Caps

[greg123]

Hi,<p>I am new to electronics, and i have a capacator question. How does it pass ac but not dc. I have seen caps in dc circuits, and if they don't pass it, why use them? Or do they store dc and then realease it when discharged? THanks

[Chris Smith]

There is no electrical path for direct current to pass through. There is a dielectric insulator stopping a DC current from flowing.<p>In AC, a magnetic flux induces a current in the opposite side conductor to create a flux or magnetic push/pull which in turn affects the circuit on the other side to generates a AC current flow. <p>Think of it like a "transformer" where each winding is not directly connected, but still generates current flow on the opposite coils.<p>Its magnetically "induced" across the cap for simple sake or explanation. <p>When positive charged electrons is at one lead, negative charged electrons on the other half migrate to the other lead. When you pus/ pull, this becomes a Ac Current flow.<p>[ September 07, 2002: Message edited by: Chris Smith ]</p>

[Ron H]

A pretty good explanation can be found at http://biz.howstuffworks.com/capacitor.htm<p>However, when Mr. Brain gets to the discussion of DC blocking (passing AC), he gets it partially wrong:<p>"A capacitor can block DC voltage. If you hook a small capacitor to a battery, then no current will flow between the poles of the battery once the capacitor charges (which is instantaneous if the capacitor is small). However, any alternating current (AC) signal flows through a capacitor unimpeded. That's because the capacitor will charge and discharge as the alternating current fluctuates, making it appear that the alternating current is flowing." <p>The fact is, in a DC blocking application, the time constant is generally long relative to the period of the voltage waveform, and the capacitor charge will NOT change for all practical purposes, so when the voltage on one terminal of the cap changes, the other terminal follows by the same change, thereby passing AC. It IS true that if you apply a fluctuating (alternating) voltage to a capacitor in a circuit where the time constant is short compared to the period of the voltage waveform, the charge on the capacitor will change, and alternating current will flow.<p>Magnetism is NOT involved in this process.<p>And yes, in DC circuits, caps are generally used for storage in one form or another, although there are applications, such as in switching regulators, and frequency compensation in some feedback regulators, where they are used for AC purposes.<p>Ron
[ September 07, 2002: Message edited by: RonH ]<p>[ September 07, 2002: Message edited by: RonH ]</p>

[Chris Smith]

Ron, <p>Electro magnetism, flux, etc, are involved in ALL current flow applications. Current can not flow, with out it. Even migration of electrons involves the property of Magnetism. <p>Atoms, electrons, protons, etc, involve magnetism.<p>"so when the voltage on one terminal of the cap changes, the other terminal follows by the same change, thereby passing AC."<p>In actual fact, nothing passes, except magnetic coupling which induces electrons on the other side which already exist, to move. <p>There is no actual current flow, or electron transfer from one side to the other. The dia-electric material sees to that.<p>[ September 08, 2002: Message edited by: Chris Smith ]</p>

[HomeBrew]

Actually, there is not a magnetic field involved in a capacitors operation. It's an electro-static field. This field has little relationship to magnetic fields such as found in inductors. DC circuits use blocking caps. to stop a DC voltage while permitting a AC signal to pass,such as in audio circuits. The AC signal is not induced such as in an inductor, it is passed through the dilectric due to the agatation of the electrons. In high power circuits, this can cause the dilectric to heat. Capacitors pass AC currents because of the constant charging and dis-charging not found in DC current.

[bwts]

Im not sure how this property arises but in DC circuits capacitors can be used to 'smooth' supply voltages that are prone either to interference or to slow powerups also to protect sensitve circuits from potentally high DC sources. AC circuits use them for filtering of frequencies (radio tuners and TV channel selectors) mostly.

[Chris Smith]

"it is passed through the dilectric due to the agatation of the electrons" ?<p>Agitation? Not a term used in physics? <p>All fields of force, are magnetic. <p>Static, is magnetic. <p>Its called "Electro Magnetic".

[HomeBrew]

A electro-static field like the one in a capacitor dilectric, is a stationary field,unlike a magnetic field, which is a moving field unless it is from a permanent source. All fields are not purely magnetic. An electro- magetic field as defined moves or changes intensity.....a static field is one that does neither. It is set up by a stationary electric charge. Capacitors are not electro-magnetic devices. A capacitors dilectric field does not depend on moving charges after the field is set up.

[Chris Smith]

Electricity is a property of magnetism. <p>The Electron exists and in doing so ehibits magnetism. <p>Magnetism is what keep the atoms together, just like gravity is a "form" of magnetism. <p>Electro static simply refers to electrons stored, static, and not flowing.<p>Like any charged plate, they attract or repel through the magnetic field within each electron. <p>Basic Physics.
Basic electron theory.
Laws of nature.
Laws of physics. <p>You can find this in any one of these subjects or books.

