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Capacitors store energy in the form of charge.
They are like electronic buckets filled with electrons or perhaps more like compressed gas cylinders.
The potential difference across a capacitor changes slowly unless large currents are flowing.

Nick Reeder's Capacitor Challenge

• Capacitors store energy in the form of charge.
  • Charge is either more or fewer electrons than normal in uncharged material.
  • Volts ( V ) push the charge around creating the excess or lack.
• Capacitance ( C ) is measured in Farads.
• Charge ( Q )  is measured in Coulombs.
• The charge Q   =   C V.
• Capacitance is defined as the number of coulombs stored per volt across the capacitor. C   =   Q / V
• The energy stored is measured in Joules.
• The energy stored = 0.5 C V²
• If the voltage is big, the stored energy can be enormous and lethal.

Capacitor Construction

Two parallel metal or foil plates insulated from each other. Many capacitors have this structure rolled up into a tube to save space.

Capacitor Symbols and Images

Left to right ... Electrolytic - Connect the right way round to prevent explosion! Tuning - A variable capacitor used for tuning a radio transmitter. Small capacitors including two electrolytics. Look for the arrows with the minus signs.

Capacitor Simulations

Click the switch to alternate between charging and discharging.

This animation has correct physics so it could be used to plot a realistic capacitor charging graph.

Capacitor Properties

The main characteristics or properties of capacitors are ...

• Capacitance is measured in Farads or more often pico, nano or microfarads.
• One Farad is an enormous capacitance and rarely found in real life.
• Capacitors store energy in the form of a stored electric charge.
  • Charge is measured in Coulombs.
  • Energy is measured in Joules.
  • Q = CV where
    • Q is the charge,
    • C is the capacitance and
    • V is the potential difference across the capacitor
    • If you double the voltage, the stored charge will double.
  • E = CV² / 2 where
    • E is the energy stored,
    • C is the capacitance and
    • V is the potential difference across the capacitor
    • If you double the voltage, the energy stored increases by 2² (four times)

More details ...

• capacitors block direct current (DC)
• capacitors pass alternating current (AC)
• The breakdown voltage (in Volts). The voltage across the capacitor should never exceed this.
• The upper useful frequency properties.
• The DC leakage current (a particular problem with electrolytic capacitors).

YouTube Video: VERY Big Capacitor vs Watermelon

Electrolytic and Tantalum Capacitors

Electrolytic capacitors use thin rolled up foil plates separated by a liquid or gel electrolyte. The insulation between the plates relies on a chemical reaction. If the capacitor is connected up the wrong way round, this chemistry fails and the capacitor works as a conductor instead. It gets hot and can explode!

• The negative pin is indicated by the arrows and minus signs.
• Electrolytic capacitors have a very large capacitance for their size.
• The working voltage is low. 12 to 160 volt ratings are common. The image above shows a 35 Volt capacitor.
• They have a significant DC leakage current, sufficient to upset some timing circuits
• They are not manufactured with good tolerance / accuracy and this can be up to 50% out
• Their capacitance is not stable and can change with time
• They are not suitable for high frequency radio applications because the coiled up foil roll acts as an inductor blocking high frequency performance
• They work well at audio and ultrasonic frequencies.

Their uses include ...

• DC power supply smoothing - This is a particular example of decoupling.
• Audio signal coupling - Block DC and pass AC.
• Audio signal decoupling - Remove unwanted AC signals.
• Timing in 555 or other timer circuits - RC resistor capacitor circuit

They can not be used ...

• In AC circuits where the polarity across the capacitor reverses
• In logic gate astable circuits because the polarity across the capacitor reverses

Capacitor Labelling