Help with very basic reaction confusion

[oldguyjohn]

Hi All,

Trying to understand reaction and realise I don't understand the basic concepts. Can someone please explain what I'm missing or doing wrong?
I worked out a simple reactive value for a small inductor using the standard text book formula.  I then assumed the value given in ohms had the same effect on the voltage / current as the equivalent value resistor.  Tried to verify by using a simple voltage divider, but the scope shows the peak voltages way off the calculated half way point?   I know the reactance changes with frequency but is a calculated reactance value different from resistive at a given frequency? I obviously don't understand the basic concepts.

Tried several times with different values of inductors, frequencies etc. never get the calculated result.  Same when I use capacitors with resistors (different formula of course). Chain of resistors just to get accurate value (tested with meter)

[IanB]

Unfortunately it's more complicated than simply treating a reactance like a resistance.

When you put an inductor in series with a resistor the combined behavior is more than the sum of its parts. You will need to look up AC circuit theory to find out the mathematics involved in calculating what happens with mixtures of resistors, capacitors and inductors in a circuit.

[Brumby]

In effect, reactance adds a time parameter (sort of) to the changes in voltage and current, so simple arithmetic doesn't cut it.

As IanB has said, AC circuit theory is where you will find the answers.

[Ammar]

Unfortunately, it is a little more complex than that (pun intended).

[Ian.M]

Yep. you need the concept of 'complex impedance' to make sense of it.  Although the simple formula for the reactance gives the magnitude (modulus) of the impedance, it doesn't give its effective phase angle (argument) so it cant simply be combined with a pure resistance in the classic potential divider formula.   Knowing that the capacitive or inductive reactance is effectively 'at right angles' to normal resistance, you can do some simple calculations on series circuits without resorting to full-on complex number theory, using Pythagoras' Theorum, but if you want to analyse anything more complicated than a simple series circuit of resistance and either capacitance or inductance, you'll need a fuller understanding of it.

See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/impcom.html
Once you have understood that, for a SINGLE FREQUENCY SINEWAVE, you can use the same rules as Ohm's Law and Kirchoff's Laws give you on circuits containing inductors and capacitors as well as resistors, just by using the complex impedance of each part where you would use its resistance in a DC circuit calculation.  Superposition allows you to theoretically handle more complex waveforms, but the maths rapidly becomes intractable as all the complex components of the impedance are frequency dependent and most non-sinusoidal waveforms are composed of a large or even infinite number of sinusoidal components.

[oldguyjohn]

Thanks to all who replied for pointing me in the right direction, and special thanks to Ian.M for explaining Ammar's pun.   It appears I will need to do a lot more study before I fully understand this.  A little embarrassed at asking this level of question especially when compared with the complexity of other 'beginner section'  questions.   Maybe there should be a 'Basics questions' section for those just starting out.

Teaching yourself a new subject can be difficult and things that must seem trivial to the more experienced can be very confusing, so many thanks to those on the forum who take time to answer raw beginners like me.

John

[Brumby]

A little embarrassed at asking this level of question especially when compared with the complexity of other 'beginner section'  questions.

We can all understand that - but don't be.  There have been some other questions here that were much more basic than yours.

Maybe there should be a 'Basics questions' section for those just starting out.

Teaching yourself a new subject can be difficult and things that must seem trivial to the more experienced can be very confusing, so many thanks to those on the forum who take time to answer raw beginners like me.

Too many sections make a forum too difficult to navigate, besides, that's what this section is all about.  It's enough of a challenge to get your question out there without having to worry about posting it in the perfectly correct section from a choice of dozens.  You can have a beginner who's just learning about Ohms Law - or a beginner who's been doing audio for years and is now getting into RF.

Yes, there are varying levels of 'beginner', but that's where the members here need to change their approach to suit.  If you feel the question you have is a 'beginner' question, then this is the right place to raise it.