Capacitive and Inductive
Reactance
Capacitive Reactance
A Capacitor at DC Conditions:
• Behaves like a battery
• Current flows into
the plates
• Plates are charged
• Capacitor will block
any further Current
flow
• Using a Multimeter
Resistance is very
high (M ohms)
Capacitor at A.C. conditions
• Using a.c. the direction
of the current changes
all the time
• The plates charge up
• Once the current changes
direction the plates
discharge then charge on
the other side (reverse
polarity)
• I leads V (leading current)
What is reactance
• Capacitor has got infinite resistance at DC conditions –
no current flows through the capacitor
• Yet when we apply a.c. the resistance seems to go down
as we observe a current passing through the capacitor
• The higher the frequency the lower the resistance
• Since this resistance is reacting to change in frequency
we call it “Reactance”
• The symbol for reactance is X or Xc for capacitive
reactance
Inductive Reactance
• A coil is pretty much the opposite to a capacitor
• At DC conditions a coil has close to 0 ohms
resistance
• As a current is applied to a coil it tries to resist
any change
• Once a magnetic field is built up the current
flows freely and unresisted
• When an a.c. current is applied – since coils
don’t like change the current seems to be
opposed
• The apparent resistance is called reactance
• The symbol for reactance is XL
Capacitive and Inductive
Reactance
Sample Calcs