Is DC resistance always higher than AC
The professor is pulling your leg as both ac and dc resistance can be adjusted, its just that it takes different techniques to do so. DC resistance is normally adjusted by logarithmic or linear potentiometers, assuming you do not want a simple fixed value resister. But you will find you cannot get a resistance less than zero, even if it is 0.001 ohm, which is a common value for current sensing. AC resistance is changed by an adjustable inductor or changing the frequency through a fixed-value inductor. What the professor might be referring to is that an inductor can create a negative voltage swing, even when fed positive pulses. When the pulse is turned off, the magnetic field it generated collapses back into the windings, but of opposite polarity. It will have the same amount of current as the positive pulse. It's apparent positive resistance would become negative for a brief period. If the negative return pulse is not clamped with a diode, it can damage the circuit. A meter would show negative inductance at that point, which is why inductance is measured with precision sine waves. The skin effect (higher resistance) only occurs at high frequencies, above 50MHZ especially. AC impedance makes sure the AC 'resistance' (not really a good term for HF AC) is dominate by a large margin. At GHZ frequencies most all of the signal is riding the surface of the wire or trace to avoid the much higher resistance deeper into the wire/trace. Some boards such as those used for Radar have the traces coated with silver, which is a better conductor than copper. The silver is then coated in gold or polyurethane to prevent tarnishing. I could write a books worth of the details of AC vs. DC behavior but that is beyond the scope of this forum. Surf the internet for more info. There is no end to what you can learn just on AC vs. DC behavior alone.