Ac voltage and current are often in phase, in something such as a resistor. However, they can become out of phase when passing through a Capacitor or Inductor. Capacitors are resistant to changes in voltage, causing voltage and current to get out of sync. Inductors are resistant to chagnes in current, causing voltage to get out of sync.
This getting out of sync is called reactance. The total reactance is called Impedance. Impedance is measured in Ohms, shown as Z
An inductor creates a magnetic field, which when the direction of the voltage switches, the field resists this change, creating a phase shift. This causes some resistance, which is why impedance is measured in Ohms. The phase shift, and therefore the resistance, is directly based on the frequency of the AC signal. The relationship is shown below
Xl = 2(pi)fL
Where Xl is the inductive reactance (in Ohms), f is AC signal frequency (in hz), and L is the inductance (in henery)
NOTE as the frequency increases, the inductive reactance increases
Inductors always make voltage lead current
A capacitor creates an electrostatic field, which swaps which plate of the capacitor each charge forms on, when the direction changes. As a result, this creates some resistance, measured in Ohms. The phase shift (in current) is based on the frequency of the AC signal. The relationship is shown below
Xc = 1 / 2(pi)fC
Where Xc is the capactive reactance (in Ohms), f is the AC signal frequency (in hz), and C is the capacitance (in f)
NOTE as the frequency increases, the capacitive reactance decreases
Internal impedance of a source limits its ablity to deliver power. A D cell for example has a lower impedance than a hearing aid battery, so despite both creating a voltage of 1.5V, the D-cell is able to deliver more current.
When the impedance is matched between two devices the power delivery is more efficent. Most ametuer equipment has an impedance of 50ohms, including antenna feed points, coax, and transceivers. When this impedance does not match, SWR is formed and can cause damage to the transceiver.
To match the impedance of two circuits, LC circuits are used to match the impedance. Two types are the pi and T network circuits, named such due to their appearance.
Impedance matching can also be performed by transformers. Impedance transformers do this exact task.