Reactance is a measure of the opposition presented to alternating current by inductance and capacitance in electrical circuits. It is one of two elements of impedance, along with resistance. While resistance opposes the flow of current in a circuit, reactance changes the phase and amplitude of the current and voltage waveforms. Reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element. It is measured in ohms, with positive values indicating inductive reactance and negative indicating capacitive reactance. An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance. There are two types of reactance: inductive and capacitive. Inductive reactance is associated with the magnetic field that surrounds a wire or a coil carrying a current, while capacitive reactance is associated with the electric field that surrounds a capacitor. The inductive reactance depends on the supply frequency and the inductance of that element, while the capacitive reactance decreases as frequency increases. The total reactance of a circuit element is the summation of inductive reactance and capacitive reactance.