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If the capacitor works by induction of charges, then does it invert an AC signal? (i.e phase shifting the input signal by 180 degrees). In my observation I dont see a phase shift between an input signal (source) and output signal (through a capacitor) in an oscilloscope. The input and output voltages are exactly in phase. Can someone explain this concept or if an keeping wrong settings in oscilloscope.

Capacitor in AC circuit

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    \$\begingroup\$ Do you mind drawing a schematic of your setup? There shouldn't be any 180 degree inversions anywhere, and the phase delay associated with a cap varies with frequency and changes from 0 degrees to -90 degrees \$\endgroup\$ Nov 30, 2019 at 4:30
  • \$\begingroup\$ During the positive half of the AC cycle, a negative charged is induced on the plate of the capacitor that is encountered first and a positive charge on the next plate. Ideally, it acts as a wire. For most cases, this happens instantaneously. So there is no phase shift. \$\endgroup\$ Nov 30, 2019 at 5:17
  • \$\begingroup\$ During the positive half of the AC the initial plate (or the first plate) of the capacitor that is connected to source develops negative charge? How that is possible? It must develop positive charge right. Im confused. \$\endgroup\$
    – Raj Raj
    Nov 30, 2019 at 6:32
  • \$\begingroup\$ Right now the cap is in a position of an AC coupling cap. This capacitor forms a highpass filter with the load impedance (in your case, the probe impedance) that blocks low frequencies and allows high frequencies to pass though un affected. In the stopband (determined by your capacitor value and your load) you will see attenuation and a +90 degree phase shift of the input signal. In the passband, you should see no attention or phase shift. The fact that you see no phase shift means that you are operating at a frequency in the passband. \$\endgroup\$ Nov 30, 2019 at 6:50
  • \$\begingroup\$ Sir, Thanks for the detailed response. Why there is no inversion? Please clear that part. \$\endgroup\$
    – Raj Raj
    Nov 30, 2019 at 7:01

2 Answers 2

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You may find it helpful to think of capacitors and inductors in this way during initial analysis of a circuit:

  • Inductors tend to keep the current through them constant in the short term.
  • Capacitors tend to keep the voltage across them constant in the short term.

In your capacitor circuit the initial voltage across the capacitor is zero so when the AC input signal "lifts" the left side of the capacitor the right side will follow. It will behave the same on the negative half cycles.

With the high input impedance of the oscilloscope very little charge will flow, the voltage across the capacitor will be close to zero and the two traces will be almost exactly the same. As you add lower values of resistance across the output you will see the voltage fall and the phase start to shift.

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No. The capacitor in your setup will form a high-pass filter with the load and in the passband an input signal will experience no attention or phase shift. In the stopband you will see a maximum of +90 degrees of phase shift (and significant attenuation). Your current setup should never invert the input voltage.


Here is a link to a high-pass filter tutorial. In it they show the magnitude and phase frequency response. If you want to verify this, find the transfer function and substitute s = jw and split into magnitude and phase angle. In doing so you should see that phase is equal to:

90 - arctan(x),

where x is some positive number, dependant of R, C, and frequency. The arctan function has an asymptote of 90 degrees as x goes to infinity, and a value of 0 when x equals 0. Therefore, the phase is always between 0 and +90 degrees.

https://www.electronics-tutorials.ws/filter/filter_3.html

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  • \$\begingroup\$ I think Iam making a fundamental mistake as far as capacitor working. I have learnt that a capacitor works by electrostatic charge induction. Means positive charge induces negative charge on the nearby conductor placed in vacuum or air media or intervened by a dielectric. Thats why I honestly ask this question that when an AC signal moves from one plate to another there must be inversion of the signal as positive induces negative and vice versa. Just to clarify I am not asking the phase shift between current and voltage. I am ctoncerned with only phase shifts between input and output voltage. \$\endgroup\$
    – Raj Raj
    Dec 20, 2019 at 5:45

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