The surface charge of a silicon nanoparticle in solution of weak silver nitrate electrolyte was studied. The charge was
determined by electrophoretic method in colloidal system strongly diluted with water. The physical model was offered,
which combined electrostatics, hydrodynamics and optics, to describe our method. With the help of our model the
approximate value of nanoparticle charge was calculated. The total surface charge of a silicon nanoparticle was
determined by a number of silver surface adsorbed ions. The method gave the value of Q ≈ 65•10-19 ≈ 40e for the
particles of about 20 nm in diameter. The layer of counterions located in liquid near the nanoparticle to neutralize its
charge, the colloidal system was in dynamical balance. Applying electric field to the colloidal system can disturbed the
balance. Thus the nanoparticles may start to move to one of the electrodes depending on value and sign of their charge.
The threshold of electric field intensity, which was necessary to disrupt dynamical equilibrium and begin the
nanoparticle movement was found in our work. To describe the nature of this threshold behavior of the colloidal system
the theoretical model was offered.