Difference Between Impedance and Resistance

Impedance: Impedance refers to the opposition that a circuit offers to the flow of alternating current (AC), accounting for the combined effects of resistance, inductance, and capacitance. It represents the total opposition in an AC circuit and is typically denoted by $Z$. Impedance is measured in ohms (Ω).

Resistance: Resistance is a component that limits current in a circuit. Once a resistor is connected to a circuit, its resistance value remains fixed. Resistance is independent of frequency and is usually associated with direct current (DC) circuits, providing a constant opposition to current flow. It is also measured in ohms (Ω).

Impedance: In an AC circuit, impedance is the ratio of the peak voltage (or RMS voltage) $U_m$ across a passive network to the peak current (or RMS current) $I_m$. This ratio is known as impedance and is represented as $Z$. The higher the impedance $Z$, the lower the current $I$ for a given voltage $U$. Impedance combines both resistive and reactive components, which means it limits current based on frequency and circuit elements.

Resistance: The resistance of a resistor is typically influenced by factors such as temperature, material, length, and cross-sectional area. The relationship between resistance and temperature is characterized by the temperature coefficient, which defines how much the resistance changes with a 1°C increase in temperature.

Impedance: Impedance is a key parameter in audio and RF systems. For example, speakers are typically designed with an impedance of 8 ohms because this provides an optimal working condition for most audio amplifiers. However, speaker impedance is frequency-dependent, and the rated impedance is usually an average value. Common impedance values in the market range from 4 to 8 ohms for most speakers.

Resistance: A resistor with a near-zero value (like a large cross-sectional wire) offers little to no opposition to current, essentially creating a short circuit. In parallel circuits, a low resistance path can bypass other branches, resulting in excessive current. Conversely, a very high resistance can create an open circuit, resulting in no current flow.