Impedance — Definition

Definition

Imagine you're trying to push a car. If the road is rough, it's harder to push, right? That 'roughness' is like resistance in an electrical circuit. Now, imagine you're trying to push a car that's also attached to a giant spring or a heavy flywheel.

The spring might push back, and the flywheel might resist changes in its motion. In an AC (Alternating Current) circuit, electricity doesn't just flow in one direction like in a battery circuit (DC). Instead, it constantly changes direction, like a wave.

When this wavy current tries to pass through certain components, they don't just 'resist' it; they also 'react' to its changing nature. This combined opposition is what we call 'Impedance'.\n\nThink of it this way: In a simple DC circuit, only resistors oppose current flow.

This opposition is called 'resistance' ($R$). But in an AC circuit, we have three main types of components: resistors ($R$), inductors ($L$), and capacitors ($C$).\n\n* Resistors oppose current flow regardless of whether it's AC or DC.

They convert electrical energy into heat. Their opposition is pure resistance.\n* Inductors (coils of wire) store energy in a magnetic field. They resist changes in current. When AC flows, the current is constantly changing, so an inductor constantly opposes this change.

This opposition is called 'inductive reactance' ($X_L$). It's higher for higher frequencies and larger inductances.\n* Capacitors (two metal plates separated by an insulator) store energy in an electric field.

They resist changes in voltage. When AC flows, the voltage is constantly changing, so a capacitor constantly opposes this change. This opposition is called 'capacitive reactance' ($X_C$). It's lower for higher frequencies and larger capacitances.

\n\nImpedance ($Z$) is the total, overall opposition to current flow in an AC circuit, taking into account both the pure resistance ($R$) and the reactances ($X_L$ and $X_C$). It's not a simple sum because reactances also introduce a 'phase shift' – meaning the voltage and current waves don't peak at the same time.

Inductors make the current 'lag' behind the voltage, while capacitors make the current 'lead' the voltage. Impedance combines these effects into a single value, telling us how much the circuit 'impedes' the current and by what angle the current's phase is shifted relative to the voltage.

Just like resistance, impedance is measured in Ohms ($\Omega$).