The document explains the behavior of capacitors with direct current (DC) and alternating current (AC). It highlights that capacitors resist changes in voltage, exhibiting capacitive reactance, and explains how they charge and discharge differently based on the type of current. Importantly, capacitors block DC while allowing AC to pass through, with reduced reactance at higher frequencies.

2. Original charges on a capacitor
+ + + +
+ + + +

3. DC current starts to flow
+++
+++
+
+
++ +
++
+ + + +
+ + + +
-

4. Charging slows as capacitor
reaches supply voltage
+
+++ +++
+++++ +++++
-

5. charged capacitor
+
++++ ++++
+++++ +++++
-

6. Capacitive Reactance
• As charges build up on capacitor plate, an
electric field is established because like
charges try to repel each other.
• As an electric field builds up on a
capacitor plate, the more the capacitor
opposes the source voltage pushing
additional charges onto the capacitor
plate.
• Opposition to voltage changes by a
capacitor is called capacitive reactance.

7. Original charges on capacitor
+ + + +
+ + + +

8. AC current starts to flow
+++
+++
+
+
++ +
++
+ + + +
+ + + +
-

9. Charging slows as capacitor
reaches supply voltage
+
+++ +++
+++++ +++++
-

10. charged capacitor
+
+++ +++
+++++ +++++
-

11. AC current reverses direction
+ ++ ++ +
+ ++ ++ ++ ++ +

12. Capacitive Reactance and AC
• With AC, capacitor plates alternate
between filling and emptying.
• If the voltage changes direction before the
capacitor plates are either full or empty,
then there is less reactance (opposition) to
the changing voltage.

13. Original charges on capacitor
+ + + +
+ + + +

14. as AC frequency increases
+ + + +
+ + + +

15. High frequency AC
+ + + +
++ ++

16. Capacitive Reactance and AC
• Current constantly flows on either side of
(not across) capacitors.
• Capacitors block DC but do not block AC.
• Capacitors resist voltage changes as
voltage on a plate becomes closer to the
supply voltage.
• As frequency increases, capacitors have
less opposition to voltage changes, less
capacitive reactance.