Trying to understand capacitative reactance - Page 1
Forum section: electronics / beginners

author: Richardcavell
author_rank: Contributor
timestamp: November 27, 2024, 09:55:04 am
content: 1. Suppose that I have a DC series RC circuit. So a battery, a resistor, and a capacitor in series. I figure that the resistor follows Ohm’s law, so a graph of the voltage in the resistor over time should have the same shape as a graph of the current in the resistor over time. And that current will be the same everywhere. Is this right?2. The nature of the equation for capacitative reactance (Xc=1/(2*pi*f*c)) is that a capacitor demonstrates different values of reactance for different frequencies. Is that right?3. As to what the word reactance means, am I right to simply think of it as resistance but in relation to alternating current? Does it produce a voltage drop and restrict current the same way that DC resistance does?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: November 27, 2024, 10:50:12 am
content: 1. correct2. correct3. not entirely correct. voltage drop and current across a capacitor (and somewhat similarly across a inductor) behave differently than those across a resistor.if you have alternating current and ideal capacitor or inductor, the voltage and current are 90° phase shifted to each other.Current through capacitor leads voltage across it by 90°, while for inductor voltage leads current by 90°. For ideal resistor voltage and current have same phase.What however is correct: you can take value for Reactance to calculate RMS/Amplitude/peak to peak current by dividing the suitable voltage (RMS / Amplitude or Peak to peak) by the Reactance

author: SteveThackery
author_rank: Super Contributor
timestamp: November 27, 2024, 11:21:04 am
content: Quote from: Richardcavell on November 27, 2024, 09:55:04 am1/ Yes, although it's better to say the voltage "across" the resistor.2/ Yes.3/ Yes, sort of. As well as working like a resistance to alternating current at the overall level (eg dropping the mains voltage to drive an LED, known as a "capacitive dropper"), the capacitor also causes a phase shift between the instantaneous voltage applied to the circuit and the instantaneous current flowing in it (the current waveform leads the voltage waveform). This phase shift might be undesirable, it might be irrelevant, or it might be just what the designer wants. Resistors don't cause any phase shift, so we use the term "reactance" for components that do cause a phase shiftI said earlier that the phase shift might be desirable. Electrical machinery often "looks" like an inductor to the mains supply. Inductors also cause a phase shift between voltage and current, but in the opposite direction - the current lags the voltage. The electricity supply companies don't like too much phase shift (for reasons we can go into another time). One way to mitigate this is to add a capacitor - with its leading phase shift - to counteract the lagging phase shift of the machine. This is called "power factor correction", the ideal result being that the supply sees something that looks just like a resistor - current and voltage in phase.As mentioned before, capacitive phase shift can be irrelevant (a capacitive dropper); desirable (power factor correction); or undesirable. I would argue that for most analogue filters the phase shift is actually undesirable, even though it is fundamental to the operation of the filter. A filter that cut off high or low frequencies but without any phase shift would be of interest to designers.We are probably at the leading edge of what this question is about, so I'll stop here. 😀