# Difference between impedance and resistance

Tags: Impedance, Resistance

Reduced to the most basic definition resistance applies to DC and impedance is the term used to describe "AC resistance" in which inductance, capacitance, resistance and frequency are all factors.

Now for the more complex part. There is no such thing as pure DC. When a DC circuit is first switched on there is a so called rise time during which the voltage and current rise to their steady-state. During this rise time (which is not instantaneous) stray inductance and stray capacitance both play a role. Typically the rise time is very short and could quite correctly be described as one half of a very high frequency AC square wave. As the steady state current and voltage are reached there is a very rapid change from the "high frequency" rise and the very low frequency (for all practical purposes zero frequency) steady state. That is the corner of the (nearly) square wave. That corner represents a very fast transition from super high frequency to effectively zero frequency. During that change a rapid scan through "all" frequencies occurs.

The opposite occurs when the DC circuit is switched off.

Keeping in mind that impedance is made up of capacitance inductance and resistance with frequency as a factor it can be argued that there is no such thing as pure resistance. That is in fact quite correct. In a similar way there is no such thing as a pure capacitor. They all have some inductance and they all have some resistance. Even an inductor has some stray capacitance and it is common knowledge that the wire that makes up an inductor (coil) has some resistance. The deviation between the theoretically pure inductor and what in practice has been achieved is referred to as the Q of the inductor. Similar concepts apply to capacitors and resistors.

At the end of the day the only real measure of "resistance" is impedance. The reality is that the effects capacitance and inductance of DC in steady state can safely be ignored hence the term resistance.

Said in other words resistance is impedance with factors influenced by frequency conveniently left out of the equation.

Now for the more complex part. There is no such thing as pure DC. When a DC circuit is first switched on there is a so called rise time during which the voltage and current rise to their steady-state. During this rise time (which is not instantaneous) stray inductance and stray capacitance both play a role. Typically the rise time is very short and could quite correctly be described as one half of a very high frequency AC square wave. As the steady state current and voltage are reached there is a very rapid change from the "high frequency" rise and the very low frequency (for all practical purposes zero frequency) steady state. That is the corner of the (nearly) square wave. That corner represents a very fast transition from super high frequency to effectively zero frequency. During that change a rapid scan through "all" frequencies occurs.

The opposite occurs when the DC circuit is switched off.

Keeping in mind that impedance is made up of capacitance inductance and resistance with frequency as a factor it can be argued that there is no such thing as pure resistance. That is in fact quite correct. In a similar way there is no such thing as a pure capacitor. They all have some inductance and they all have some resistance. Even an inductor has some stray capacitance and it is common knowledge that the wire that makes up an inductor (coil) has some resistance. The deviation between the theoretically pure inductor and what in practice has been achieved is referred to as the Q of the inductor. Similar concepts apply to capacitors and resistors.

At the end of the day the only real measure of "resistance" is impedance. The reality is that the effects capacitance and inductance of DC in steady state can safely be ignored hence the term resistance.

Said in other words resistance is impedance with factors influenced by frequency conveniently left out of the equation.

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