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473
474

Why a capacitor blocks DC? (i.imgur.com)

submitted ago by mahendragr

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[–]BattleHardened 41 points42 points ago

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Can't explain that.

[–]willisan2069 1 point2 points ago

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Shocking

[–]ifyouwereahotdog 3 points4 points ago

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no miscommunication.

[–]EmperorSorgiva 13 points14 points ago

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Love it. My class just went over capacitors a couple weeks ago and this cheered me up. :D

[–]Nowihaveagun2 1 point2 points ago

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This might be the most innocuous comment I've ever seen. Why would anybody downvote it? Here's an upvote to get you out of the red.

[–]Contero 6 points7 points ago

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I have two wires that aren't connected at all, why does DC not travel through them?

[–]despaxes 2 points3 points ago

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Because Marvel is better.

[–]Fearlessleader85 0 points1 point ago

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But they're close together.

[–]shadowsoflife 12 points13 points ago

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As an engineering student with circuits classes, that picture makes me cry inside.

[–]DELTATKG 13 points14 points ago

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As an engineering student with circuits classes, this picture makes me lol so hard.

[–]aussie_bob 12 points13 points ago

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As an art graduate with circle glasses, this picture makes me think of a post-modern Hegelian dialectic disempowering current paradigms.

[–]chemistry_teacher 1 point2 points ago

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I am a chemistry teacher and what is this?

[–]AeBeeEll 3 points4 points ago

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As a former Engineering student, my professor showed me this same joke in class.

[–]TheKorovaMilkbar -2 points-1 points ago

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As a non engineering student, this also makes me lol.

[–]funnynickname 1 point2 points ago

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DC circuits - "Hey this isn't that bad."
AC circuits - "WHAT THE HELL, I DON'T EVEN...."

[–]red_cat_dicks 35 points36 points ago

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Q: How does a capacitor blocks dc current?

A: It is often said that capacitors block DC or equivalently, that a capacitor is an open circuit at DC. But is this true? Actually, no, it is not true. What is true is that the DC steady state current through a capacitor is identically zero. In DC steady state, all circuit voltages and currents are constant. By the fundamental capacitor equation:

i=C*(dV/dt)

we can see that this is the case.

Examining the equation answers your question, however. In steady state DC all currents and voltages are constant, so the deriovative of a constant voltage potential = 0, simple enough. On the other hand, when an AC voltage is applied, voltage is alternating at some frequency so this derivative is not zero (it's only zero at the peaks of the positive and negative cycle, at values of +- signal amplitude (it's just a sine wave signal) - everywhere else the derivative is non-zero so a current flows across the capactor by definition.

QED

Source: Yahoo Answers

[–]this1 26 points27 points ago

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Maybe an easier way of understanding this is as follows. A capicitor is two conductive plates seperated by a barrier.

The polarity on the plates is opposite, so when you have DC, this is fine, one plate stays +, the other -. AC though, creates negative and positive voltages, so on plate flips from + to -, as the other flips the other way, thus causing a current.

This is more an observation of the effect as opposed to an explanation of the cause, but whatever, the question sounded pretty basic, so I would think that would suffice.

[–]Wooknows 1 point2 points ago

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Ha, a real explanation of the real world and no math bs, shaggy2dope would approve.

[–]BonutDot 0 points1 point ago

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It's easy if you think of capacitors like buckets and electricity like water. A capacitor is a bucket that fills up with water, then dumps out ready to be filled again. DC is like a faucet, it always blasts water into the bucket and doesn't give it time to empty. AC is like a pulse-jet shower head, alternating between filling the bucket and resting. The bucket gets time to empty during the pulse of the jet, so it performs its function properly.

Tadaa!

[–]uptwolait 22 points23 points ago

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TIL Yahoo Answers actually has answers.

[–]threefifths 14 points15 points ago*

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You can get some good info from there as long as you're not looking up how babby is formed.

[–]uptwolait 6 points7 points ago

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They need to do way instain mother>

[–]Narissis 6 points7 points ago

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...who kill their babby, because those babby can't frihgt back...

[–]FreeBribes 0 points1 point ago

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The equation only defines what happens - in math terms, derived from the reality of how a capacitor works... It doesn't actually say why the capacitor does what it does.

