Re: Though there is insulator inside a capacitor,how current passes through it?

Greetings:

When I first started out into the field of electronics 50 years ago
I had the same question as you have about capacitors and how they work.
I’ll answer your question and also provide a number facts about
capacitors that I know will puzzle all beginning students.

All capacitors operate in the same way; however, the size of the capacitor
and it’s location in the electronic circuit will determine it’s function.
The unit of capacitance is called the Farad; however, in
electronics circuits we tend to use capacitors in micro farads or
smaller (a micro farad = one millionth of a farad).
Large capacitors in the microfarad size are generally made from two large
sheets (or plates) of metal foil seperated by an insulator such as sheet
of beeswaxed paper or Teflon. The three sheets are the rolled up into a
small cylinder for packaging. Other capacitors are made of metal plates
separated by air or oil and some low voltage capacitors use a chemical film
to make a microscopically thick insulator between conducting sheets. The
electrical size of a capacitor is determined by the area of the sheets or
plates, the spacing between the conducting plates and a property of
the insulating material called the dielectric constant. Larger plates,
smaller spacing and greater dielectric constants all make capacitors have
greater electrical values in Farads.

However, all insulators (dielectric) have a breakdown voltage where an
electrical arc can burn through the insulator. So the maximum voltage
that a capacitor can be charged to is also an important parameter which
limits how small a spacing (insulator thickness) that you can make within
the capacitor. For example, a one microfarad capacitor in a 6 volt
transistor radio will be about the size of a pencil eraser. A one microfarad
capacitor used in a one hundred thousand volt power station will be about
the size of an automobile. The difference in size is determined by the
greater spacing needed between high voltage plates to eliminate possible
arcing between the plates.

Now, to answer your question. We know that like electrical charges repel
each other (- and - or + and +) and unlike electrical charges attract each
other (- and + or + and -). Electricity is caused by the flow of electrons
from atom to atom within a conductor. Electrons have a negative charge and
so they repel one another. Ions, atoms missing and electron, have a net
positive charge and they repel one another and they also attract electrons.

A battery (and other power sources)is an electron pump, because of an internal
chemical reaction an excess of electrons is accumulated at the negative
electrode of a battery and these excess electrons result in an excess of
positive ions that are missing electrons at the positive terminal of a
battery. The internal working of a good battery will not let electrons flow
back to the ions internally, they have to find an extewrnal path (a circuit).
We call this electrical pressure between the battery terminals voltage.

Now if we connect the leads of a capacitor to a battery the electrons at
the batteries negative (-) terminal will begin to flow (current) toward
the + terminal of the battery through the circuit. When the electrons reach
the insulator in the capacitor they cease to flow and they stack up on
the capacitor plate (a negative charge). This negative charge produces an
electrical field that passes through the insulator and repels
electrons from the atoms in the second plate (This is similar to the
field of a magnet passing through paper or plastic insulators to hold metal).
These free electrons flow to the positive ions at the battery terminal.
This current continues until all of the ions in the battery + terminal
are filled with electrons and then the current stops. At this point the
capacitor has the same potential (voltage) across it as the battery.

Now if you remove the capacitor from the battery it will stay charged at
the battery voltage for a long period of time. A good quality capacitor
can stay charged for days or weeks. The capacitor can then be connected to
another circuit and it will supply electrons (current) to the circuit and
act like a battery until all of the electrons stored on the - plate of the
capacitor return to the + plate of the capacitor. At which time the capacitor
will be discharged and will have no voltage potential between its terminals.
This discharge can occur in an instant causing a spark of high current
if you directly connect the leads together a short circuit), or it can take
a much longer time to discharge if you place a resistor the circuit between
the capacitor leads to reduce the current flow between the plates.

So in answer to your question, a capacitor will enable a current to flow
until it is fully charged or discharged at which time the current will
cease to flow.

If you reverse the battery terminal connections to the capacitor before
it is fully charged, the capacitor will discharge through the battery and
then recharge in the opposite polarity of the new battery connections.
If we repeat this process often we have an alternating voltage
and an AC current passing back and forth through the capacitor;
however, if we stay connected to one polarity of the battery long enough,
the current will stop when the capacitor is fully charged. In this mode,
which is often used in amplifier circuits, the capacitor passes AC current
from transistor to transistor for amplification while it and stops DC bias
current from one transistor passing through to another transistor.

These processes can all be mathematically modeled; however, this is
beyond the scope of this answer.

Best regards, Your Mad Scientist
Adrian Popa