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# Resistance vs. Impedance

Can someone tell me the difference between Resistance vs. Impedance ?
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ZeusGod
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Commented:
RESISTANCE (R)- Resistance to the flow of current (Unit of measure: OHM)

RESISTANCE - The opposition to flow of charge through a material, expressed in ohms (). Resistance is similar in many respects to mechanical friction. The resistance of a wire depends on its material, length, thickness and temperature.

RT = Total Resistance
Series circuit RT=R1+R2+R3+... etc.
Parallel circuit RT=(R1xR2)/(R1+R2) or 1/[(1/R1)+(1/R2)+(1/R3)+(...)]

Resistivity of
various materials  (higher =
more resistance)
Silver 9.9
Copper 10.37
Gold 14.7
Aluminum 17.0
Tungsten 33.0
Nickel 47.0
Iron 74.0
Carbon 21,000

impedance

The resistance and reactance a wire or component offers to a change in current as the current runs down the length of the wire, also measured in ohms.

Thus the difference is as the current changes the resistance changes and this is expressed as ohms but is impedence.
Hope this helps Dave

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Commented:
Resistance is the ohmic (DC) impedement to currnet flow. Impedience is the AC component that takes into account both inductive and capacitive effects. Impedience tends to vary with AC frequency. Inductors pass DC but exhibit more and more impedience as the frequency increases. Capacitors block DC and show less and less impedience as frequency increases.

This is easily understood when you recall that electricity flowing has both an E (electric field) and H (Magnetic field) both at 90 degrees to the direction of current flow and each other. An inductor interacts with the magnetic field and the capacator interacts with the electric field.

If you were to plot inductive reactance (the equivalent 'resistance' of the inductor) on one axis and the the capacitive reactance (the equivalent 'resistance' of the capacitor) on the other axis the net impedience, Z, would be the vector sum of the two (solve the triangle). In any system the dynamic (AC) impedience is a complex interaction of these two values. At DC, the inductive reactance drops to zero (the coil of wire is essentially a dead short) and the capacative reactance is an open circuit (no DC flow between the plates other than dilectric leakage).

Some items have a characteristic impedience. For example, coaxial cable (RG-58U, etc), by carefully balanced design, has the inductive and capacitive aspects of it's electrical properties balanced so it shows a constant impedience over a wide range of frequencies (typically 50 or 75 ohms) but *NOT* at DC or other very low frequencies (ie audio) where it behaves like normal wire.

Got it?

M
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Author Commented:
But do people know why we have those resistance & impedance ? And also I need some website can show me the figure or chart about this. Thanks
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Commented:
Question is to broad in that do people know why resistance and Impedance. For those that design or troubleshoot electronics the answer is yes! Your question is not specific enough to know what you are trying to do with this information. Here are a couple of web sites, you can also use a search engine and visit different web sites to obtain possibly what you are looking for. Normally resistance is a given in that you can take an ohm meter and ring out a circuit and find out what the resistance is, or calculate using charts that define characteristics of certain components or wiring. Impedance requires special monitoring equipment or monitoring current and voltage and doing some calculations. Here are a couple of web sites:
Impedance http://computerservices.smsu.edu/Networking/impedanc.htm
http://www.scimedia.com/chem-ed/electron/defintns.htmhttp://assetteducation.com/Assettpg2.htm
Hope this is what you are looking for and it helps. Dave
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Commented:
Impedance is just used for electricity.  Resistance can be used for non-electrical stuff - example - He had resistance to buying a new car.

Impedance can depend on frequency.  Resistance does not depend on frequency.  You cannot speak of the "resistance" of a capacitor or inductor since they depend on the frequency of the current.  Capacitors and inductors exhibit impedance which depends on frequency.  A plain old resistor exhibits resistance (or impedance) which does not depend on frequency.

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Commented:
impedance is including the resistance R,if the cicuit is DC ,then the impedance will be the same as resistance (no changing in current flow directions so no effect for capacitors or coils reactances)
impedance (X),
(X)=Xr=R .
but in AC circuits impedance will be three parts:
1) resistance (R) 0 degree
2) capacitor reactance (Xc)-90 degree
3) inductor reactance (Xl)+90 degree
impedance (X):

(X)=Xr+Xc+Xl
in this case impedance will be MORE GENERAL than resistance because of reacting components like(capacitors and inductors effects) .hope this will clear the case.
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Commented:
noba,

Welcome to E-E!

