Charging a capacitor with AC voltage

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Discussion Overview

The discussion revolves around the concept of charging a capacitor using AC voltage, exploring the mechanisms involved and the implications of alternating current on capacitor behavior. Participants examine both theoretical and practical aspects, including specific applications like Tesla coils.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant questions how a capacitor can charge with AC voltage, noting the rapid switching of voltage at 60Hz.
  • Another participant states that capacitors store charge, not current, and suggests that AC must be converted to DC for effective charging, indicating that in an AC circuit, the capacitor alternately charges and discharges.
  • A different participant describes the charging process of a capacitor during the positive half of an AC waveform, explaining that it reaches maximum charge at a specific point in the cycle and then discharges as the voltage decreases.
  • One participant reflects on the rapid nature of the charging and discharging process, speculating on the purpose of the capacitor in a Tesla coil circuit, linking it to energy release.

Areas of Agreement / Disagreement

Participants express differing views on the ability of a capacitor to charge with AC voltage. While some acknowledge that charging occurs in intervals, others emphasize the need for rectification to retain charge effectively. The discussion remains unresolved regarding the overall efficacy of charging a capacitor with AC.

Contextual Notes

Participants do not fully explore the implications of frequency on charging intervals or the specific role of components like diodes in the circuit. There is also a lack of consensus on the practical applications of capacitors in AC circuits.

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Homework Statement


Can somebody explain how it is possible to charge a capacitor using ac voltage? The way I see it is the capacitor wouldn't have time to charge as the voltage is constantly switching. In the case of 60Hz - 120 times a second.

I seen a schematic of a tesla coil where the capacitor is in series with a primary coil the source is a transformer. A spark gap is in parallel with the source and capacitor/transformer. There were no diodes.

Homework Equations


The Attempt at a Solution

 
Last edited:
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Capacitor stores charge and not current. In genereral,you must convert AC into DC using rectifier diode and then charge the capacitor.

But, if u connect a capacitor with AC source,then it will alternately get charged and discharged determined by frequency of the source. This is obviously because the current isn't in one direction. So it can't be charged i.e charge can't be retained/stored in capacitor in AC curcit
 
just consider for example the AC voltage where the the voltage is given by V(t) = Vsin(t), for the duration of the positive part of the wave from 0 to pi/2 the capacitor would be 'charging' for that whole process and be at maximum charge at pi/2.

then from pi/2 to pi, the voltage will be decreasing across the capacitor and so the charge will be decreasing. this will be the same process for the other bit of the wave except it will be charged the other way round eg the plate that was originally positive will be negative from pi to 2pi .

So the capacitor is still charging just for very short intervals depending on the frequency
 
Yep. I get the picture now. I just didn't appreciate the speed at which this is happening. Basically the spark gap must be firing twice per cycle. I wonder why the cap is there in the first place? Is it because of w=.5cv^2? A quick release of energy into the coil.
 

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