Reactance is the property of resisting or impeding the flow of ac current or ac voltage in inductors and capacitors. Note particularly we speak of alternating current only ac, which expression includes audio af and radio frequencies rf. NOT direct current dc.

Inductive Reactance

When ac current flows through an inductance a back emf or voltage develops opposing any change in the initial current. This opposition or impedance to a change in current flow is measured in terms of inductive reactance.

Inductive reactance is determined by the formula:

2 * pi * f * L

where: 2 * pi = 6.2832; f = frequency in hertz and L = inductance in Henries

Capacitive Reactance

When ac voltage flows through a capacitance an opposing change in the initial voltage occurs, this opposition or impedance to a change in voltage is measured in terms of capacitive reactance.

Capacitive reactance is determined by the formula:

Some examples of Reactance

What reactance does a 6.8 uH inductor present at 7 Mhz? Using the formula above we get:

2 * pi * f * L

where: 2 * pi = 6.2832; f = 7,000,000 Hz and L = .0000068 Henries

Answer: = 299 ohms

What reactance does a 33 pF capacitor present at 7 Mhz? Using the formula above we get:

1 / (2 * pi * f * C)

where: 2 * pi = 6.2832; f = 7,000,000 Hz and C = .0000000000033 Farads

Answer: = 689 ohms

Now in the real world we don't use big numbers like that, we use exponentials on our pocket calculator to get numbers like this:

For inductive reactance

where: 2 * pi = 6.2832; f = 7 X 10⁺⁶ Hz and L = 6.8 X ^-6 Henries

Answer: = 299 ohms

For capacitive reactance

1 / (2 * pi * f * C)

where: 2 * pi = 6.2832; f = 7 X 10⁺⁶ Hz and C = 33 X ^-12 Farads

Answer: = 689 ohms