[billdar]

hehe.. I've been reading this and cant resist...<p>If there are electrons, then you get an electric flux, and hence a magnetic field... Then one could resonably concieve that a charged capacitor pulled from it's circuit would behave like a magnet. And furthermore, a battery would have similar properties... A diode would too, without the charging...<p>I think not.<p>Magnatic Fields and Electric fields are linked, but they are not the same. Electric fields are created/destroyed when electrons are moving. A proportional magnetic field is created perpendicularly to the electric field.<p>ie, a wire with DC current trough it has an electric field similar to a cylinder radiating from the center wire to the cylinder wall. The magnetic field is perpendicular and orients itself to the direction of the current.<p>Once a system has stabilized (ie cap or battery) you are left with a potential between the plates, but no current. Hence, no electric/magnetic fields. <p>Try it for yourselves. Get a magnetic compass, a wire, battery, resistor, and a cap. Hook the components in series and put the compass next to it. You'll see it.<p>let the flames begin... <p>
ps. electrons = gravity = electricity = magnatism <-- thats funny stuff, I'm going to use it on my buddies.

[Ron H]

Billdar said what I was in the process of composing, but I'll throw in my (additional) two cents anyway.<p>Magnetic fields are created by moving charges. If the charges are static, there is no associated electromagnetic (EM) field - only an electrostatic one.
Do a Google search for "electrostatic flux". It is analogous to, but different from, electromagnetic flux. If current is flowing in a capacitor, there will be an EM field set up, but it is generally incidental to the use of a capacitor.<p>Ron<p>[ September 09, 2002: Message edited by: RonH ]</p>

[greg123]

So, the cap is blocking both AC and DC from going from one lead to the other directly, but the AC charges and discharges the cap so it appears to pass. The dc charge is charged on the cap, but there is no way to dischage it (with ac) so it "blocks" it from moving to the other side?

[Chris Smith]

Newbie basically Yes<p>To the other wanna be physics students, you havent passed your exams yet! Keep trying! <p>Each election relies on a magnetic field to hold it together, called orbiting the Nuclei. So where does it go when no current is flowing in the wire? Its still there,... If it disappeared, the electron wouldn't hold its orbit, and the atom would fall apart.<p> As for the compass, not even a good "one pony trick" because both negative and positive reside in the cap, thus attracting and repelling at the same time. <p>May I suggest a good "first year" coarse in either electronics, or preferably physics?<p>PS... Neg and POS refer to what?
Attraction and repeling refere to what?<p>Magnetic attraction is why lint is attracted to static charges, non moving charges. Magnetism is what pulls the electrons to the other side of the cap. Its ALL Magnetic and its created by the motion of the electron in orbit around the atom.<p>[ September 10, 2002: Message edited by: Chris Smith ]</p>

[billdar]

Hehe... the physics bully pushing around us wanna-be's. I guess the old addage is correct:<p>"Never argue with stupid people, they will bring you down to their level and beat you with experience."<p>I know I can't change minds (especially on internet) but I would like to plant the smallest seed of doubt in you rock solid beliefs. <p>I just have a hard time believing that I met the person that can difinitivly describe everything from quantum physics to planetary motion in terms of a capacitors function. A lot of people I know would be interested in such unifying physical principle.<p>Just for sh*ts and giggles I looked up some equations to see whats going on. Here's what I found.<p>
F= qE
F = force vector
q =charge of electron
E = electric field vector<p> E = k*Q/r^2
E = magnitude electric field
Q = total electric charge on target
r = distance
k = columbs constant<p>Looks to me like the electric field is a function of charge and distance. And you get a force vector associated with this electric field. maybe that force is what attracts the charged lint? Let's see...<p> F= qv x B
F = force vector
q = charge on electron
v = charge velocity vector
B = magnatic field vector
x = cross product (ie, qv*B*sin(angle) )<p>So, we can see that the magnetic field is a function of charge, velocity of that charge, and force. If the velocity = 0, then the magnetic field is 0 (at least the magnitude). This also shows that the greatest magnetic field is perpendicular to the electrical field, which would be difficult if they were the same.<p>I know, I cheated on this 'exam'. My cheat sheet/notes can be found in and around http://theory.uwinnipeg.ca/physics/<p>[ September 10, 2002: Message edited by: billdar ]</p>

[Chris Smith]

So what your saying is that the electron doesn't orbit, thus no motion, thus = 0? <p>Is this about right? <p>Nice formulae, but you applied it to the wrong thing! Try again?<p>Ps what is the "electrostatic force" that attracts? Wind? <p>Your flaming out bad, in fact you just crashed.<p>[ September 10, 2002: Message edited by: Chris Smith ]</p>