What is Impedence? (along with Reactance and ohmic resistance)

[onetwothree4]

Yo whats up, being the 12.18am here yo

Been hearing people use the word impedence everywhere and can't be finding a good explanation for what it is, heres what i got so far from google
GOOGLE DEF:

1. the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

so it is something relating to a circuit/compoent's resistance to ac current...... but what the damn is ohmic resistance and reactance?

someone explain.. without the maths ?

 

[cowboybob]

Not a simple question.

...(impedance is) ... "the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance. ... so it is something relating to a circuit/compoent's resistance to ac current...

Yes.

but what the damn is ohmic resistance and reactance? ...

Ohmic resistance is a material's (carbon, for instance) opposition to the flow of DC electric current; measured in ohms (Ωs)

Reactance is the opposition to the flow of AC current caused by the inductance and capacitance in a circuit rather than by simple resistance alone. This is measured as Impedance, also expressed in Ohms.

Both restrict the flow of electrons (current) in a circuit. While this value is rather simple to determine for a DC circuit (Ohm's Law), it is considerably more difficult to determine in an AC circuit since the frequency of the AC signal plays a huge part in determining the reactance value.

Might also help for you to investigate how inductors and capacitors respond to DC and AC, both as individual components and as combined circuits.

<EDIT>4:37PM - Clarification.

 

[MrAl]

Hi,

It sounds like what you should do is learn what resistance is first, then go from there.
Resistance is more basic than impedance, and resistance is used quite a lot too so it is good to know more about this, and because it is part of impedance it will help you understand impedance later.
Learn Ohm's Law too, for resistances only, then go from there.

 

[crutschow]

Impedance is the combined effect of the ohmic resistance and reactance in the circuit, although sometimes it just refers mainly to the resistive component (as the input impedance is 75 ohms).
Due to the reactance component it can vary with frequency.

 

[Flyback]

Inductive reactance describes how when "changing" or "alternating" current trys to pass through an inductor it gets kind of slowed up, -as if it was wading through mud.

Capacitive reactance describes how high frequency currents, or quickly changing currents can easily fly through big capacitances.....the bigger the capacitance, the easier they can go quick through the cap......in fact, very high frequency currents can even fly quickly through small capacitances.

its as if high frequencies get their strap on jet pack when they go through capacitors.

iNDUCTORS "SLUg" current.
capacitors "slug" voltage.

 

[k7elp60]

Simple impedance is calculated by simple 90 degree right angle math. The vertical line is the reactance, the horizontal line is the resistance and the angle line is the impedance. Using electronic symbols, R for Resistance, X for reactance, and Z for impedance we have R^2 +X^2=Z^2. The reactance is an alternating current resistance of capacitors and inductors. The value of the reactance varies with frequency, or cycles per period of time.
I hope this helps your understanding.

 

[onetwothree4]

see see, I so far only know a bit about how capacitance and resistance works with dc current.So i will check that out before learning reactance and impedance.

Ok impedance is calculated with Pythagoras

 

[rfranzk]

Ok impedance is calculated with Pythagoras[/QUOTE]

Not really, Check here for a basic description of calculating inductive and capacitive reactance and then total impedance.

Look up phasor diagrams to see how phase angles with alternating current are graphed and represented.

 

[Flyback]

I think it should maybe said though that all this talk of impedance is a means to an end.....the first thing is to work out what that end is.......ie to make an electrical product that works.....the best way to get understanding is to actually do that and to tackle impedance etc along the way.
Remember that all of the standard laws on electronics are "strictly speaking", inaccurate.......electrons don't move through conductors carrying electricity.....all electrical energy actually transfers as EM fields, inside and around the conductor.....the standard laws of electricity that we know and love, are all "low frequency approximations"....at microwave frequencies, you can no longer get away with using them as they become to inaccurate.

In fact, all these laws are just "models" that we have invented to try and pin down electrical behaviour just so we can use electricity in applications.........they are now saying that all our models are inaccurate, and that its actually "string theory" which is commanding everything, but nobody understands that.

 

[Colin]

Here is an understanding for beginners.
Suppose you have a coil of wire wrapped around a piece of iron.
Connect one end to the 0v of a battery and touch the other end to say the 12v positive terminal.
What will happen is this:
When you touch the 12v on the coil, a current will flow in the winding and this will produce magnetic flux. This flux is called EXPANDING FLUX and it will cut all the turns of the coil and produce a voltage in the turns that has an opposite potential to the applied voltage.
The voltage in each of the turns will add up and maybe result in a "back voltage" of 11.9v. This means the forward voltage is not 12v but only 0.1v.
This is the voltage that is really entering the coil and because the voltage is so small, only a very small current will flow.
This is why the initial current is very small.
The magnetic flux keeps expanding and opposing the incoming voltage but as it increases, it is not as effective at producing the same amount of back voltage and thus the forward voltage increases and the current increases. But this current does not produce the same amount of back voltage and so the current increases further.
This keeps happening and the flux keeps increasing and the current keeps increasing but the flux does not produce the same amount of back voltage as it did in the beginning. Eventually we get to a situation where the flux reaches a maximum but it is not EXPANDING FLUX (but STATIONARY FLUX) and it no longer produces ANY back voltage . So all the 12v from the battery is accepted by the coil and it produces the maximum magnetic flux but no back voltage is produced.
That's just the beginning of the explanation.

