Could some one show me how capacitor/inductor really work ?

[smarty]

I can understand how electron moves throw resistor in physical but about capacitor and inductor.what's really happened ? Please don't give equation i can't image how electron moves inside and why phases is out in AC .

 

[yungman]

Capacitor store charge. Physically, it consist of two plates close together. If you ground one side and put a +ve voltage on the other plate, electrons leaving the plate that attach to the +ve so there are net protons on the surface of the plate. This in turn will attract electrons onto the other plate that connect to the ground. If you open the contact of the +ve side to the +ve plate, the plate is going to be still possitively charged and therefore still keeping the electrons on the other plates. So in this sense, it is a charge storing device.

Capacitor can serve as the DC blocker because no charge particles can really pass from one plate to the other physically. But if you keep changing the voltage on one plate, the other plate will follow. ei. If you put -ve voltage on one plate, you put excess electrons onto that plate, the -ve charges will repel electrons out of the other plate into the ground. If you keep changing the voltage on the first plate from +ve to -ve and back and so on, the other plate will to the same thing back and fore. A current is form on the other plate as if the current passing from the first plate to the second. This is called displacement current. Therefore capacitor pass AC current/voltage and stop DC current/voltage.

Regarding phase lag of capacitor, it would be harder to explain without equation. Let try this way. Current alway lead the voltage of the resistor, that is the reason we say there is a phase lag of the voltage of the capacitor compare to resistor because the voltage on the resistor is in phase with the current and voltage of the capacitor is lag the current. So when you have a current passing through the resistor in series with a capacitor, the voltage of the resistor automatically leading the capacitor.

You can think of this way, again assume one side of the cap is grounded. The moment you inject current into the open side of the cap, you start pulling charge onto the grounded plate as explained before, the displacement current start flowing immediately as if it is real current. But actually no current can physically go from one plate to the other. The incoming current only manage to charge up the plate and increase the voltage of the plate. But voltage take time to build up, so it is more like the delay reaction. So you can see that the moment you start supplying current on one plate, current start flowing, but the voltage on the plate take a while to build up. So the voltage is always lagging the current.


Inductor is a magnetic storage device. The energy stored is in form of magnetic energy. I don't have an easy explanation for this, I'll let somebody else take a try.

 

[falbani]

Think the capacitor as a black box you see from outside. If you make any change in the applied voltage across this black box, you see electrons going into the device from one side, and electrons (the same amount) leaving the device from the other, so, you describe it as a current flowing through the device.

Now, you open de black box and find the capacitor, who clearly "insulates" the two terminals. This strikes you at first, because your description involved a current flowing through... what? air? This apparent contradiction is solved by remembering that the capacitor "stores charge" (actually it stores energy by separating charges... net charge always remains zero!). The "illusion" of a current going through the device rests on the fact that the electrons going into it are actually being stored on one plate (but it could be any shape) and electrons leaving it were "idling" in the other plate and now are leaving space for "positive charges" (lack of electrons, in this case).

(yungman points out that this current is called "displacement current", but I do not agree. "Displacement current" is a not-so-fortunate name given to the derivative of the D field, which in simple cases is proportional to the Electric field. I think the current we are talking about here is the ordinary one.)

Interesting conclusion: the "insulation" that a capacitor provides is only for constant voltages (usually called DC). Constant voltage ---> no current flows (you actually have to waits some time after the moment you apply it to let the charges accommodate). With varying voltages, charges never stop accommodating and you always have some current. Indeed, for very rapid changing voltages, the capacitor is like a short-circuit.


Electrons flow in an inductor in the same way the do in resistors (indeed all resistors have some inductance and all inductors have some resistance*), with the addition that they "feel" an additional force counter-acting upon them. This force is the result of an "induced" electric field because of the varying magnetic field, which in turn is generated by the very flow of the electrons (who came first? chicken or egg?). Energy is stored in an inductor by setting a current through it. Faraday's Law tells us that any change you attempt in that current, will be opposed by the fields, so you need energy to do it. Once the current is set, you can cut it very fast and energy will be liberated in form of a spark maybe (if a few conditions are met).


I think yungman gave you a fair idea of why the voltage is "behind" current in a capacitor. Remember that voltage is proportional to charge (V = Q/C) and charge is the "accumulation of current", so you need some current to go first to build up some charge to sustain voltage.

An inductor has some "current inertia" because any attempt to change it is counteracted by an induced electric field. If you apply some voltage across an inductor you have to wait some time before charges react to it, i.e., to the opposed field to go to zero as the current is established.

The exact dynamic of this is hard to tell without equations.


* At sufficient high frequencies, everything behaves as having resistance, inductance and capacitance. This happens when the physical dimensions are comparable with the wavelength of the "voltage" wave. The very notion of voltage stop making sense and you have to put Kirchhoff laws aside and work with distributed models or directly with Maxwell Equations.


I don't know if this was the explanation you were looking for. Sorry if I told you things you already known.

 

[yungman]

Check out p 325 in "Field and Wave Electromagnetics" 2nd edition by David K Cheng or

P322 in "Introduction to Electrodynamics" 3rd edition By David Griffiths.

Both relate current passing through capacitor as Displacement current.:

The term comes from

$$\nabla X \vec B \ ;=\; \mu\vec J_{free} \;+\; \mu\vec J_D \;=\; \mu \vec J \;+\; \mu \frac{\partial \vec D}{\partial t}$$

Where

$$\vec J_{free}$$

is the conduction current density and

$$\mu \frac{\partial \vec D}{\partial t}$$

is displacement current density.

 

[falbani]

Yes, that is true... I know... but there are no electrons going through the capacitor, so it is not a fortunate name. I was afraid that smarty misunderstood it as if real charges were involved "jumping the gap". It didn't seem he had the proper background for introducing the Maxwell-Ampere's Law and it might be confusing because he may conclude that what he was not understanding was because of ignoring Displacement current, and that's not the case.

 

[tiny-tim]

hi smarty!

Please don't give equation i can't image how electron moves inside and why phases is out in AC .

the electrons flow "the long way" round the circuit, back and forward in AC

current is like speed, and voltage is like force (or acceleration), so you'd expect them to be out of phase in the same way that speed and force (or acceleration) are out of phase in ordinary shm (eg a spring oscillating about equilibrium … maximum force is at zero speed, zero force at maximum speed )

 

[smarty]

For capacitor :
I think know that how electric field work inside of capacitor (and how dielectric matter in capacitor reduce electric field to avoid hazard).

give that we got 2 things A, B , capacitor with 2 plates Ca,Cb .wires and everything're made by same matter .
When we connect like : A-----Ca Cb------B
A will lost 1 electron/second and B will gain 1 electron/second ,then after 60s B will lost 1 electron/second and A will gain 1 electron/second => we have dynamic EMF on A,B with frequency's120 seconds => we got AC .assume that rate of electrons flow (current) 1 electron/second .
what will happen ?
I think that electron moves from Ca to A until EMF of A equal to Ca .Electron also from B to Cb
until EMF of B equal to Cb => so in abstraction we have current flow from A to B.
But time for an electron left Ca or enter to Cb is depend on speed of electron why do they always lag 90 deg ?

 

[falbani]

smarty: please rewrite your message with proper punctuation marks and better organized (and leave blank lines to separate ideas)... I couldn't understand anything from it.

Sorry... :$

 

[BrockLee]

get spice and geda, They will allow you to simulate
(from a database they come with) electrical symbols.
so you don't have to guess how they work, because you will know what they do.