Problem — Water jet deflected by a rod

Problem

You rub a plastic rod with a wool cloth and bring it close to a thin jet of water from a tap. The jet curves. What does this indicate? Is the water electrically charged?

Solution

What happens. By rubbing, the rod becomes charged (say, negatively). The water molecule is a permanent dipole: the charges within it are already separated (the oxygen slightly negative, the hydrogens positive). As the rod approaches, the molecules of the jet orient themselves, turning the side of opposite sign towards the rod.

Why the jet bends. On the near side (opposite sign) the attraction is stronger than the repulsion on the far side, because the force goes as $1/r^2$:

$F \propto \frac{1}{r^2},\qquad r_{\text{near}} < r_{\text{far}} \;\Longrightarrow\; F_{\text{attr}} > F_{\text{rep}}$

The net result is a net attraction and the water stream curves towards the rod.

Is the water charged? No. The water remains globally neutral: no net charge is transferred to it. The deflection is due to the orientation of the dipoles (polarisation), not to a charge on the water.

$\ev{\text{orientation of the water dipoles}\Rightarrow\text{attraction; the water remains neutral}}$

Links

Topics: Elettrostatica Concepts: Dipolo elettrico · Carica elettrica