The voltage drop across the resistor is in phase with current.
An oscilloscope is just a fancy voltmeter that graphs voltage vs time. You cannot see current directly. To see the phase relationship between voltage drop across capacitor and current, you use a small resistor. V_R will be in phase with I (0°), while V_C will lead I by 90°. V_C will lead V_R by a quarter of a cycle.
So it's a kludge to allow current to be visible on a voltage device to measure phase angles.
You need a sensing resistor only when measuring current. If you want capacitor current, the resistor is put in series with the capacitor and similarly for inductor current.
Oscilloscopes measure voltage so no sensing resistor is needed. You would use a resistor to measure current with an oscilloscope. The IR drop across the resistor would create the voltage an oscilloscope would read.
Well it doest have to be used for that, but its just more practical to regulate current with it, because it's changed by force applied on it, instead of having to use a button or something.
when measuring AC voltages on capacitors and inductors with an oscilliscope on parallel circuits, when and where to you put the sensing resistor? What is the sensing resistor for?
