Q: Could a simple cup of coffee be heated by a hand held device designed to not only mix but heat the water through friction, and is that more efficient than heating on a stove and then mixing?
In fact, that’s exactly how scientists (Joule) figured out how to equate heat energy and kinetic energy in the first place. When you introduce turbulence to a system the energy flows from large scale eddies into smaller and smaller scale eddies. Efficiency is defined as , where is efficiency, is the energy put in, and is the energy lost to heat. So the nice thing about trying to create heat intentionally, is that you’ll always be 100% efficient (if you lose some heat to heat, would you notice?). You’d be better off with a normal mixing device that has a heating element built in, or heating on a stove first.