The polarity of water?
The deflection of a stream of water by an electric charge is often cited as a demonstration of the polarity of water molecules. The idea has even found its way into science text books.
What’s wrong with the explanation?
Water molecules are polar; this is true. However it is also true that they electrically neutral and that they are very small. If a negatively charged rod is held 10 cm away from a water molecule then the attractive force on the hydrogen side and the repulsive force on the oxygen side are, for all practical purposes, equal and opposite.
Here are some simple demonstrations that clearly show that the ‘polarity argument’ is unsatisfactory, the first being the most obvious.
No deflection occurs if the water becomes a stream of separate drops, unless the charged rod is sufficiently close to the point where the stream becomes discontinuous.
(This refutes the polarity argument.)
Get a 500 ml plastic drinks bottle. Using an awl or nail make a hole (about 1 mm in diameter) in the side of the bottle a few centimetres from the bottom. Place the bottle on a large insulated block (e.g. styrofoam container) near a sink. Fill the bottle with water and arrange it so that a stream of water flows into the sink. If a charged rod is brought near the stream it will not show a continuous deflection. (It may show a slight deflection initially but it will not last.)
Clamp a salt cellar on a retort stand so that the salt can flow out. Bring a charged rod near the salt stream. It does not deflect. (The charge separation in NaCl is greater than that of water; so are the charges. NaCl is about five times more polar than water but it is not deflected.)
(Don’t try this one.) A continuous stream of mercury would be deflected by a charged rod even though mercury atoms are not polar.
A stream of iron filings is not deflected by a charged rod.
These results cannot be explained by the polarity argument.
The right answer
The right answer is perfectly simple. Continuous conductors may be deflected by an electric charge. Water is a conductor and so if, for example, a negatively charged rod is brought near an unbroken stream of water coming from a tap then a positive charge can be induced on the part of the stream near the charged rod. The force of attraction between the two will deflect the stream. This cannot happen if the stream is broken into separate drops.