What is reactance?
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:
1 / (2 * pi * f * C)
where: 2 * pi = 6.2832; f = frequency in hertz and C = capacitance in Farads
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