One point of order tho. Here at E-E use of the "Answer" option is discouraged unless you are providing a definitive, absolutely correct answer. In normal operations we all post Comments because the questioner can accept any comment as an answer. Additionally selecting "Answer" moves the question from the "Awaiting Answers" category to the "Locked Pending Grading" category and reduces the number of experts who will see it. This does a disservice to the questioner when the "Answer" is not an absolutely correct response.

Although your answer is correct, it is both similar to other comments and does not fully delve into the computation of capacitive and indutive reactance.

ZeusGod,

All electronic components tend to exhibit all three aspects, resistence, inductance and capacitance. In a lot of elements one aspect will swamp the others, but normally all three are present.

Even a simple resistor will have a certain amount of inductance due to the leads and parasitic capacatance between the component body an the circuit card. Normally these values are negligible but in certain cases can become significant. For example some resistors are formed by winding a length of wire on a round insulator. The exact length of the wire controls the resistance value, but unless the wire is wound, folded back at the midpoint and wound back in the other direction the resistor will also exhibit a significant amount of inductance. Wether this is a problem or not depends on the application.

Computation of inductive/capacitance reactance (Xl, Xc) involves a 2pi*f term (circular frequency) that scales the reactance term by the frequency of the signal. I don't recall the exact formulas off the top of my head but you should check any good basic electronics text for AC Theory. This is fairly dense stuff for a short posting.

AS for what is Xl/Xc good for, well all sorts of tuned circuits use these values. Applications include radio, TV, cell phones, powerline filters/surge suppressors, telephones, all sorts of applications. In fact Xl/Xc rears it's head in most any application more complex than a flashlight.

M
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Commented:
I could provide you as others can more info, however as I indicated above "Your question is not specific enough to know what you are trying to do with this information". We are just spinning our wheels until we know more and how technical you are. I try to use the KISS principal in that "keep it simple sir" is how I answer. Later Dave
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Commented:
http://www.ti.com/calc/docs/act/92eng2.htm

Xl = 2 * pi * f * L

Where L is inductance in Henry's and f is frequency in hertz.

Xc = 1 / ( 2 * pi * f * C )

You can see that they're reciprocal effects with frequency.

M
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Author Commented:
OK,
I know about DC circuit. This is what I have:

U
I =---
R
and I also know about DC series and parallel.

But I have no idea about AC circuit. As what you post above, I can't understand how you guys get the formular. Can someone tell me any web site that prove or provide more detail about that formular or AC circuit. The problem I asked you about Resistance & Impedance because of my Network Class. I'm learning about Impedance but have no idead about how AC circuit works. Now I need a website that give me more information on AC and DC. Thanks
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Commented:
Formula is *definition* of Xl/Xc! Isn't *derived* from anything.

You've been given several sites. You need to read up on AC Theory.

M
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Commented:
http://ourworld.compuserve.com/homepages/g_knott/elect40.htm

http://www.audioreview.com/techtalks/general/messages/113681.html

We could fill this page with sites, however, better if you just do a search and this way you can read if it applies to what you want or go to next site. Later Dave
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Author Commented:
But why does it exitst ? Can someone give me some explanation on it ?
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Commented:
What do you mean by "why"?  Impedance is a consequence of Maxwell's equations, but why Maxwell's equations exist may be a philosophical question beyond the scope of this forum.
Simple resistance can be thought of as an infinite coax cable.
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Commented:
Flowing electricity (current) consists of current flowing at the speed of light along the axis of flow with a pair of interlinked waves, each 90 degrees to each other about the same axis. One is the electric wave, one is the magnetic wave. Electric waves interact with capacitors, magnetic waves interact with inductors. The current itself is opposed to it's flow by resistance. Thus the three aspects of electricity (current, electric wave and magnetic wave) are linked and are each represented by different terms in the various equations.

Current flow is represented by Ohm's law which is *defined* as E = I * R. The power law is P = I * E. From these you can solve for any given term in units of the other. The power law can be stated as P = I^2 R for example.

When a DC current flows the Electrical and Magnetic fields don't vary. They spring out of the conductor when the current flow starts and their magnitude and direction doesn't change as long as the current flow doesn't change.

