# Building intuition on the behavior of inductors and capacitors with both voltage and current

1. I understand that an inductor cannot change current instantaneously, however I am not sure how that relates to the voltage. In a high pass RL circuit, for example, I know that the inductor presents a high impedance to high frequency signals. However isn't this impedance only defined with the frequency of current through the inductor, and not the frequency of voltage? So do these RL circuits filter currents, and also filters voltages as a result of filtering currents?

When one says that an inductor has a "high impedance to high frequency signals", is this referring to both current and voltage, because both have the same frequency? Also, how does the EMF generated by the inductor to prevent sudden changes in current affect the output voltage of a filter? Does this have an effect on the filtering of the voltage with an RL circuit?

I tried to think of an RL circuit as a voltage divider with the inductor having an equivalent impedance at a certain frequency, however I am confused because I thought the impedance is only defined with the current frequency.

1. Secondly, I know that a capacitor stores energy as an electric field, as accumulated charge on one plate and the same magnitude of the opposite charge on the opposite plate. I also know that a capacitor presents a lower impedance to high frequency voltage, because with a high enough frequency the capacitor does not have time to charge up and accumulate an electric field in either direction, since the direction is always changing fast. Doesn't this depend on the size of the capacitor? Or is this only the case if a resistor is in series with it, because the time constant is RC? Also, what is the direction of voltage across a capacitor does not change direction quick, rather there is a voltage spike in the same direction as the DC voltage? We can't use the same analogy for constantly changing voltage direction right? How does a capacitor in this case supply current to "suppress"

If someone could provide an intuition on the voltage and current behaviors of inductors and capacitors, that would be greatly appreciated. I guess I have always been thinking of inductors and their relation to current, and capacitors with their relation to voltage, however not the other way around, and it is hard to find good intuition online because most websites associate inductors with current and caps with voltage. I have been trying to connect the dots on my own, however it has not been helping much. Thanks!

Beyond inductor, it seems you are confused by voltage / current.

An overall great analogy to electricity is water, it behaves quite similarly, as the current is the water current and voltage the pressure (height of the water).

While most components (resistor -> small pipe, capacitor -> cup) can be easily represented in this way, inductor is a bit more tricky, but you can imagine it as water spinning a wheel, for the water to get through it has to spin the wheel first, and the wheel if rotating will suck water, imagine this wheel to have momentum.

An inductor won't really have any effect on the voltage, what it does is resist the change of the current flow, very much like a heavy rotating wheel. Heavy to start, heavy to stop.

If you have an inductor with a varying voltage source, and no load, so no current, the inductor will have no effect.

When one says that an inductor has a "high impedance to high frequency signals", is this referring to both current and voltage, because both have the same frequency?

It refers to the current only, BUT, usually an inductor will be followed by a capacitor (typical buck converter, filters...

• Thus since the inductor slows down the current, the capacitor will take time to charge.
• Thus the voltage will be affected. In this situation, the voltage is the resultant of how much current passes through the inductor and charging the capacitor.

Also, how does the EMF generated by the inductor to prevent sudden changes in current affect the output voltage of a filter?

The EMF on the core acts like a spinning flywheel, when the current change, it has to change the field first, which takes time, very much the same as you try to change the speed of a flywheel as you are adding or removing angular momentum.

Does this have an effect on the filtering of the voltage with an RL circuit

Rather LC or RLC circuit, yes, because since the current cannot abruptly change, it will smooth out ripple.

For the capacitor, you can simply relate to water, as the current is the water current and voltage the pressure, and a capacitor a cup that you fill.

As you fill the cup, the pressure at the bottom will increase and you have a certain amount of charges in it.

because with a high enough frequency the capacitor does not have time to charge up and accumulate an electric field in either direction, since the direction is always changing fast. Doesn't this depend on the size of the capacitor?

It very much is.

Also, what is the direction of voltage across a capacitor does not change direction quick, rather there is a voltage spike in the same direction as the DC voltage?

The capacitor has to be discharged before it can be charged in the reverse direction, which will create a current "increase".