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Author Topic: In electricity, what is the difference between impedance and resistance?  (Read 8420 times)

Offline Joe L. Ogan

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In electricity, what is the difference between impedance and resistance?  Thanks for comments.  Joe L. Ogan


 

Offline peppercorn

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Impedance is a function of frequency. It relates to ac waveforms.
 

Offline Joe L. Ogan

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Does AC have resistance?  Does DC have imedance?   Thanks for comments.  Joe L. Ogan
 

Online evan_au

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Simple Electrical Circuits are often represented by Resistors, Capacitors, Inductors and Voltage Sources.
  • Resistors have a Resistance to DC, and an Impedance to AC which equals the Resistance at all frequencies.
  • Capacitors have infinite resistance to DC, and an Impedance to AC which reduces with increasing frequency.
  • Inductors have zero resistance to DC, and an Impedance to AC which increases with increasing frequency.
  • Voltage sources may be DC (simpler) or AC (more complex):
    • Direct Current voltage sources means that the voltage has a constant value, and current always flows in the same direction. DC circuits mostly have resistors and DC voltage sources. The current and voltage at each point in the circuit can be represented as "real" numbers. In a DC circuit, an inductor looks like a zero resistor, and a capacitor can be considered an infinite resistor (if you wait long enough).
    • Alternating Current means that the voltage is continually changing, usually in a "sine wave", which is shaped like ripples on a pond. The "amplitude" represents the amount that the top & bottom of the ripple differs from the "average" value. AC also has a "phase", which represents where the AC cycle is in its range from top to bottom (and back again) and a "frequency", which represents how quickly it cycles through these conditions. The current and voltage at each point in the circuit must be represented as "complex" or "imaginary" numbers ("imaginary" is an unfortunate and obsolete term that was coined before lots of really practical applications were found for them). AC currents can flow through capacitors, inductors and resistors to different extents depending on the frequency, and experience different phase shifts depending on frequency.
    • There is an intermediate form of "transient analysis", where you apply a DC signal to a circuit, and the voltage on capacitors and inductors jumps around for a while until it settles down to a normal DC circuit. This has some characteristics of DC and AC circuits

In one sense, a DC voltage source can be considered to be an AC source with zero frequency, which means it will easily flow through inductors, and somewhat easily through resistors, but not through capacitors.

Often, a voltage will represent a complex shape like a voice, which can be considered as DC plus sine waves of many different frequencies added together (Fourier analysis).

Practical circuits contain components like transistors, whose properties change over time, dependent on input voltage or current; this can produce even more complex behaviours like sustained oscillations or chaotic behaviour (or a Turning machine).
 

Offline alancalverd

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Impedance Z is the quotient voltage/current. In general Z is frequency-dependent and complex (current waveform may not be in phase with the driving voltage) but when the frequency is zero (a DC circuit) Z = R (resistance).
 

Online evan_au

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At DC, resistance is most important.

But when you deal with AC, impedance is a more "general" measure than resistance, in that impedance can represent resistance, but resistance cannot represent impedance.

Not all components are "ideal":
  • At DC, every voltage source has some internal resistance. Every wire has some resistance. Every inductor has a small amount of resistance, and every capacitor has a small amount of leakage current.
  • When dealing with high frequencies, every resistor also has a small amount of capacitance and inductance, and every capacitor has a small amount of inductance. So you need to consider the impedance of all components at these frequencies.
  • When you deal with very high frequencies (where the length of the wire is greater than around 10% of the wavelength), even the wires of a circuit will cause a significant phase shift, so the wires must be treated as transmission lines, as a combination of resistance, capacitance and inductance. Signals at these frequencies act more like vibrations propagating through a network of strings and springs.
 

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