 

[crutschow]

.........................................
When you touch the 12v on the coil, a current will flow in the winding and this will produce magnetic flux. This flux is called EXPANDING FLUX and it will cut all the turns of the coil and produce a voltage in the turns that has an opposite potential to the applied voltage.
The voltage in each of the turns will add up and maybe result in a "back voltage" of 11.9v. This means the forward voltage is not 12v but only 0.1v.
This is the voltage that is really entering the coil and because the voltage is so small, only a very small current will flow.
This is why the initial current is very small.
The magnetic flux keeps expanding and opposing the incoming voltage but as it increases, it is not as effective at producing the same amount of back voltage and thus the forward voltage increases and the current increases. But this current does not produce the same amount of back voltage and so the current increases further.
.............................

Unfortunately that explanation, while interesting, is not really correct.
Assume a coil with zero resistance.
When you suddenly apply 12V to the coil the resulting magnetic flux generates a back voltage equal to 12V (not 11.9V or any lesser value) according to the formula V = L di/dt where V is the back EMF, L is the coil inductance and di/dt (amps per second) is the rate that the current increases with time.
This increase in current will continue until/if the coil saturates.
At that point the inductance drops to zero and the current is now limited only by the circuit external series resistances.

(A good analogy is that inductance rather looks like mechanical inertia. A steady force suddenly applied to a stationary, friction-less, inertia mass will cause it to steadily accelerate at a constant rate from a dead stop as long as the force is applied.)

 

[Colin]

When you suddenly apply 12V to the coil the resulting magnetic flux generates a back voltage equal to 12V (not 11.9V or any lesser value)
This is entirely UNTRUE.

How can a coil produce a magnetic flux if the opposing voltage is 12v ???? ?????????????????????

 

[steveB]

I'd like to explicitly stress one thing that his been implied in all the good descriptions above. Impedance is something defined for pure sinusoidal signals only. Of course everyone has stated it's for AC and mentions frequency, and even phase, which implies pure sinusoidal waves. But, it's important to stress the point because impedance makes no sense without pure sinusoidal waves.

MrAl is correct to stress that one must understand resistance first because impedance is defined as an expansion of the idea of resistance. Ohm's law formula (V=IR) can be extended to sine waves (V=IZ) where the Z (impedance) is a complex number that captures the magnitude and phase relationship between the voltage sine wave and the current sine wave.

 

[KeepItSimpleStupid]

OK my turn and a little math and ASSUMING frequency is a PURE sine wave.

Resistance is the part of the circuit that follows ohms law = R = V/I for all frequencies measured in ohms.

Xc and XL are impedances that vary with frequency for capacitors and inductors. You can "cancel" the effects of capacitance with inductance and vice versa.

Capacitance tends to resist the change in voltage. Inductance tends to resist the change in current.

Simple Inductors are generally "coils of wire". Simple Capacitors are parallel plates separated by a dielectric (could even be air).

Models of a typical real inductor consists of an ideal inductor and a series resistance.

A model of a capacitor consists of an ideal capacitor in parallel with a parallel resistance and a series resistor.

We typically learn about DC circuits then advance to sine wave signals and finally advance to arbitrary waveforms.

We can't cover AC and DC signals in a single paragraph.

With sinusoidal AC voltages and sinusoidal AC currents, power calculations may not follow P=V*I. There will exist a phase difference between v(t) and i(t) and the direction of that difference is dependent on whether the capacitive or inductive terms dominates. For sine wave voltages and currents P=V*I*cos(theta).

The V and I used are the V and I that would exist if it were a DC circuit. This is called the RMS value or Root Mean Squared value. Not all AC meters read this value for arbitrary waveforms and frequencies.

 

[MrAl]

When you suddenly apply 12V to the coil the resulting magnetic flux generates a back voltage equal to 12V (not 11.9V or any lesser value)
This is entirely UNTRUE.

How can a coil produce a magnetic flux if the opposing voltage is 12v ???? ?????????????????????

Hi,

This is another one of those very very good points to bring up. I like to see things like this come up because in theory we often overlook some points in order to make a more important point clear.

In this case the original statement is actually TRUE, at least in THEORY. Note we have to add the qualifier, "THEORY" because that's what we want to use most of the time in order to calculate something about a circuit. We leave the PRACTICAL for later when we feel like looking into the finer points.

