Reactance Question

[Pyaden]

Why is it necessary to determine Xc from measured values of current and voltage as opposed to measuring it directly with a digital multi-meter? Our first week in capacitors, I know how to calculate reactance, I know that mulimeters apply some voltage to check resistance.

 

[duke37]

Because the reactance is a function of frequency.

 

[Pyaden]

I realize it is affected by a change in frequency, but it is still a resistance value..;is'nt it??

 

[hevans1944]

I realize it is affected by a change in frequency, but it is still a resistance value..;is'nt it??

No it is NOT a resistance value. And for capacitors, your multimeter will indicate an open circuit (eventually) as the DC voltage supplied by the multimeter charges the capacitor. The multimeter may measure (probably will measure) a resistance for an inductor, but this is also NOT the reactance of the inductor, it is the ohmic resistance of the winding of the inductor. Also, reactance does not dissipate any power. Resistance always dissipates power.

 

[Pyaden]

That helps clear up the fog. I knew that the multimeter supplied voltage to check resistance but was a little usure how the capacitor reacted to the supplied voltage. Thanks very much...

 

[Colin Mitchell]

It's a resistance value but it is called AN EFFECTIVE RESISTANCE VALUE because it changes according to the frequency of the signal.

 

[Ratch]

It's a resistance value but it is called AN EFFECTIVE RESISTANCE VALUE because it changes according to the frequency of the signal.

Wrong, the resistance does not change with frequency. The impedance changes, but the resistance remains the same.

Ratch

 

[dorke]

I realize it is affected by a change in frequency, but it is still a resistance value..;is'nt it??

The units of reactance is indeed ohm units ,
like that of a pure(real) resistor.

But reactance is an "imaginary" resistance not a "real" one.
i.e. real and imaginary in the sense of the impedance plane.
Like so:


As mentioned before the X(reactance) values are dependent on frequency.
For a capacitor XC = 1/jwc= -J/wc ; |XC|=1/wc ohm
For an inductor XL = jwL ; |XL|=wL ohm

One very important thing to understand :
Reactance values are defined for the sinusoidal steady state activation of an inductor and capacitor from the general form of the relation between voltage and current through them.
iC=C*dv/dt
vL=-L*di/dt

same as for a resistor R=v/i
we have:
XL= vL/iL
XC=vC/iC

 

[hevans1944]

I realize it is affected by a change in frequency, but it is still a resistance value..;is'nt it??

No! It's only a "resistance" in the sense that reactance opposes (limits) the flow of current in AC circuits without undergoing any power dissipation. Real resistors also oppose (limit) the flow of current in AC or DC circuits, but in all instances real resistors dissipate real power in opposing the current: P = I²R. Reactance never dissipates any power because the current flowing through a reactance is always ninety degrees out of phase with the voltage across the reactance. This product of voltage and current is called VAR or Volts Amperes Reactive. When feeding power to real equipment, the current passes through real conductors that have resistance and those conductors DO dissipate power. This is why the VAR rating is important: you have to size the conductors to carry the maximum reactive OR resistive current, whichever is greater.

 

[Ratch]

The units of reactance is indeed ohm units ,
like that of a pure(real) resistor.

But reactance is an "imaginary" resistance not a "real" one.
i.e. real and imaginary in the sense of the impedance plane.
Like so:
View attachment 25525

As mentioned before the X(reactance) values are dependent on frequency.
For a capacitor XC = 1/jwc= -J/wc ; |XC|=1/wc ohm
For an inductor XL = jwL ; |XL|=wL ohm

One very important thing to understand :
Reactance values are defined for the sinusoidal steady state activation of an inductor and capacitor from the general form of the relation between voltage and current through them.
iC=C*dv/dt
vL=-L*di/dt

same as for a resistor R=v/i
we have:
XL= vL/iL
XC=vC/iC

Reactance is not imaginary resistance (whatever that is) or any other kind of resistance. Reactance and resistance both inhibit current, but they do it by two different ways. Resistance inhibits current by converting the charge energy density into heat, thereby causing an energy loss, which results in less current. Reactance inhibits current by causing a reverse voltage, which reduces the applied voltage, causing less current to exist in a circuit, but causing no net energy loss.

Ratch