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If the quarks in a neutron are (up,down,down), why isn't it negatively charged? Excuse the silly question, just wondering.

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Because the quarks have fractional charges that cancel. –  Brandon Enright Feb 14 at 6:24
    
It's worth mentioning that while its net charge is zero, the neutron has a non-null charge distribution, something like this. –  giordano Feb 15 at 19:21
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4 Answers

up vote 18 down vote accepted

The up quark has a charge of $+2/3$, the down has a charge of $-1/3$. If you have a bound state of charged particles, the total charge is just the charge of the elementary constituents. The neutron consists of one up quark and two down quarks, so the total charge $Q$ is:

$$Q = 2/3 + 2 \times (-1/3) = 0$$

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The names up and down don't refer to electric charge $Q$ but are rather references to isospin charge $I_3$.

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A neutron consists of three quarks $u d d$(up down down quarks). The up quark(u) carries charge $2e/3$ and the down quark(d) carries a charge $ -e/3 $. Thus $2e/3-e/3-e/3=0$

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Because $2/3-1/3-1/3=0 $.

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