We describe two principles of continuity. One for capacitors and also one for inductors.

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These two ethics are suspect by the $i$-$v$ equations because that capacitors and also inductors.

The principle of continuity of capacitive voltage says: In the lack of infinite current, the voltage across a capacitor cannot change instantaneously.

The dual of this is the principle of continuity of inductive current: In the lack of boundless voltage, the present through an inductor cannot change instantaneously.

Written by Willy McAllister.


Capacitor Inductor


When talking to an experienced engineer you might hear something choose “The voltage top top a capactitor cannot adjust instantaneously.”

What does this mean, and where does it come from?

An instantaneous change means the voltage transforms from to speak $1\,\text V$ to $2\,\text V$ in $0$ time. The voltage has an abrupt vertical step.

This can take place for a resistor. Ohm’s law places no border on how rapid $i$ and $v$ can readjust in a resistor. But that’s not the instance for a capacitor. Here’s why,

Capacitor $q$-$v$ equation

We determine the voltage ~ above a capacitor with this equation,

$q = \text C \,v$


$v = \dfracq\text C$

This is the fundamental definition of a capacitor. The voltage counts on the lot of charge $q$ save on computer on the capacitor’s plates.

Charge is constantly associated through some kind of particle, commonly an electron in the circuits us study. One electron is actual stuff the exists on the capacitor plates.

Suppose you try to make the voltage readjust instantaneously. You room saying that real stuff (a bunch that electrons) has to instantly show up or disappear. That happens in sci-fi movies, but not in actual life.

This is why us say the voltage ~ above a capacitor cannot readjust instantaneously. The voltage ~ above a capacitor never has actually an abrupt action up or down. It constantly changes gradually.

A capacitor is favor a bucket

A an excellent mental photo for a capacitor: A capacitor is prefer a bucket because that charge.


The stored electron $q$ are choose the water in a bucket. The voltage $v$ throughout the capacitor correpsonds come the water level in the bucket. The capacitance $\text C$ corresponds to the dimension of the bucket.

The water level (voltage) changes gradually together you to water water in or out of the bucket utilizing a water tap or ladle.

If you turn the bucket all the method upside down the water pours the end really fast, but it no disappear in $0$ time.

same story, with capacitor current

We have the right to reach the exact same conclusion about an altering voltage by thinking about capacitor current.

Current is the motion of charge. Let"s make the idea more formal. You deserve to define existing as,

$I = \dfrac\Delta Q\Delta t$

where $\Delta Q$ is part amount the charge and $\Delta t$ is a time interval. So existing is the amount of charge relocating past a point in some amount of time. Come measure existing in systems of amperes, we normalize the moment interval to one second. The lot of fee is measure in devices of coulombs.

If we let $\Delta t$ acquire smaller and smaller, the quantity of fee passing by gets smaller, too. In the limit, both $t$ and $q$ end up being "infinitissemally" small, (but no zero). And this is wherein we acquire the an interpretation of present in derivative notation,

$i = \dfracdqdt$

Now think about the present in a capacitor. $q = \text C v$ tells us exactly how to uncover the stored charge if we know the capacitor voltage. If us take the derivative the both sides v respect to time we"ll get,

$\dfracdqdt = \text C \dfracdvdt$

The left next is $dq/dt$, which we just decided was the an interpretation of current. Lets replace $dq/dt$ v $i$,

$i = \text C \dfracdvdt$

We just obtained the well-known $i$-$v$ partnership for a capacitor. The existing in a capacitor is proportional come the price of adjust (the slope) that the voltage throughout the capacitor.

Instantaneous change of voltage

A concern you could ask is, "what present do you require to reason an instantaneous change of voltage top top a capacitor?". An "instantaneous" readjust is a change from one voltage to another that happens in zero time. One minute the voltage is $1\,\text V$ and also the next minute BING! the is $2\,\text V$. That"s an instantaneous change. The adjust of voltage is some finite difference, prefer $\Delta v = 2-1$. The adjust of time is $\Delta t = 0$. The capacitor $i$-$v$ equation tells us what the existing has to be,

$i = \text C \dfrac2 - 10$

$i = \infty$

To achieve an instantaneous readjust of voltage calls for an limitless current!

