Impedance vs Resistance: Understanding the Distinction in Circuit Elements

In summary: When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance...This statement is not quite correct...in a steady state condition, this is true, but when DC is initially turned on...or shut off...there ARE reactance effects.DaveIn summary, electrical impedance is the measure of the opposition that a circuit presents to the passage of a current when a voltage is applied. Resistance is used in a DC circuit, while impedance is used in an AC circuit. Impedance is determined from the inductive and capacitive reactances, while resistance is used in a steady state condition. Mutual inductance and self inductance can also contribute to impedance.
  • #1
digipony
35
0
Just wondering here... What is the difference between impedance and resistance? Is resistance typically used to describe the nature of a circuit element, while impedance is of an entire circuit? I have looked up the definitions for both, and I just don't see the difference between them. Thanks!
 
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  • #2
really basically...

impedance is used in an AC circuit ie ... resistance to an AC current
impedance is determined from the inductive and capacitive reactances

resistance is used in a DC circuit ... resistance to a DC current

look up
Characheristic Impedance
Inductive Reactance and
Capacitive Reactance

cheers
Dave
 
  • #3
Impedance rolls together resistance, inductance, and capacitance into one complex value.
 
  • #4
Electrical impedance is the measure of the opposition that a circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current (AC) circuit. Impedance extends the concept of resistance to AC circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude.

http://en.wikipedia.org/wiki/Electrical_impedence

Next line in wikipedia:

When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance...

This statement is not quite correct...in a steady state condition, this is true, but when DC is initially turned on...or shut off...there ARE reactance effects.

This is a nice/accurate synopsis from the above source:
"There are an additional two impeding mechanisms to be taken into account in AC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). "
 
  • #5
There is one more difference not so far mentioned.

Inductance comes in two flavours.

Self inductance
Mutual inductance

The contribution of inductance to impedance refers only to the former.
 
  • #6
Studiot said:
There is one more difference not so far mentioned.

Inductance comes in two flavours.

Self inductance
Mutual inductance

The contribution of inductance to impedance refers only to the former.

Even a network of pure resistors can have a similar effect, though; the voltage across one of the resistors can be a scaled version of the voltage across another resistor.
Mutual Impedance is a more general term for the relationship between the volts across one element of a circuit / structure and the current passing through another - see the theory of multiple element antennae.
 

FAQ: Impedance vs Resistance: Understanding the Distinction in Circuit Elements

1. What is the difference between impedance and resistance?

Impedance and resistance are both properties of circuit elements that determine the flow of current, but they have different definitions. Resistance is a measure of how much a material or component resists the flow of electrical current. On the other hand, impedance takes into account both the resistance and the reactance (ability to store energy) of a circuit element.

2. Can impedance and resistance be measured in the same units?

Yes, both impedance and resistance can be measured in ohms (Ω). However, the calculations for impedance and resistance are different, and they have different effects on the flow of current in a circuit. In simple circuits, resistance can be used to calculate current, while in more complex circuits with capacitors and inductors, impedance must be used.

3. How does impedance affect the flow of current in a circuit?

Impedance affects the flow of current by limiting or opposing it. In a circuit with only resistance, the current will decrease as the resistance increases. In a circuit with both resistance and reactance (due to capacitors or inductors), the impedance will determine the total opposition to current flow. This can result in phase shifts and changes in the amplitude of the current.

4. Why is it important to understand the distinction between impedance and resistance?

Understanding the difference between impedance and resistance is crucial in designing and analyzing circuits. In more complex circuits, such as those found in electronic devices, impedance plays a significant role in determining the behavior of the circuit. Ignoring impedance can lead to incorrect calculations and potential damage to the circuit.

5. How can I calculate impedance and resistance in a circuit?

Impedance and resistance can be calculated using Ohm's Law, which states that the current (I) in a circuit is equal to the voltage (V) divided by the resistance (R) or impedance (Z): I = V/R or I = V/Z. To calculate impedance, you must also take into account the reactance of the circuit, which can be calculated using the capacitance (C) or inductance (L) and the frequency (f) of the circuit. The formulas for calculating impedance vary depending on the type of circuit element, such as capacitors or inductors.

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