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Capacitors store energy in the form of a stored electric charge.

Capacitance is measured in Farads

One Farad is an enormous capacitance and rarely found in real life. Normal capacitors are measured in Microfarads (µF), Nanofarads (nF) and Picofarads (pF). SI Prefixes

Q = CV where Q is the charge, C is the capacitance and V is the potential difference across the capacitor.

E = CV² / 2 where E is the energy stored, C is the capacitance and V is the potential difference across the capacitor.

Capacitors are used for timing, coupling, decoupling, smoothing, filtering and tuning.

Capacitors ...

• pass alternating currents (AC) and block direct current (DC)
• pass high frequencies more easily than low frequencies so they can be used in filter circuits
• combined with an inductor form a tuned circuit used in radio tuning
• couple AC signals between modules while blocking DC potentials
• are used for decoupling. This gets rid of unwanted AC signals by shorting them out to ground.
• are used to control precise time delays
• store charge so the potential difference across them can not change suddenly
• are used to smooth the "lumpy" DC from a power supply rectifier diode
• current leads the voltage across the capacitor (is phase shifted by 90^o)
  Using a water analogy, the current is the water flow and the voltage is how full the bucket is. If you tip water into a bucket, there is a large water flow but it still takes time to fill the bucket. Once the bucket is full, the water flow stops.
  Capacitors are the same. A current is needed to charge the capacitor. The current flows before and while the capacitor is charging. Once the capacitor is charged, the current flow drops to zero. Current leads the voltage.

Capacitor Simulation

Click the switch to start the simulation. Full Screen

 

 

Water Analogy ...

• It takes time to fill the tank / charge the capacitor
• The tank/capacitor fills fast at first and the fill rate slows down, the fuller it gets
• The tank/capacitor empties fast at first and the emptying rate slows down, the emptier it gets
• Eventually the battery/resevoir would be drained
• The water level/capacitor charge can not change suddenly
• If there were more pressure/voltage, the tank/capacitor would fill faster
• The tank fills until the water levels are equal. The capacitor charges until the voltages are equal. Full Screen