I have a small three-phase motor being mechanically driven to act as a generator. My goal is to measure power delivered to a load, which in my case would simply be a 4 ohm resistor, using an oscilloscope. I've seen others with the same goal have measured the voltage drop over a shunt resistor to measure the current and then measured voltage across their load in order to use P=VI.
Why is that extra resistor used to measure current? If I have a known resistance and I then measure voltage across it, can I not use P=V^2/R to get power delivered to load? I know it is probably a simple question but any help would be appreciated.