[–]Pinhedd 8 points9 points ago

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I'd like to add, since this is the only actual explanation here, that when a DC system is in steady state that means that nothing more is changing, more appropriately the capacitor is at peak capacity (or as close as it will go) and thus the electric field generated by the charge on the capacitor plates cancels out the electric field generated by the voltage source. Effectively, in steady state the capacitor has a voltage drop equal to the voltage gain in the relevant circuit terminals while dissipating no power (the power is stored in the electric field) and passing no more current.

If the circuit starts out in zero state (no voltage applied, no charge on the capacitor) and a EMF is applied to the circuit, the circuit will not be in steady state until the capacitor is fully charged and thus, current will flow through it.

[–]red_cat_dicks 1 point2 points ago

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These three big comments (yours, this1's, and mine) sum up a good portion of circuits in physics class. Good show!

[–]UncertainHeisenberg -2 points-1 points ago

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It took five years to complete a microelectronic engineering/computer science dual! Those bastards must have fabricated all the other stuff they taught me. And here I was thinking it was all useful cos it works...

[–]VernonLittle -1 points0 points ago

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What is happening here? A guy paid a compliment to two other commenters, suggesting that they basically covered highschool physics circuits with their comments, and then you showed up to tell everybody that you took CS/EE and are smarter than everybody else? Please.

[–]UncertainHeisenberg 0 points1 point ago

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Who said I was smarter? I was just trying to indirectly say that there is a lot more to these things than people may think. I was joking, not trying to be offensive, and upvoted some of the descriptions.

[–]apersoncommenting 1 point2 points ago

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tl;dr: Impedance Z = 1/(jwC) = infinity for jw = 0 (DC). Infinite impedance means no current flow.

Some things are simpler in the frequency domain.

[–]CaptainExtravaganza 1 point2 points ago

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I beg to differ.

You have not QED as I still don't understand and you have therefore demonstrated shit all.

Argue with that logic.

[–]ceri23 1 point2 points ago

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Well done red_cat_dicks

[–]uzsbadgrmmronpurpose 1 point2 points ago

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Improper use of QED which means "which was to be demonstrated" and is usually used at the end of a mathematical proof, not an answer to a question. I know you didn't write it, just FYI

[–]red_cat_dicks 0 points1 point ago

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Good to know. Thanks.

[–]triplecats 2 points3 points ago

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As an EE, I approve of this.

[–]goofoffering 2 points3 points ago

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So that is how electrons get through the double slit.

[–]diviave 9 points10 points ago

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This post is too current

[–]saffir 7 points8 points ago

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Watt should we do about it then?

[–][deleted] 4 points5 points ago

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We should resist it

[–]Xarddrax 5 points6 points ago

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I'm amped about that idea!

[–][deleted] 2 points3 points ago

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IC what you did there

[–]Brody_Broderson 1 point2 points ago

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Let's Re-Volt!

[–][deleted] -1 points0 points ago

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Yes, LEDs

[–]diviave 0 points1 point ago

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Nope Chuck Tesla

[–]BlueberryButts 4 points5 points ago

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As a person with little to know electrical circuitry experience or knowledge,

Doesn't know if trolling

Or possibly legitimate.

[–]apersoncommenting 1 point2 points ago

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I think there's a corresponding one for an inductor somewhere (involving the DC current being able to go straight-through while the AC current gets tangled up in the coils). But of course it's not exactly the opposite case since AC current still flows through inductors (Z = jwL)...

[–]guttata 1 point2 points ago

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That doesn't sound right, but I don't know enough about capacitors to dispute it.

[–]MeltBanana 1 point2 points ago

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I have no idea if this is right/wrong/irrelevant/funny.

[–]putyourdumbnamehere 1 point2 points ago

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AC doesn't give a shit.

[–]evdev 0 points1 point ago

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I hate when my AC starts eating snakes.

[–]PlacentaSoup 1 point2 points ago

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thats how it was taught to me when I was in high school

[–]cheekytriangle 3 points4 points ago

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Is this... Wrong? What's a capacitor? Wait, how do magnets work?

[–]autom8r 2 points3 points ago

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They asked why, not how, BTW.

I think a better answer to why is that DC's a prick and AC always brings the bitches.

[–]apersoncommenting 1 point2 points ago

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Unless you're an inductor and swing the other way =P

[–]evdev 0 points1 point ago

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MAN BITCHES

[–]DarthContinent -1 points0 points ago

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ಠ_ಠ

[–]DarthContinent 1 point2 points ago

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Blocked. Again.

