# inductive reactance

Discussion in 'Electronic Basics' started by conrad, May 3, 2007.

1. ### conradGuest

With inductive reactance, the phase shift that
is induced where voltage leads current by
90 degrees or pi/2 radians, is the
voltage of the self-induced emf vs'
the current from our applied voltage?

2. ### Phil AllisonGuest

"conrad"

** You have mixed up two separate situations.

1. Where an AC voltage is APPLIED to an inductor causing current to flow
with phase lag.

2. Where an AC current is made to flow in an inductor from a source that
has a series impedance of its own.

In case 1, the reference AC voltage & phase is simply that of the source.

In case 2, the reference AC voltage & phase is that appearing across the
inductor.

........ Phil

3. ### conradGuest

And when they say voltage leads
current by 90 degrees, is this the
voltage from case 2 and the current
in case 1? or no?

4. ### Phil AllisonGuest

"conrad"
"Phil Allison"

** In either case, a 90 degree shift exists between the AC voltage *across*
and the current flowing * through* an ( ideal) inductor or capacitor.

The confusion exists is only in your mind.

........ Phil

5. ### Paul E. SchoenGuest

It may be easier to think of just the current lagging the voltage for
inductance, as this is what happens when you apply a voltage to an inductor
(which is usually the case). For a capacitor, it may be easier to
conceptualize voltage lagging the current. Leading and lagging are just
different ways of saying the same thing, but I find it easier to picture a
second quantity lagging (happening later in time than) the first quantity,
rather than the concept of leading, which implies the second thing is
happening before the first thing. I hope that helps, although in rereading
it might be even more confusing.

Paul

6. ### Rich GriseGuest

An inductance opposes a change in current, and a capacitor opposes a
change in voltage.

When you first apply a voltage to an inductor, the current begins to
rise, as the voltage decreases due to series resistance in the supply,
or by design in your circuit. (the series resistance drops more voltage,
leaving less for the inductor.)

When you feed that circuit with AC, the result turns out to be a 90 degree
phase lag for the current. i.e., when you change the applied voltage, the
current doesn't change right away, like it does with a resistor.

A capacitor is just the opposite - try to apply a voltage to it, and for
a very short time it looks like a short circuit - the current first goes
as high as it can, and the voltage can follow it as it looks less like
a short circuit - ergo, the voltage lags the current in/on a cap.

Hope This Helps!
Rich

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