters

Annihilation lives in **[Unit 15](/ap-physics-2-revised/unit-15 "fv-autolink"): Modern Physics, Topic 15.7 (Fission, Fusion, and Nuclear Decay)** and directly supports learning objective **15.7.A**, which asks you to describe how conservation laws constrain interacting subatomic particles. The CED's essential knowledge says energy in nuclear processes can be released as [kinetic energy](/ap-physics-2-revised/key-terms/kinetic-energy "fv-autolink") of products or as photons, and that mass and energy may be exchanged via E=mc². Annihilation is the cleanest example of both ideas at once.

It also connects to **15.7.B** through [beta-plus decay](/ap-physics-2-revised/unit-15/8-types-of-radioactive-decay/study-guide/yQriAHMZEcNvi6kE "fv-autolink"). A nucleus like fluorine-18 emits a positron during radioactive decay, and that positron annihilates the first electron it runs into. This chain (decay → positron → annihilation → photons) is exactly how PET scans work, and it's one of the AP exam's favorite real-world applications of modern physics.

## Connections

### [E=mc² (Unit 15)](/ap-physics-2-revised/key-terms/e=mc²)

Annihilation is [E=mc²](/ap-physics-2-revised/key-terms/e=mc² "fv-autolink") with nothing held back. Plug the total mass of the particle-antiparticle pair into the equation and you get the total photon energy released, because the entire mass converts.

### Beta-plus decay and half-life, N = N₀e^(-λt) (Unit 15)

Annihilation needs a positron, and positrons come from beta-plus decay. The decaying [isotope](/ap-physics-2-revised/key-terms/isotope "fv-autolink") (like fluorine-18 in a PET scan) follows the exponential decay law, so the rate of annihilation events in a sample drops off with the isotope's half-life.

### [Conservation of momentum (Units 4 and 15)](/ap-physics-2-revised/key-terms/conservation-of-momentum)

The same momentum conservation you used for collisions explains why annihilation produces two photons moving in opposite directions. If the pair starts roughly at rest, total momentum is zero, so the photons must cancel each other out.

### Photons and quantization (Unit 15)

The energy from annihilation comes out as gamma-ray photons, each carrying energy E = hf. Annihilation is where the particle side of modern physics (mass) hands its energy over to the wave-particle side (photons).

## On the AP Exam

Annihilation shows up almost entirely in multiple-choice questions tied to Topic 15.7, and the PET scan is the go-to scenario. A typical stem describes a positron from beta-plus decay (often fluorine-18) meeting an electron in body tissue, then asks what happens or what form the released energy takes. The answer they want is that the pair annihilates and the energy is released as photons (gamma rays), which detectors pick up.

You should be ready to do three things: (1) identify annihilation as total mass-to-energy conversion governed by E=mc², (2) state that the products are photons, not kinetic energy of leftover particles, since nothing is left over, and (3) use conservation of momentum to explain the two oppositely-directed photons. No released FRQ has used the term verbatim, but it's fair game anywhere the exam tests mass-energy equivalence or conservation laws in nuclear processes.

## annihilation vs Fission and fusion

Fission and fusion release energy because a small fraction of the mass goes missing (the mass defect) and converts to energy, but the products are still particles with nucleon number conserved. Annihilation converts ALL of the mass into photon energy and leaves no particles behind. Another way to see it: fission splits a nucleus, fusion combines nuclei, but annihilation requires a matter-antimatter pair and produces only light.

## Key Takeaways

- Annihilation occurs when a particle and its antiparticle, like an electron and a positron, collide and convert their entire mass into photon energy.
- The energy released is calculated with E=mc², using the total mass of both particles, because the mass-to-energy conversion is 100 percent.
- Conservation of momentum explains why electron-positron annihilation produces two photons moving in opposite directions rather than a single photon.
- PET scans work because beta-plus decay (such as fluorine-18) emits a positron that annihilates with an electron in body tissue, producing detectable gamma-ray photons.
- Annihilation differs from fission and fusion, which convert only a small mass defect into energy and still leave particles behind.

## FAQs

### What is annihilation in AP Physics 2?

Annihilation is the process where a particle and its antiparticle (like an electron and positron) collide and convert all of their mass into energy as photons, following E=mc². It's covered in Unit 15, Topic 15.7.

### Does annihilation destroy energy?

No. Annihilation destroys the particles, but energy is fully conserved. The mass of the pair converts into an exactly equal amount of photon energy, which is the law of conservation of energy plus mass-energy equivalence working together.

### How is annihilation different from nuclear fission or fusion?

Fission and fusion convert only a tiny fraction of the mass into energy and produce new particles, with nucleon number conserved. Annihilation converts 100 percent of the mass into photons and requires a matter-antimatter pair, leaving no particles at all.

### Why does annihilation produce two photons instead of one?

Conservation of momentum. If the electron-positron pair is roughly at rest, total momentum is zero, so a single photon flying off would violate momentum conservation. Two photons traveling in opposite directions keep the total momentum at zero.

### How does annihilation relate to PET scans?

A radioactive isotope like fluorine-18 undergoes beta-plus decay and emits a positron. That positron annihilates with an electron in body tissue, producing two gamma-ray photons that the scanner detects, which is exactly the scenario AP multiple-choice questions like to use.

## Related Study Guides

- [15.7 Fission, Fusion, and Nuclear Decay](/ap-physics-2-revised/unit-15/7-fission-fusion-and-nuclear-decay/study-guide/nVULWHgdGjkiGhbI)

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