It is a fact that in a parallel resonant circuit, the impedance is at

a maximum. Is this due to the fact that - at resonance - the energy

flows between the cap and coil are so large as to be able to repell

any current from the external energy source, thereby rendering its

path effectively blocked?

Steve, I've been watching your postings as you strive to understand

a somewhat difficult subject--AC circuit theory. I can see that you

are struggling with it, but I admire your perseverance! Keep at it,

and you'll eventually get it. I'll add my input to the help others

have been giving you.

I think it's helpful to realize that what are called two-terminal

circuit elements (R, L, and C are the fundamental components)

*enforce* a relationship between voltage and current. The voltage

*across* a component and the current *through* it are not independent.

It is what two-terminal components do; they establish a relationship

between voltage and current, *for that component only*.

So, if some two-terminal circuit elements are in series, the

current in each of them *must* be identical. If they are in parallel,

the voltage seen (applied across) by each of them *must* be identical.

Thus, if they are in series, the currents must be the same and

*only* the voltages across each can be different. If they are in

parallel, only the *currents* in each can be different; the voltage

seen by each is the same.

For components in series, since the current in all of them is the

same, it makes sense to use current as a reference, and speak of the

phase of the voltages *across* (not to ground) each component with

respect to the current through all of them.

For components in parallel, it is appropriate to use the voltage

*across* them as the reference, and speak of the phase of the current

in each with respect to the voltage across all of them.

Now, since for a C and L in parallel the voltage across the two

components is the same, only the phase of the currents can differ.

The current in one is 180 degrees out of phase with the other, and

when those currents are added by the parallel connection, they tend to

cancel. If the magnitude of the currents is identical, which is what

happens at a frequency such that the reactance of each is the same

(this is resonance), then we get complete cancellation of the currents

(for ideal L and C). Thus the current into the parallel combination

of the L and C is zero, even though we have applied some non-zero

voltage to the two of them. When we have a circuit that has the

property that no current (or very little) is produced with a finite

applied voltage, we say that the impedance of that circuit is high.

It's not that the parallel combination of L and C at resonance repel

the applied voltage. In fact, a current does exist in both the L and

C, but the two currents are 180 out of phase, and completely add to

zero at the connection of the L and C.

The same thing happens in a series resonant L and C circuit, but

with the roles of current and voltage reversed. For a given current

through the L and C, if the applied current is at a frequency where

the reactance of the L and C is the same, then the magnitude of the

voltage *across* the inductor and *across* the capacitor is the same

and since these voltages are 180 degrees out of phase (for ideal

components), they completely cancel (add to zero). Remember that the

current in the L and in the C is the same, since they are in series.

Thus we have a circuit with (almost) no voltage across it even though

a current is passed through it. We say that such a circuit has a low

impedance. This circuit doesn't repel the applied current; it's just

that the voltage *across* one component cancels the voltage *across*

the other, giving a resultant of zero *across* the series combination.

I hope this helps.