Capacitor & Inductor?

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  • #1
Q-1: Why capacitor blocks DC and allows AC to flow?

Q-2: Why inductor blocks AC and allows DC to flow?

Answers and Replies

  • #2
Staff Emeritus
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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
Chi Meson
Science Advisor
Homework Helper
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
Science Advisor
Gold Member
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 thanx 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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