Teacher Guide

Avaliable with permission from the University of Wisconsin Press, and author Bassam Shakashiri.

A stream of water is deflected in the electric field of a charged rubber rod and a charged glass rod.

MATERIALS

• overhead projector
  
• 50 mL water
• mirror, ca. 15 cm square
• 50-mL buret, with stopcock and stand 
• 250-mL beaker
• hard rubber rod 
• cat's fur
• glass rod silk cloth
• hard rubber comb (optional)

PROCEDURE

Preparation

Place the overhead projector on its side and attach the mirror to its lens head to deflect its beam onto the screen. Mount the buret in the stand and position it in the field of the projector so the image of its tip is in focus on the screen. Fill the buret with water. Place the beaker below the buret to catch the water when the buret's stopcock is opened.

Presentation

Open the stopcock of the buret. Rub the rubber rod with the cat's fur and hold the rod about 3 cm from the stream of water. The stream deflects toward the rod. Repeat the presentation with a glass rod rubbed with a silk cloth. The deflection is again toward the rod. A rubber comb run through someone's hair can be used as well.

DISCUSSION

In this demonstration, a stream of water is deflected by an electrically charged rod. The rod is either glass or rubber. The glass rod is charged by rubbing it with a piece of silk, and the rubber rod is charged by rubbing it with cat's fur.

When a liquid composed of polar molecules is placed in an electric field (gener­ ated by a separation of charges, for example) the molecules tend to align, producing a macroscopic separation of charge that opposes the charges generating the electric field. Even liquids composed of molecules without permanent dipole moments undergo shifts of electron and nuclear positions, which produce a macroscopic charge separation, al­ though the separation is smaller than that in polar molecules. These displacements typi­ cally occur in a time scale of picoseconds or less. A rough gauge of the magnitude of the charge separation is the dielectric constant of the liquid [1] . (The dielectric constant of a liquid is the ratio of the capacitance of a condenser filled with the liquid to the capacitance of the condenser when it is empty.) Although the water molecule has only a moderate dipole moment, the hydrogen-bonded structure of liquid water allows a large charge separation and a concomitantly large dielectric constant.

In a uniform electric field, the force tending to move the positive charges which develop is exactly balanced by oppositely directed force, which attempts to move the negative charges. Therefore the liquid remains in position. In a nonuniform electric field these forces are unequal, and the liquid moves toward the region of higher field strength. This effect, dielectrophoresis, has been used in separation methods [2]. Di­ electrophoresis is related to the polarity of the molecules which compose the liquid via the dielectric constant. Onsager's equation relates the dielectric constant, the molecular dipole moment, and the polarizability, which is determined by the ease of distorting the nuclear- and electron-charge distributions [1] . If the dipole moment, µ., is 0, the rela­ tive dielectric constant is about 2. For substances with typical values for permanent dipole moments, the relative dielectric constant is larger by about a factor of 3 or more. The relative dielectric constant of water is unusually large (78 at 25°C), a fact that has stimulated numerous attempts to compute the relative dielectric constant of water via statistical mechanics (with mixed success). Thus, dielectrophoresis occurs in liquids of both polar and nonpolar molecules, although the effect is enhanced in polar liquids. A fairly prevalent misconception is that a stream of oil cannot be deflected in the manner of this demonstration because the molecules of oil are nonpolar. This claim focusses on the wrong parameter, the molecular dipole moment instead of the dielectric constant of the liquid. In fact, an easily noticeable deflection of a stream of mineral oil can be produced using a narrow glass rod (8 mm in diameter) rubbed with silk. By using a narrow glass rod, the electric field can be concentrated and the nonuniformity en­ hanced. The dielectrophoretic effect is fairly feeble relative to the force on bodies carrying a net charge. However, measurements with electrometers show that sufficient charge densities are found on the rods to produce deflections in the stream of liquid. The effect is larger when the rod is placed closer to the liquid stream, because the elec­ tric field gradient across the sample increases. Greater charging of the rod and smaller rod diameters enhance the effect. Thus, a highly charged rod with a narrow diameter held close to the liquid stream can produce a deflection in a liquid with a small dielectric constant.

When cat fur and the rubber rod are placed in contact, a negative charge develops on the rubber rod. Rubbing enhances the effect, which is called triboelectricity. The charged rod is a source of a nonuniform electric field. The field intensity is proportional to l/ r, where r is the distance from the axis of the rod. The distance between the lines shown in the figure is a representation of this dependence. The stream of water is polar­ ized in the field as shown in the figure. The attractive electric force (the product of the charge multiplied by the electric field) exceeds the repulsive force, and the column is deflected toward the rod.

With the glass rod rubbed with silk, a positive charge is produced on the rod. The polarization of the water molecules in the stream is reversed, but the imbalance of forces again creates a net attraction of the rod for the liquid [3 , 4 ] .

The charging of neutral matter by contact electrification and rubbing is a compo­ nent of the process of electrophotography- a major commercial process [5]. The phys­ ics of the charging process is incompletely understood. A list of the "triboelectric se­ ries" showing the direction of charge transfer when two materials are rubbed together has been published [5].

If the liquid to be deflected is electrically conductive- that is, contains mobile ions-a charge separation as shown in the figure can occur by ion displacement, and the stream will be deflected toward the rod in the nonuniform electric field as with the dielectrophoretic mechanism. A discussion of this possibility appears in the literature [6], where it is claimed that induced (net) charge is the primary cause of the deflection. Partial support for this point of view is that drops from the falling stream collected in a metal cup connected to an electroscope carry a charge opposite in sign to that of the rod. We would explain this observation not in terms of acquisition of a net charge, but by noting that the charged rod causes a vertical as well as horizontal movement of ions, resulting in the bottom part of the stream (which breaks off and is collected) being charged oppositely to the top of the stream and to the rod. The (separation of charge) effect is readily observable with water and is even larger with salt water. Our experi­ ments did not indicate that the deflection of salt water was significantly greater than that obtained with pure water under the same experimental conditions. In addition, we col­ lected drops from a deflected stream of n-hexane (commercial grade) and observed no separation of the electroscope leaves, indicating no detectable charge separation (or net charge acquisition). n-Hexane has an immeasurably low conductivity and cannot be deflected by a mechanism involving ionic charges. It is, however, an unfavorable liquid for observing dielectrophoresis. Nevertheless, deflection of the stream can be observed, suggesting that the dielectrophoretic mechanism is predominant in the demonstration as performed.

REFERENCES

1.  N. E. Hill, W. E. Vaughan, A. H. Price, and M. Davis, Dielectric Properties and Molecular Behaviour, Van Nostrand-Reinhold: London (1969).
2.  H. A. Pohl, Sci. Am. 203 (Dec.):106 (1960).
3.  A. D. Moore, Sci. Am. 226 (Mar.):46 (1972).
4.  H. A. Pohl, Dielectrophoresis:  the Behavior  of Neutral M atter in Nonuniform Electric Fields, Cambridge University Press: New York (1978).
5.  D. M. Burland and L. B. Schein, Phys. Today 39:46 (1986).
6.  I. D. Brindle and R. H. Tomlinson, J. Chem. Educ. 52:382 (1975).