About the reactance of capacitor

[lindacheung66]

For inductors, it is the induced e.m.f. which resists the current flow.
However, I don't quite understand how capacitors resist the current flow.
Why is there capacitive reactance?
What does 'A built-up electric field resists the change of voltage on the element' mean?

 

[sophiecentaur]

For inductors, it is the induced e.m.f. which resists the current flow.
However, I don't quite understand how capacitors resist the current flow.
Why is there capacitive reactance?
What does 'A built-up electric field resists the change of voltage on the element' mean?

That last quote doesn't really make strict sense. Using the word "resists", in this context, implies resistance (a mechanism for losing energy as current is conducted).
A capacitor, being basically an open circuit, will not carry on conducting forever, when connected to a battery. Initially, however, some charge can flow into it. As the charge increases, an increasing potential difference will form across the terminals and, once this PD is equal to the supply voltage, no more current will flow (there is an exponential change). The PD across the capacitor 'opposes' the supply voltage, rather than "resisting" it. (Here, I am talking in terms of the usage of those words in the context of electrical circuits and not in general English language use).
That's why we say that a capacitor 'blocks' DC. For an alternating current , particularly at a high enough frequency, the direction of the applied AC voltage changes rapidly enough for the capacitor never to get fully charged and so the capacitor 'lets through' an alternating current. The higher the frequency, the less the opposing PD becomes so the Reactance decreases proportionally.

 

[lindacheung66]

Thank you very much.
You have difinitely dispersed my misunderstanding.

 

[sophiecentaur]

Good good. Keep 'em coming.

btw, I should have included, in my hand waving model of charging a capacitor, some amount of Series Electrical Resistance in the source of the Voltage. There will always be finite resistance, in practice, so it is a reasonable thing to do. Without the added resistance, you get other awkward things happening and more headaches!

 

[sardar]

I can help clarify the concept of reactance in capacitors. Reactance is essentially the opposition to the flow of electrical current in a circuit. In capacitors, this opposition is caused by the build-up of an electric field between the two plates of the capacitor.

To understand this, let's first review how a capacitor works. A capacitor is made up of two conductive plates separated by an insulating material, called a dielectric. When a voltage is applied to the capacitor, one plate becomes positively charged and the other becomes negatively charged. This creates an electric field between the plates.

Now, when an alternating current (AC) is applied to the circuit, the voltage and current are constantly changing direction. As the voltage changes, the electric field in the capacitor also changes, which in turn resists the change in voltage. This opposition to the change in voltage is what we refer to as capacitive reactance.

In contrast, inductors resist the change in current flow. This is because inductors are made up of a coil of wire, which creates a magnetic field when current flows through it. This magnetic field resists any change in the current flow.

So, to answer your question about why there is capacitive reactance, it is simply a result of the nature of how capacitors work. The build-up of an electric field between the plates creates a resistance to the change in voltage, which is known as capacitive reactance.

Lastly, the statement "A built-up electric field resists the change of voltage on the element" means that the electric field in a capacitor opposes any changes in voltage. This is why capacitors are commonly used in circuits to regulate or filter out changes in voltage.

I hope this explanation helps clarify the concept of reactance in capacitors. Please let me know if you have any further questions.