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Q-value
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Energy released in a reaction.
Q = (m_i−m_f)c²Q = (m_initial − m_final)c² is the energy released (Q > 0) or required (Q < 0) in a reaction.
D + T → He + n releases 17.6 MeV.
Use masses in u × 931.5 MeV.
In more detail
The Q-value of a nuclear reaction is the energy released (positive) or absorbed (negative), given by the difference in mass between reactants and products times c squared.
Real-world uses
- Q-values predict whether a reaction releases or absorbs energy.
- Reactor designers use Q-values to estimate energy output.
- Engineers calculate the energy released in a nuclear reaction from the mass difference between the initial and final states.
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Frequently asked questions
What does a positive Q mean?
The reaction releases energy and is exothermic.
What does a negative Q mean?
The reaction needs energy input, such as from a particle accelerator.
Is Q positive for an energy-releasing reaction?
Yes. A positive Q means the reaction releases energy, while a negative Q means energy must be supplied.
Related topics in Atomic & Nuclear Physics
Radioactive series · Carbon dating · Nuclear reactions · Nuclear fission · Chain reaction · Nuclear reactors