# Alternating currect

1. Aug 24, 2006

### momomo_mo

I want to ask the relationship between "IMPEDANCE" and "REACTANCE" with resistance in the simple R circuit.
I don't know why we should create so many terms .They are all come from OHhm's law(ie V=IR)
Is the difference is due to voltage and current is at the peak in impedance and reactance??
Also,I see some reference books stated that impendance =Vrms/Irms??
Whcih one is correct?

Last but not least, Can anybody help me to clarify this sentences concerning imedance----the ration V/I is defined as the impedance of the circuit which measure its opposition to a.c.???

Thank you

2. Aug 25, 2006

### Clausius2

The Impedance is usually defined as a complex number $$\mathcal{Z}$$ such that $$\mathcal{U}=\mathcal{Z}\mathcal{I}$$. The real part of the impedance is the Resistance, and its imaginary part is the Reactance. The Reactance is caused by coils and condenser in a circuit and their magnetic fields, which generate Reactive Power. Those elements not only cause a change in modulus of voltage and current, but also a shift on the phase of the waves of both of them.

3. Aug 25, 2006

### rbj

momomo_mo, do you know about what we electrical engineers call "phasors" (not to be confused with Star Trek ray-guns)?

judging from these two questions, i am not sure what it is that you do know, so to find the best answer (and to save time) for you.

4. Aug 25, 2006

### Clausius2

The thing of the star trek is funny. Yeah, just to clarify, my big U and big I (complex numbers) are in this context phasors.

5. Aug 25, 2006

### momomo_mo

I am a secondary school students and I do not need any professional answers.
I want to know Last but not least, Can anybody help me to clarify this sentences concerning imedance----the ration V/I is defined as the impedance of the circuit which measure its opposition to a.c.???

Also,mathematically what are imepdance and reactance defined?? Vo/Io
(o--peak) or Vrms/Irms or V/I (at any time instant) ????

THank you

6. Aug 26, 2006

### NoTime

V/I is the impedance of any circuit DC or AC.
In a steady state DC circuit resistance = impedance.
In an AC circuit impedance = resistance + reactance.

Peak is just that. The maximum voltage peak of the AC voltage swing.
RMS is the DC equivilent voltage of the Peak V.
To understand reactance you nead to learn how to deal with vectors.

7. Sep 3, 2006

### Slayer26M

Impedence is the total opposition to current flow. You can add resistance and reactance vectorally or just do an impedance triangle. Resistance is your x axis, reactance is your y axis, and the impedance is your hypotenuse, and the angle between converts to your power factor. But as Notime said, you should learn about vectors, and how inductive and capacitive loads affect the circuit. (ELI the ICE man, lead and lag, and all that other stuff we electricians have to learn but never use in the field)