The key difference between resistance and impedance is that Resistance is the opposition to current flow in both DC and AC systems, defined simply as R=V/IR = V/IR=V/I while, Impedance extends this concept to AC systems by including frequency-dependent reactance, making it a complex quantity represented as Z=V/IZ = V/IZ=V/I.
What is Resistance?
Resistance in electricity refers to the property of a circuit or a component within a circuit that converts electrical energy into heat while opposing the flow of electric current. This occurs due to collisions between the charged particles carrying the current and the fixed particles within the conductor’s structure. While resistance is most notable in devices like lamps, heaters, and resistors, it is present in every part of a circuit, including connecting wires and transmission lines.
The conversion of electrical energy into heat, even in small amounts, impacts the electromotive force (EMF) or driving voltage needed to produce a specific current through the circuit. Electrical resistance RRR is quantitatively defined as the voltage VVV (measured in volts) across a circuit divided by the current III (measured in amperes) flowing through it, expressed as R=V/IR = V/IR=V/I. For instance, if a 12-volt battery drives a 2-ampere current through a wire, the wire has a resistance of 6 ohms, calculated as 12 volts divided by 2 amperes. The ohm (Ω) is the standard unit of electrical resistance, equivalent to one volt per ampere.
Resistance in a conductor is directly proportional to its length and inversely proportional to its cross-sectional area. It also varies depending on the material of the conductor. When conductors are cooled to extremely low temperatures, some exhibit zero resistance and allow currents to flow indefinitely, a phenomenon observed in superconductors.
The reciprocal of resistance, denoted as 1/R1/R1/R, is called conductance, measured in units of reciprocal ohms, known as mhos.
What is Impedance?
Impedance, denoted by the symbol Z, quantifies the opposition to electrical flow and is measured in ohms.
In direct current (DC) systems, impedance and resistance are equivalent and defined as the voltage across an element divided by the current (R=V/I).
However, in alternating current (AC) systems, impedance incorporates “reactance,” which arises from the frequency-dependent effects of capacitance and inductance. Although impedance is still measured in ohms and expressed by the equation Z=V/I, both the voltage (VVV) and current (III) are influenced by frequency.
Resistance vs Impedance
The major difference between resistance and impedance is given below:
| Parameter | Resistance | Impedance |
|---|---|---|
| Definition | Opposition to the flow of electric current. | Total opposition to the flow of alternating current, encompassing both resistance and reactance. |
| Symbol | R | Z |
| Units | Measured in ohms (Ω). | Also measured in ohms (Ω). |
| Type of Current | Applies to both direct current (DC) and alternating current (AC). | Mainly pertains to alternating current (AC). |
| Dependence on Frequency | Independent of frequency. | Varies with frequency and includes both resistive and reactive components. |
| Components | Consists solely of resistance, a real component. | Includes both resistance (real) and reactance (imaginary), forming a complex quantity. |
| Phasor Representation | Represented as a real number in phasor diagrams. | Represented as a complex number in phasor diagrams, indicating both magnitude and phase. |
| Ohm’s Law | V = I × R (Voltage = Current × Resistance). | V = I × Z (Voltage = Current × Impedance), incorporating both resistance and reactance. |
| Energy Dissipation | Energy is dissipated as heat in resistive components. | Energy is dissipated as heat in resistive components and is stored and released in reactive components. |
| Example | A light bulb’s filament has resistance. | A circuit containing both resistors and inductors or capacitors has impedance. |
| DC Circuit Behavior | Determines the behavior of direct current circuits. | Reactance is irrelevant in DC circuits; resistance alone determines the behavior. |
| AC Circuit Behavior | Affects both magnitude and phase in AC circuits. | Impedance, including both resistance and reactance, influences both magnitude and phase in AC circuits. |
| Application | Used in both DC and AC circuits. | Primarily used in AC circuits where reactive components are significant. |