We can say the statement is true in theory because of:
V=L*di/dt

and also because the flux increase follows the current increase, and thus we have from Faraday:
V=N*d(phi)/dt

where phi is the flux.

What this means is that the voltage is proportional to the time rate of change of the flux, not the absolute level of the flux itself. This is very important because that means to know the voltage we have to know the time rate of change of flux not the flux itself.

The flux is just:
phi

while the time rate of change of flux is:
d(phi)/dt

so the time rate of change of flux is the first derivative of the flux, not the flux. The flux is proportional to current in a linear inductor, so we have:
phi=K*i

Now looking again at V=L*di/dt we can solve for di/dt:
di/dt=V/L

and so with a constant voltage of 12v and an inductance of 1 Henry we have:
di/dt=12/1=12 amps per second.

Another important thing to note is that this happens even at t=0+ which is an infinitesimally short time after t=0. This means any measurable change in flux occurs immediately after t=0 in theory, so right from the start we do actually have a change in flux and therefore we actually do have a voltage.
Putting this 12 amps per second back into the original equation we have:
V=L*12
and again with L=1 Henry we have:
V=12 volts.

So this is actually a simple concept but it requires using theory rather than actual practice, and we do that often and it comes out similar to what we really see in practice, with some usually minor alterations.
The alterations we are talking about here come in the form of the parasitics like inter-winding capacitance, and of course a little series resistance. This means some current will flow immediately, limited only by the series resistance, although its path will be through the capacitances anyway not the coil turns until some very short time later. But in theory we ignore this to simplify the expressions and get something that works very similar anyway.

We talked about voltage and current near t=0 in another place at one time, and because it has been proven that the force that voltage brings about on a charge occurs without ANY delay, in theory we can say that as soon as the voltage appears the force on the charges appears simultaneously. This leads us to believe that the charge starts to move instantaneously as well, and so we can always say that the di/dt is non zero at t=0+ for a pure inductance.

As Steve nicely pointed out however, when we talk about impedance we are restricting our analysis to the AC steady state solution and ignoring transient as well as DC solutions.

I must also point out again that i think we are getting a little too deep here for the OP's liking. I believe they would do well to get a firm grasp on how resistance works first, perhaps solving a few circuits with resistances and maybe some DC voltages. This is the usual coarse of study anyway...learn DC circuits first then move on to AC circuits.

 

[crutschow]

When you suddenly apply 12V to the coil the resulting magnetic flux generates a back voltage equal to 12V (not 11.9V or any lesser value)
This is entirely UNTRUE.

How can a coil produce a magnetic flux if the opposing voltage is 12v ???? ?????????????????????

Au contraire. It is entirely TRUE.
That is the whole purpose of the changing magnetic flux. It is the changing magnetic flux that is generating the opposing opposing voltage and that opposing voltage has to be equal to the applied voltage since that is all there is to generate the opposing voltage in an ideal inductor.

If it were less than that what is generating the rest of the voltage???????????????????????????????
And how would you calculate its value? Show us the equation for that.

 

[Ratchit]

Yo whats up, being the 12.18am here yo

Been hearing people use the word impedence everywhere and can't be finding a good explanation for what it is, heres what i got so far from google
GOOGLE DEF:

1. the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

so it is something relating to a circuit/compoent's resistance to ac current...... but what the damn is ohmic resistance and reactance?

someone explain.. without the maths ?

You should be thoroughly confused by now. Well meaning folks have been trying to explain impedance by describing how to calculate it, or explaining what causes it, or describing it in a philosophical way. Others are stating things that are just flat out plain wrong. I believe you want a correct definition of impedance, right? Impedance consists of several parts, but its overall effect is to impede the flow of electrical charge. Let's start out with several factoids about electrical impedance.

0. Current does not flow, it exists. Charge can flow, however. Current is charge flow.
1. Impedance consists of resistance and reactance combined in a particular way.
2. Reactance comes in two flavors, inductive reactance and capacitive reactance.
3. Both resistance and reactance are measured in ohms.
4. Both resistance and reactance impede the flow of charge carriers, but they do it by different methods.
5. Resistance impedes the flow of electrons by dissipating the energy electrons possess while traveling through a wire. During this time they encounter the ionic core atom of the conductor along with other charge carriers. This causes the electron to lose energy and the opposite particle to gain energy. This energy exchange causes the atoms to vibrate faster and the drift velocity of the electrons to slow down. Therefore the conductor heats up and the drift velocity slows, resulting in less current. The heat is lost energy from the electrical circuit.
6. Reactance impedes the flow of electrons by causing a back-voltage to form. This does not cause the circuit to lose energy. Instead energy is stored and released cyclically with no energy loss to the circuit.

So to iterate, impedance is a component of a circuit which slows down charge flow. After you realize this, you can study how different electrical components do this. Go ahead and ask a question, but see if you can find out the answer by reading and thinking first.

Ratch