  A general research on chiral symmetry restoring phase transitions at zero
temperature and finite chemical potentials under electrical neutrality
condition has been conducted in a Nambu-Jona-Lasinio model to describe
two-flavor normal quark matter. Depending on that $m_0/\Lambda$, the ratio of
dynamical quark mass in vacuum and the 3D momentum cutoff in the loop
integrals, is less or greater than 0.413, the phase transition will be second
or first order. A complete phase diagram of $u$ quark chemical potential versus
$m_0$ is given. With the electrical neutrality constraint, the region where
second order phase transition happens will be wider than the one without
electrical neutrality limitation. The results also show that, for the value of
$m_0/\Lambda$ from QCD phenomenology, the phase transition must be first order.
