In a DC circuit the current and voltage are steady values, and the equationcan be used because it is know what the values ofandare. In an ac circuit the current and voltage are constantly changing. There is more than one value of the current we could use – the peak current, the current at an instant, the average current, the magnitude of the average current or a quantity called the root mean square current. The one we would wish is one that makes calculations easiest, and if there are components with inductance and capacitance, which could could make calculations a great deal more complicated, this is accounted for. There is such a value of the current – the root mean square or rms current – for which- all the ordinary equations of dc electric circuit have simple ac equivalents.

dc |
ac |

The quantityin equations on the right hand or ac column is called the inductance. It is analogous to resistance. It represents the resistance of a circuit to ac current. It occurs because components like inductors and capacitors may not have anything like inherent resistance – though the windings of an inductor have resistance, it is in effect Lenz's Law that resists the flow of current, and a capacitor is effectively a break in a circuit, so it's dc resistance would be infinite.

The Impedance of a resistor is it's effective resistance. If there is no Capacitance or Inductance in a circuit the equations on the right hand side reduce to those on the left hand side.