12. Thermodynamics
From the soothing warmth of a heating pad to the dangerous burning flames of an acetylene torch, heat is used for many functions. It can cut, it can melt, and it can weld. But seriously, the difference between sound and heat is difficult to explain. In The Dominators , the Second Doctor was the first to use the screwdriver as a cutting torch to burn through a metal wall. He needs to burn through the wall as if he had an acetylene torch. In no time, there is a hole cut in the wall. In Terror of the Zygons , the Fourth Doctor first used it to heat-up the Zygon fire alarm to facilitate his escape. But how would someone use sound to cut through metal? Ultrasound cannot interact directly with molecules as its wavelengths are too long, but it can affect them in another manner. Sonochemistry can causes cavitation on the surface that in turn causes extremes in temperature and pressure. This could be controlled to produce either a mild heating effect that warms up an object or cause enough agitation to melt through metal.