Inductors prevent change in current, capacitors prevent change in voltage - Page 1
Forum section: electronics / beginners

author: Pcmaker
author_rank: Regular Contributor
timestamp: November 04, 2017, 01:25:06 pm
content: In basic terms, is this accurate? I'm having a hard time wrapping my head around the basics of electronics and someone told me this and it's the best explanation I've had given to me.

author: Ian.M
author_rank: Super Contributor
timestamp: November 04, 2017, 02:24:42 pm
content: That's only true for infinitely large inductors or capacitors. However if you change it to "Inductors resist change in current, capacitors resist change in voltage" its accurate.It all comes down toV=L*dI/dtandI=C*dV/dT. The latter will be more familiar in its integrated form asQ=C*V.N.B. 'resist' as a synonym of 'oppose', not anything to do with Ohmic resistance.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 04, 2017, 04:52:23 pm
content: Water offers a good analogy if you are struggling imagining how electronics work.Imagine a tree trunk floating in a lake and now throw a stone at one side of the tree trunk, it will make waves but they will not pass the tree trunk except in certain special circumstance such as resonance or extremely large waves. This is exactly analogous to a capacitor between your signal wire and ground.Imagining logs, pipes, bags recepticles and baffles in various situations on/in/full of water covers most passive components one way or another and gives you a good broad grasp of the way things work.

author: kosine
author_rank: Regular Contributor
timestamp: November 04, 2017, 05:37:25 pm
content: Here's how I (try!) to explain it to people in simple terms...Think of a capacitor as an electron bucket. It has a cross sectional area (the capacitance) and a height (the voltage it's filled up to). But just like a water bucket, it takes a little time to fill to this voltage level, and that's determined by how quickly the electrons are flowing into it (the current - essentially electrons per second). The time it takes to discharge the bucket is likewise determined by the rate at which the electrons flow out. Obviously for any given flow rate, larger buckets (capacitors) are slower to fill/empty.Think of an inductor as a really long pipe (wire), usually wound into a coil to make it more compact*. The same current can flow through a wire of any length, but longer wires (physically) contain more electrons, and it takes a little time for them all to "get up to speed".Moving electrons have a kind of momentum known as magnetic flux. (Don't worry about the terminology.) Basically, longer wires have more electrons moving through them, so they contain more momentum. You have to put in energy to build up that momentum, so need to leave it hooked up to the voltage source for longer. After a certain period of time the momentum has built up and current will have increased to a maximum, as determined by the resistance of inductor.This is why long wires (inductors) delay any change in current - it's all that electron momentum being changed, which takes time. Very short wires don't have this effect to any noticeable degree, because they don't contain as many electrons: Change the voltage and the current changes almost instantly, just like in an ideal resistor. Longer wires contain more electrons and aren't so cooperative.So why would anyone want to use a really really long wire (i.e. an inductor?) One common use is generating large voltages. If you allow the current in an inductor to build up (basically give it some time), and then suddenly stop the flow (ideally instantly), all those electrons come to an abrupt halt. Their built-up momentum (kinetic energy) has to go somewhere, and it gets converted into potential energy, otherwise known as voltage. It's the electrical equivalent of water hammer and is the basic principle behind Tesla coils, and on a somewhat less dangerous level, many switching mode power power supplies. (Boost converters.)* I'm ignoring the magnetic effect as it tends to confuse more than enlighten. Adding an iron core to a wire coil basically makes it behave as if it were much longer than it actually is.

author: TimFox
author_rank: Super Contributor
timestamp: November 04, 2017, 07:15:24 pm
content: In my first electronics course, the professor summarized:"You can't change the voltage across a capacitor instantaneously (that requires infinite current) and you can't change the current through an inductor instantaneously (that requires infinite voltage)."After that, use the equations discussed above.

author: Zero999
author_rank: Super Contributor
timestamp: November 04, 2017, 08:46:57 pm
content: Quote from: TimFox on November 04, 2017, 07:15:24 pmIt isn't possible to change the voltage or current in anything instantaneously.

author: Dave
author_rank: Super Contributor
timestamp: November 05, 2017, 01:59:43 am
content: Quote from: Hero999 on November 04, 2017, 08:46:57 pmIt is in the plain mathematical model of a capacitor or inductor, ignoring the parasitics.

author: Brumby
author_rank: Unknown
timestamp: November 05, 2017, 02:15:45 am
content: The word "prevent" is too strong. Changes in inductor current and capacitor voltagedooccur - but they occur over time. How much time depends on the various circuit elements involved. It can be long or it can be very short - but that time cannot be zero.IMO, this is the best short answer so far:Quote from: TimFox on November 04, 2017, 07:15:24 pm

author: TimFox
author_rank: Super Contributor
timestamp: November 05, 2017, 08:36:08 pm
content: If you change the voltage across aresistorinstantaneously, the current will change instantaneously.Here, the two states have finite voltage.We're talking about idealized components here. If you like, you can express this as a mathematical limit, taking the limit as the transition time from one state to the other goes to zero. If that requires an infinite current (or voltage), then that solution is not physical.My statement was a simplification to guide the understanding of the effect of fast changes on the components. Specifically, if you have a C-R high-pass filter (series capacitor to shunt resistor) and you hit it with a fast voltage rise into the capacitor, then the initial behavior at the junction from capacitor to resistor is a fast voltage rise across the resistor.For full details, refer to the differential equations discussed above.