In AC, the current is (usually) sinsodial It starts at zero, increases to a maxima, swings back thru zero to a minima and returns to zero. This is because moving a wire in the presence of a static magnetic field (ie from a magnet) induces a current in the wire. Mechanically it's convienient to rotate the loop of wire and the number of magentic lines of force cut by the loop changes in a sinsodial fashion, generating a sinsodial current flow. The varying electric and magnetic fields now interact with the world in ways that are different than the DC current does. The Terms Xl and Xc (Inductive Reactance and Capacitive Reactance) are used to describe the effects of the fields on coils of wires (inductors) and conductive plates separated by an insulator (capacitors). The inductor interacts with the magnetic field by Xl = 2 * pi * f * L and the capacitor reacts with the electric field by Xc = 1 / (2 * pi * f * C). Since a current generates *BOTH* the electric field *AND* the magnetic field, the two terms interact with one another. Together they generate a term called Impedience which is the vector sum of the two terms. To completly describe a circuit we must therefore descibe it both in the DC terms (resistance) but also in AC terms (impedence).

As you look at the spinning loop of wire in the static magnetic field that is used to generate the current the loop sweeps thru 2 * pi radians for each rotation - this is why there is 2 * pi in the reactance equations. To normalize the units to 'per second' we introduce the f term. Thus we can now talk about the frequency component or the AC (Alternating Current). If you look at the Xl equation, you'll notice it's effect gets LARGER with increasing f. This means that the impedience will start off low (0 when f = 0) and will increase as f increases. The " 1/ " portion of the Xc equation makes the effect of C undefined or effectively infinite at very low f's (an open circuit) and drops as f increases.

These results are also fairly intuitive if you look at inductors and capacator. An inductor is nothing more than a coil of wire consisting of one or more turns. Since wire has a low DC resistence, when we measure the ohmic value of a coil we read essentially a dead short (Xl = 0 when f = 0). When we look at a capacitor, it is made up of two parallel (non-contacting) plates separated by an insulator. Since there is no DC current path, a capactor reads essentially infinite (open circuit) when we meter it with our ohmmeter (Xc = infinity when f = 0)

The effects of Xl and Xc are very real and measurable. They are not trivial parameters that "drop out" of equations. If anything Xl and Xc are the dominant terms in most real world applications.

M
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Commented:
Following...
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Commented:
Wow! Back to high school and Delco electronics yearly training! Consumer electronics 101!

Mark, Thanks for that!

My \$.02: the easiest way for me to think of impedance is kind of an "angular resistance" meaning that the effect of impedance is similar to resistance, but resulting from capacitors or coils. I.E.- if resistance is thought of in terms of a water line...where your Voltage is a water line going left to right, resistance is a line going right to left, keeping the water in the line from going as far as fast, but increasing pressure on the water.....   Impedance is a line coming in at a 45 degree angle which keeps the water from moving as far as fast, but in a different way....simplification, I know and I'm sure mark can correct/add to help me in my oversimplification.

Rick
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Commented:
Used to teach this stuff. Wait until someone asks "how do diodes/transistors/IC's work?"... Can explain semiconductor effect completely without maths in a single class session.

I love the look on their faces at the end of the class when they all understand I *then* I tell 'em that they've just mastered the basics of "solid-state subatomic theory". They're all amazed that they're smart enough to understand such an "impossible" concept. If I introduce the class as "solid-state subatomic theory" they don't do nearly as well as they're *sure* they couldn't understand anything so complex sounding - so they don't. But if I just plunge in without preface they'll get it every time.

Water analogy only goes so far. Sometimes serves to confuse. Like the old joke about calling a technically illerate person and saying you're from the electric company and telling them that their E bill is high because the electricity is "leaking out" thru any outlet that doesn't have something plugged into it... :-)

M
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Author Commented:
Too complicated.
I will read & understand later.
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Commented:
ZeusGod,

You need to study!

It's only *six* paragraphs!

It's not complex because we want it to be, it is because it is.

You asked "But why does it exitst ?", thats as short and yet complete answer as I can give.

You didn't like the *four* paragraph answer.

M
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Commented:
Changing E fields cause B fields.
Changing B fields cause E failds.
E and B fields store energy.
(actually, magnetic fields are just a relativistic transform of electric fields)
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