ಥ︵ಥ

[–]average_AZN 1 point2 points ago

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EE here, capacitors never block current... Even with DC they still charge. to say they block all DC current is just flat wrong.

[–]TheRemix 3 points4 points ago

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At steady state they do. Ignoring any negligible possible leakage current of course.

[–]average_AZN 0 points1 point ago

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Well of course, but saying they ALWAYS block current is what i have a problem with

[–]evdev 1 point2 points ago

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You also just said they NEVER block current, which I have a problem with.

[–]TheRemix -1 points0 points ago

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EE high five!

[–]cavercody 1 point2 points ago

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EE here. First, this is a joke. I laughed.

Second, is there such a thing as true DC? In any realizable circuit, DC is turned on at some point in time, often modeled with the unit step function. But if you want to get an unrealizable situation and say there has been DC since t -> -infinity and will exist to t -> infinity is there ever current flow? It's the same principle around monochromatic light or ideal brick wall filters.

[–]canton7 1 point2 points ago

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It's a simplification to make the things easy to deal with. Call it an Engineering Approximation, if you will.

In real life, nothing corresponds exactly to the models. The models only hold given a set of assumptions, which might never all be valid. Doesn't make the model worthless, though.

[–]canton7 0 points1 point ago

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Capacitors never let current pass through them, which can be interpreted as saying they block current.

Of course they can still charge, but since current isn't passing through, can this be interpreted as "not blocking"? Depends who you're talking to.

[–]bobbinika 1 point2 points ago

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I had a teacher in tech school draw cartoons. DC was a tiny guy so he couldn't jump across the gap but he was small enough to walk through inductors.

AC was a tall guy and could just step over the gap but too talk to walk through an inductor.

best teacher ever

[–]YourCreepyUncle -1 points0 points ago

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All this time I thought that teaching the subject in a factually accurate manner is what makes a good teacher, when really it's just the ability to draw cartoons and make up explanations so dangerously inaccurate that they make Yahoo Answers sound like MIT.

[–]bobbinika -2 points-1 points ago

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hahaha but his cartoons were the best and its not like you need to know electronic principles in the Air Force.

[–]grud315 1 point2 points ago

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As a student studying electrical and computer engineering, it is good to know that there are students out there in the same field who are dumber than me. Lets me have the good jobs.

[–]themali 1 point2 points ago

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Or more creative then you...

[–]BlueberryButts 1 point2 points ago

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As a person with little to know electrical circuitry experience or knowledge,

Doesn't know if trolling

Or possibly legitimate.

[–]SanDiablo 1 point2 points ago

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That doesn't sound right -- but I DON'T KNOW ENOUGH ABOUT CAPACITORS TO DISPUTE IT.

[–]clumsymechanic 0 points1 point ago

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if any current flows through a capacitor then it's said to be leaking. IE a capacitor does not allow current to flow through the dielectric barrier.

charges can accumulate and leave the conducting plates, causing a temporary flow of current in or out of the leads, but can not travel from one lead to another. if potential accross the device is alternating with zero net bias in either direction, current will flow in and out of each plate of the device. this current will be out of phase with the applied voltage and will not at any time be flowing through it.

[–]4ofN -2 points-1 points ago

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nice pic. too bad it doesn't illustrate the principle at all.

[–]TrollKy 28 points29 points ago

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thatsthejoke.png

[–]crosscut 1 point2 points ago

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It does get the idea across.

[–]Projectile_Muffin -1 points0 points ago

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Care to elaborate?

[–][deleted] 0 points1 point ago

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Makes sense.

[–]FuriousJ 0 points1 point ago

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Seems legit.

[–]deyv 0 points1 point ago

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This is fucking genius.

[–]iamstarwolf 0 points1 point ago

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AC what you did there

[–]aslyter45 -1 points0 points ago

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I'm from r/trees. I'm really confused right now. Um.

[–]evdev 0 points1 point ago

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Not trying to "shoo" you off, but this isn't the thread you're looking for.

[–]aslyter45 -1 points0 points ago

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I'll find my way home now.

[–]evdev -2 points-1 points ago

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Good travels to you, sir! Don't trip on the math on the way out!

[–]aslyter45 0 points1 point ago

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Much obliged.

[–]snakrause -1 points0 points ago

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Yupp, that stands up to scrutiny.

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