Capacitor transient response, what happens in the dielectric?

[Lavabug]

I'm trying to marry the concepts from my EM course with what I've been doing in my lab course. Correct me if I'm wrong, in a plain RC circuit, when closed with a DC source, the capacitor initially acts as a short (voltage is zero across the terminals, while I is max), until after a few microseconds the voltage increases exponentially to the one I'm administering (consequently I drops down to 0 ideally).

What's happening to the dielectric in the capacitor in these first few moments? The current through the cap is initially nonzero, meaning charge is flowing through it and the dielectric is quickly becoming polarized, but why is the voltage zero at the terminals initially? There's a circulation of E between the capacitor plates, so why isn't there any difference in potential on the terminals?

 

[ashishsinghal]

Initially ie just after switch is closed, the current develops and the charge on the capacitance is very low (it builds up with time). Since charge tends to 0 initially potential also tends to zero.

 

[Lavabug]

Not sure I understand you, could you please elaborate a bit more?

 

[Evil Bunny]

What's happening to the dielectric in the capacitor in these first few moments? The current through the cap is initially nonzero, meaning charge is flowing through it and the dielectric is quickly becoming polarized, but why is the voltage zero at the terminals initially? There's a circulation of E between the capacitor plates, so why isn't there any difference in potential on the terminals?

I'm not sure it's completely accurate to say that charge is flowing through it. It's more like charge is building on the plates and the dielectric is shifting from an equilibrium state to a polarized state in response to the charge buildup.

You kind of hit on it yourself... It acts like a short initially. So when you short something out, for example you place a wire between the two terminals on a battery, you have a great deal of current and (almost) no potential difference between the terminals.

When the voltage builds in the cap to the applied voltage, the source can't "push" any more charge to the plates, so it then acts like an open.

It sounds like you know most of this, but I hope maybe that helped a little...

 

[sardar]

In a capacitor, the dielectric is the insulating material between the two plates. When the capacitor is connected to a DC source, the dielectric initially behaves as an insulator, preventing any flow of charge. This is why the voltage across the terminals is initially zero. However, as the current starts to flow through the capacitor, the dielectric starts to become polarized, allowing charge to build up on the plates. This process takes a few microseconds, as you mentioned, and as the charge builds up on the plates, the voltage across the terminals also increases exponentially.

The reason why the voltage across the terminals is initially zero is due to the fact that the dielectric has a finite resistance. This means that there is a delay in the polarization of the dielectric, causing a delay in the buildup of charge on the plates. As a result, the voltage across the terminals is initially zero until the dielectric becomes fully polarized and the charge on the plates reaches its maximum value.

Additionally, the circulation of electric field between the plates is not enough to create a potential difference on the terminals. This is because the electric field is evenly distributed between the plates, resulting in no net potential difference across the terminals. It is only when charge starts to build up on the plates that the potential difference across the terminals becomes noticeable.

I hope this helps to clarify the behavior of the dielectric in a capacitor during the initial transient response. It is important to note that this behavior may vary depending on the type of dielectric material used and the specific circuit setup. Further experimentation and analysis may be needed to fully understand the behavior of the dielectric in a specific capacitor circuit.