How would you explain the difference between impedance and resistance?

Question

to someone without very little background in electronics?

EXCELLENT answers. Probably the best I have ever gotten in any Yahoo question.

So kudos to everyone. Problem: I can't make up my mind which is the best one.

Logically, this calls for a vote.

Answer 1

For someone with little background, there would effectively be no difference.

There is the formal definition of resistance for DC, and impedance for AC. But if you think about it resistors are dimensionless, you can stick a resistor into an AC or DC circuit and it will offer the same resistance in either case.

The real reason for impedance being more useful in the AC case is because impedance stems more from electromagnetic effects:

In general, resistance is referring to the property of the material that electricity is passing through that causes electric energy to dissipate in the form of heat. Ie electrons bouncing around and hitting metal ions in the wire.

Impedance however, is in general referring to an 'effective' resistance that occurs due to electromagnetic effects.

For example, capacitors will have a high 'impedance' once they are near fully charged (fully charged it looks like an open circuit which is another topic). Effectively, it will look like a huge resistor is in the way. However, whereas resistors would just be dissipating the energy in the form of heat, capacitors would be storing it for later use, but impeding the current in the same fashion.

And in AC currents, there is also power dissipation due to capacitances in the wires.

Oftentimes, companies that use large loads of electricity (like car manufac. plants) will actually put capacitors on the return end of their powerlines, so they can match the capacitive effects of the powerplant, and return more of the electricity to save money.

Answer 2

Generally speaking, both of these terms are similar and describe the degree
to which an electrical device (resistor, capacitor, inductor, etc...) will
dampen the flow of electrons.

RESISTANCE ( R ): Units of Ohms... usually used to describe dampening of
current in DC circuits (w/ Resistors).
IMPEDANCE: ( Z ): Units of Ohms... usually used to describe dampening of
current in AC circuits (w/ Inductors and / or Capacitors).

Slight differences between IMPEDANCE and RESISTANCE.
Side note: Capacitors and Inductors IMPEDE the flow of AC current.

DC circuit example:
9v battery wired to a 9000 Ohm resistor will allow 1 mA of current to flow.

AC circuit example:
If you were to apply a 9VAC (rms) voltage to this same resistor you would
also have the same amount of current and power being dissipated by the
resistor. You would also refer to the 9000 Ohm resistor as a 9000 Ohm
RESISTANCE.

However, if you were to use any AC voltage source and hook it up to a
capacitor or an inductor you would then refer to the current dampening of
the AC current as an IMPEDANCE.

The following formulae will let you calculate the actual IMPEDANCE value.

Z(capacitor) = 1 / (jwC) ........... Ohms
Z(inductor) = jwL ............. Ohms

j = square root of (-1)
w = 2*pi* [ frequency of voltage source ] in radians / sec
C=capacitance in F [Farads]
L=inductance in L [Henrys]

Once calculated, if you are now wondering what the imaginary portion of the
result is...convert it to polar notation. This new result will give you an
impedance along with an angle (phase angle). This angle is simply a measure
of how the electrical current is either leading or lagging the voltage.

Answer 3

Although similar they are different

Electrical impedance or simply impedance, is a measure of opposition to a sinusoidal electric current. The concept of electrical impedance generalizes Ohm's law to AC circuit analysis. Unlike electrical resistance, the impedance of an electric circuit can be a complex number.

Electrical resistance is a measure of the degree to which an object opposes the passage of an electric current. The SI unit of electrical resistance is the ohm. Its reciprocal quantity is electrical conductance measured in siemens.

The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit.

Answer 4

I'm kinda surprised that nobody mentioned that impedance involves a frequency dependent phase shift between the applied AC voltage and the current flowing throuigh the impedance, or that the power actually dissipated by an impedance is *not* the simple product of the RMS voltage and current but also reguires that it be multiplied by the 'power factor' (cosine of the phase angle between the voltage and current).


Doug

Answer 5

Impedance is a property of components in an alternating current (AC) circuit; resistance, of those in a direct current (DC) circuit. A direct current flows in one direction only, and resistance measures a components resistance or opposition to this flow. An alternating current, however, changes direction cyclically (specifically, the current function is a sinusoidal curve) and impedance measures a component's opposition to such a current.

Answer 6

I wouldn't. Try changing the subject ;-)

Seriously, it's pretty hard to do if someone hasn't had DC and AC theory. If you really must do it, try using audio speakers as an example, and compare how different it would be than if you just hooked the speakers to a battery...impedance of speakers with AC might be 8 ohms, but DC resistance would be almost zero.

Answer 7

Only a resistor has resistance. A capacitor or inductor, which shifts the phase of an alternating current has impedance even if it has no resistance. Its resistance is an imaginary number of ohms.

Answer 8

Inductance and Capasitor which are addition to resistance are blocking the ac current. That is call impedance.
If there is resistors only, that is resistance.

Answer 9

Resistance is constant, it has no relation to signal or power frequency, whereas impedance is the resistance at a certain frequency.

Answer 10

Resistance is the resistance to current flow of Direct Current.
Impedance is the resistance to current flow of Alternating current flow.normally inductance resistance, capacitance.