As lengthy as you don"t have a source of boundless current, the voltage on a capacitor will not change instantaneously. That is the rule of voltage continuity for a capacitor.


We have the right to tell the same story around an inductor, but this time the story is around current instead of voltage. The principle of continuity because that an inductor is a little harder come understand due to the fact that it is based on magnetism, and also magnetism is constantly tricky to number out.

This is an example of a dual relationship. A dual is a really exciting idea i have seen largely in electronics; not too frequently elsewhere. Duality recognizes the similarity in between two relationships. It constantly involves a switcheroo in between variables. In this story, us exchange $\text C$ and also $v$ because that $\text L$ and also $i$. We finish up through a similar (but contrasting) conclusion. Dual is kind of hard to define, however I hope this instance helps you watch what it means. Duals are typical in circuit theory. When you acquire the concept, friend will begin to uncover them on her own. I always feel smart when I acknowledge a dual.

Inductor $i$-$v$ equation

The administer $i$-$v$ equation because that an inductor is,

$v = \text together \dfracdidt$

The voltage appearing across an inductor is proportional come the price of readjust $di/dt$ (the slope) of the present through the inductor. I’m no going come prove this equation because it’s pretty tough to do, but I want you to believe it anyway.

Compare the inductor’s $i$-$v$ equation come the capacitor $i$-$v$ equation, $i = \text C dv/dt$. They room duals of each other. Notice how they room similar, however we swap $i$ for $v$, and $\text C$ because that $\text L$.

$d$ notation

The $d$ in $di/dt$ is notation native calculus, it way differential. You deserve to think that $d$ as an interpretation "a tiny adjust in ..."

For example, the expression $dt$ method a tiny change in time. When you check out $d$ in a ratio, favor $di/dt$, that means, "a tiny change in $i$ (current) for each tiny adjust in $t$ (time)." one expression favor $di/dt$ is referred to as a derivative, and it is what you examine in Differential Calculus.

In calculus, $d$ to represent a little amount the change, so tiny it ideologies $0$. Periodically you will see change indicated v a $\Delta$ symbol. We use $\Delta$ to indicate a huge change, prefer $1$ meter or $1$ second. And also we usage $d$ to suggest tiny nearly-zero-sized change.

Instantaneous readjust of inductor current

Quite often, an inductor is component of a circuit that also includes a switch. It is really simple for the switch to “request” one instantaneous adjust of current. All you need to do is open up or nearby the switch. Throw the move from closed to open could mean a adjust of present going indigenous $100\,\textmA$ come $0\,\textmA$ in no time $(\Delta i = 0 - 100\,\textmA$ when $\Delta t = 0)$, because that example. Inductors don’t like this. Here’s why,

Let’s look in ~ the inductor $i$-$v$ equation for this example,

$v = \text together \dfrac\Delta i\Delta t$

$v = \text together \dfrac-100\,\textmA0$

$v = -\infty !$

To accomplish an instantaneous adjust of present requires an boundless voltage! Whoa!

If girlfriend don’t have actually a resource of limitless voltage, the present through one inductor will not change instantaneously. The is the rule of continuity because that an inductor.

In physics terms, over there is energy stored in the magnetic ar surrounding the inductor. That magnetic power cannot show up or disappear in an instant. It has to be included or taken away gradually, similar to the water in the capacitor bucket.

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This is why everyone says, “The current in one inductor never transforms instantaneously.” It constantly changes gradually.

Inductor references

Inductor - just how it functions Inductor kickback Inductor i-v equations Inductor kickback (1 that 2) Inductor kickback (2 the 2)


When we develop circuits through capacitors and also inductors we have to account because that time, since the $i$-$v$ laws for $\text C$ and $\text L$ include time. Two principles the continuity impose certain limits ~ above what can take place to voltage and current,

The voltage top top a capacitor can’t readjust in one instant. The existing through one inductor can’t adjust in an instant.