Why Capacitor Blocks DC & Inductor Blocks AC?

In summary, a capacitor behaves like an open circuit to DC, while an inductor behaves like a resistor to AC. The difference is that a capacitor takes a while to charge, while an inductor does not.
  • #1
Q-1: Why capacitor blocks DC and allows AC to flow?

Q-2: Why inductor blocks AC and allows DC to flow?
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  • #2
A capacitor consists of 2 conductive plates separated by a non conductive dielectric material. There is no path for DC currents, it is an open circuit as far as DC is concerned. AC is able to pass because electric charge is stored on the plates, energy can be exchanged through the changing E&M fields that exist between the plates.

An Inductor is merely a piece of wire so it is seen by DC as simply that, a wire. To AC on the other hand the continually changing E&M fields attempt to maintain a constant current, thus it resists the AC changes, imposing an impedance that depends on the frequency of the AC current.

A Capacitor also presents an impedance to AC which depends on the frequency.
  • #3
Some clues:

Think in terms of electrons. In a capacitor, can electrons jump the gap across the plates? Can the electrons exert a force across the gap?

In an inductor, what must a magnetic field do to induce a "back emf" ? What must happen to the current to make the magnetic field do this?
  • #4
Think in terms of electrons. In a capacitor, can electrons jump the gap across the plates? Can the electrons exert a force across the gap?

Of course. You must think in terms of electrons but in a wave form. In the fourth Maxwell's Equation there is a "displacement current" that reffers to this effect. The electrons can be transported inside a conductor, but between the plates too. Think in what it's the real meaning of the electromagnetic force, only an exchanging of some information (virtual photons) between two particles.
  • #5
Regarding your first question:

In a capacitor there is never any
flow of current across the plates.
A capacitor doesn't actually let
alternating current "flow". What
happens is that, when the current
is going in the first direction
the plates are charged according-
ly; one side positive, the other

If the current continued in the
same direction the capacitor would
become fully charged and the flow
would stop. Instead the current
changes direction. The side that
was negatively charged is now
positively charging, and visa

The illusion that the capacitor is
allowing alternating current to
flow across the plates is caused
by the fact it takes a certain
amount of time for the capacitor
to charge. The flow of current is
actually flowing into "storage"
so to speak, not across the
plates. When the current is
reversed the "full" side is emp-
tied and the "empty" side is
refilled. (The "full" side would
be the "negative" side: it is full
of electrons which carry what we
have decided to call a "negative"
  • #6
well thanks to all of you !

thanx to all of you for quick replies, are those enough answers to my question?Can we go more in depth?

please suggest me any website where I can read all about Capcitor and Inductors in detail.I am looking for more in depth knowledge.

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1. Why does a capacitor block DC but allow AC to pass through?

A capacitor is made up of two conductive plates separated by an insulating material, also known as a dielectric. When a DC voltage is applied across the capacitor, the electrons in the circuit begin to accumulate on one plate, creating a negative charge, while the other plate becomes positively charged. This separation of charges creates an electric field that opposes the flow of electrons in the circuit, effectively blocking the DC current. However, since the electric field is constantly changing in an AC circuit, the capacitor is able to charge and discharge, allowing the AC current to pass through.

2. What property of an inductor causes it to block AC but allow DC to pass through?

An inductor is made up of a coiled wire, which creates a magnetic field when a current flows through it. In an AC circuit, the direction of the current is constantly changing, resulting in a constantly changing magnetic field. This changing magnetic field induces a voltage in the inductor that opposes the flow of current, effectively blocking the AC current. However, in a DC circuit, the current is constant, and thus, there is no changing magnetic field or induced voltage, allowing the DC current to pass through the inductor.

3. How does a capacitor affect the voltage in a circuit?

A capacitor has the ability to store and release electrical energy, which can affect the voltage in a circuit. When a capacitor is connected to a voltage source, it charges up to the same voltage as the source. However, as the capacitor charges, the voltage across it increases until it reaches the same voltage as the source. When the capacitor is discharged, it releases the stored energy, causing a drop in voltage. This ability to store and release energy can help regulate and stabilize the voltage in a circuit.

4. Can a capacitor or inductor completely block DC or AC currents?

While capacitors and inductors are often described as blocking or allowing DC and AC currents, they can never completely block or allow them. This is because, in real-world circuits, there will always be some leakage or resistance that allows a small amount of current to pass through, even if it is very small. However, the amount of current that is allowed or blocked can be significant enough to affect the functioning of the circuit.

5. How are capacitors and inductors used in electronic circuits?

Capacitors and inductors are important components in electronic circuits as they can help regulate and filter voltage, store and release energy, and block or pass AC and DC currents. They are commonly used in power supplies, filters, oscillators, and many other electronic devices. Their properties and behavior can be used to design and control the flow of electricity in a circuit, making them essential for many electronic applications.

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