Mass–Energy Equivalence
E = mc² connects mass and energy.
About this topic
Einstein's equation E = mc² says mass and energy are equivalent: a small amount of mass corresponds to an enormous amount of energy because c² is huge. One gram of matter equals about 9 × 10¹³ J.
That is roughly the energy released by a 20 kiloton bomb, from a paperclip's worth of mass. It explains the energy released in nuclear reactions, where the products have slightly less mass than the reactants, and how the Sun shines, losing 4 million tonnes of mass each second. In particle physics energy can become matter and vice versa.
In PET scans an electron and a positron annihilate, turning their entire mass into gamma rays, which are detected to build an image. Particle accelerators use the reverse process, converting kinetic energy into new particles.
A common error is to think E = mc² means that any object can be turned into energy easily. Only in nuclear and particle processes does a measurable mass change occur. Even burning coal changes mass by a billionth of a percent, too small to notice.
Key ideas
- A little mass is huge energy
- Nuclear reactions convert mass
- c² is enormous
Quick check
Use cases: where Mass–Energy Equivalence helps
- Homework: review the key idea behind Mass–Energy Equivalence before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Mass–Energy Equivalence without notes.
- Visual learning: play with the interactive lab to see how Mass–Energy Equivalence changes when you adjust its inputs.
- Teaching a friend: explain Mass–Energy Equivalence in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Mass–Energy Equivalence?
E = mc² connects mass and energy.
What will I learn in Mass–Energy Equivalence?
A little mass is huge energy. Nuclear reactions convert mass. c² is enormous.
Is Mass–Energy Equivalence part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Mass–Energy Equivalence lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Mass–Energy Equivalence?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.