Friction makes heads smoke
Shuffle across the floor and you set off tiny sound waves, the energy of which is quickly converted to heat. These waves cause most of the friction, some physicists say. However, in certain cases, the resistance of small electrical currents generated when two objects rub against each other can also have a slowing effect. The first evidence of this electrical friction appears in the February 23, 1998 Physical Review Letters. Physicists have found that solid nitrogen on a lead surface loses 50% of its normal friction when the lead loses its electrical resistance.
To find an example of electrically generated friction, physicist Jacqueline Krim and two students at Northeastern University in Boston conducted what Krim called "the simplest experiment imaginable". They placed a thin sheet of solid nitrogen, cooled to about -188 oC, on a small lead-coated quartz crystal. When an electric current flows through the crystal atoms, they vibrate at a specific frequency. If the nitrogen sticks to the lead, it will put its weight on the quartz crystal, causing it to vibrate at a lower frequency. The frequency can therefore serve as a measure of the friction.
The assembly is cooled to below 7.2 Kelvin (-265.96 oC) - the temperature at which lead becomes a superconductor, i.e. loses all electrical resistance –, any electrical friction should disappear. In Krim's experiment, the frequency of the vibrating crystal abruptly dropped by 50%. Consequently, half the friction between the lead and the nitrogen must be caused by electrical currents.
The experts are extremely puzzled by the dramatic effect."I don't know how to explain that," says Uzi Landman, a physicist at the Georgia Institute of Technology in Atlanta. It's strange, he notes, that the strong vibrations of the quartz crystal don't provide enough energy to completely destroy the superconductivity and maintain high friction. "Whatever's going on, it's very exciting," says Landman, "and will spark a lot of heated debate."
