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| {{Other uses|Q value (disambiguation){{!}}Q value}}
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| {{DISPLAYTITLE:''Q'' value (nuclear science)}}
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| In [[nuclear physics]] and [[nuclear chemistry|chemistry]], the '''''Q'' value''' for a [[nuclear reaction|reaction]] is the amount of energy released by that reaction. From energy conservation of the simple reaction,
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| :<math> a+X\rightarrow Y+b</math> | |
| one finds,
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| :<math> m_a c^2 + K_a + m_X c^2 + K_X = m_Y c^2 + K_Y + m_b c^2 + K_b</math>
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| where ''K'' is the kinetic energy and ''m'' is the rest mass. Rewriting the expression above as
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| :<math>\left(m_a+m_X - m_b - m_Y\right) c^2 = K_b + K_Y - K_a - K_X </math>
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| enables the general definition of Q:
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| :<math>Q = K\left(\text{Final}\right) - K\left(\text{Initial}\right) = \left(m(\text{Initial})-m(\text{Final})\right) c^2</math>
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| A reaction with a positive ''Q'' value is [[exothermic]], i.e. has a net release of energy, since the kinetic energy of the final state is greater than the kinetic energy of the initial state.
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| A reaction with a negative ''Q'' value is [[endothermic]], i.e. requires a net energy input, since the kinetic energy of the final state is less than the kinetic energy of the initial state.<ref name="Krane">
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| {{cite book
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| |author=K.S. Krane
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| |year=1988 | |
| |title=Introductory Nuclear Physics
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| |page=381
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| |publisher=[[John Wiley & Sons]]
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| |isbn=0-471-80553-X
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| }}</ref>
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| Q values are also featured in [[particle physics]]. For example in [[Sargent's rule]], which states that the reaction rate of weak interactions is proportional to ''Q''<sup>5</sup>. The ''Q'' value is the [[kinetic energy]] released in the decay of the particle at rest. For example, for neutron decay:<ref name="Martin">
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| {{cite book
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| |author=B.R. Martin and G. Shaw
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| |year=2007
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| |title=Particle Physics
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| |page=34
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| |publisher=[[John Wiley & Sons]]
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| |isbn=0-471-97285-1
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| }}</ref>
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| :<math> Q = (m_\text{n} - m_\text{p} - m_\mathrm{\overline{\nu}} - m_\text{e})c^2</math> | |
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| where ''m''<sub>n</sub> is the mass of the [[neutron]], ''m''<sub>p</sub> is the mass of the [[proton]], ''m''<sub>{{overline|ν}}</sub> is the mass of the [[electron antineutrino]] and ''m''<sub>e</sub> is the mass of the [[electron]].
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| ==See also==
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| *[[Fusion energy gain factor]]
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| ==Notes and references==
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| {{reflist}}
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| == External links ==
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| * [http://www-nds.iaea.org/queryensdf Nuclear Structure and Decay Data – IAEA] with query on decays' ''Q''-values
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| [[Category:Nuclear physics]]
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| {{chem-stub}}
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| {{nuclear-stub}}
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| {{particle-stub}}
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