Paul J. Dolan, Jr., & Charles W. Smith, Dept. of Physics & Astronomy, 5709 Bennett Hall, University of Maine, Orono, Maine 04469-5709 *
Abstract:
A recent article in "The Physics Teacher"(1) discusses
the fact that an object floating on water, by buoyancy, will be attracted
when approached by a charged rod, whereas an object floating primarily
by surface tension will be repelled by the same charged rod. The effect
is stronger than the article suggests, and can be seen with many different
materials. We will discuss methods of demonstrating the repulsion and present
a mechanism for this effect. A method of demonstrating the mechanism that
causes the repulsion will also be discussed.
Dindorf recently presented as a "Trick of the Trade", the fact that a buoyant object, such as an aluminum ‘boat’, while floating will be attracted to a charged rod; this would be expected due to the induced charge separation in the ‘boat’. However, when a flat piece of the same material floats due to surface tension, it will experience a net repulsive force from the charged rod; that this will occur is not intuitively obvious. We have experimented with a number of different materials, and have also shown that the effect may be well displayed using a clear liquid (water) in a clear dish, atop an overhead projector. Our preferred object is an aluminum 1-yen coin (2), and the repulsion seen is quite dramatic.
Consider that there are two primary mechanisms by which an object will float on a liquid: buoyancy and surface tension. An object floating due to the buoyant force is partially submerged in the liquid, but the induced charge separation primarily occurs in the portion of the object above the water. (One can see this by the manner in which an aluminum boat will also tip towards the charged rod.) An object floating due to surface tension does so because the liquid does not ‘wet’ the object easily. Which of the two effects is dominant may be seen on the overhead display: a buoyant object shows little deflection of the water surface, while around the object floating due to surface tension, there is a distinct depression in the water surface, visible both on the projected image, and by eye.
Consider one further detail: water is a polar liquid; thus, it too will have an effective ‘charge separation’, and be attracted to the charged rod; this is another ‘trick of the trade’, that can be easily demonstrated by deflecting a thin stream of water with a charged comb. Thus, the water will ‘mound up’ on the side of the charged rod. When the object is (partially) beneath the liquid (buoyancy), this mounding does not produce a strong net force. However, when the object is on top of the water (surface tension), this effect is dominant, and the object tends to slide down the mound, and thus the net effect is a force away from the charged rod.
With a little care, one can demonstrate the crossover between the two regimes. Find a small piece of plastic which will float in water, but which water does not wet readily. When floating, the plastic will demonstrate an attraction to the charged rod (or at most a weak repulsion). Then, add mass to the plastic, such as a paper clip – not enough to completely submerge it, but enough that the depression on the water surface from surface tension becomes evident. Now, the charged rod will strongly repel the plastic; thus, we have changed from the regime where buoyancy is dominant to the one in which surface tension is dominant.
It is also possible to demonstrate the ‘mounding up’ of the water. Place a clear grid on the overhead beneath the tray of water. With the thin object floating in it, one can clearly see the ‘defocusing’ due to the change in the shape of the surface of the water. Now, bring up the charged rod – with some care, one can see that the defocusing id different on either side of the (repelled) object – adjustment of the focus of the projector should convince one that the surface of the water closest to the charged rod is raised, as compared to the remainder of the surface.
We have demonstrated that this effect is independent of
the sign of the charged rod (as would be expected), and can be seen in
many different materials, when surface tension is the dominant force (metal,
including paper clips & aluminum foil & coins), thin sheets of
plastic, and as noted above, buoyant objects subjected to an additional
mass to enhance the surface tension.
*Permanent address: Physics Department, Northeastern Illinois University, Chicago.
1.) "Tokar’s Question", The Physics Teacher 38, 488 (November 2000).