Capacitance

• For reading from a graph:

  In one time constant (RC seconds), during discharge, p.d. falls to 1/e (=0.37) of its original value, V₀.
• For reading from a graph:

  In one time constant (RC seconds), during charging, p.d. falls to 1-1/e (=0.63) of its original value, V₀.
• Dielectrics are insulating materials which become polarised in the presence of an electric field.
• As dielectric molecules have their own electric fields, when they align anti-parallel to the electric field of the plates, another electric field is created which opposes the capacitor's electric field.
• The larger the permittivity, the larger the opposing field of the dielectric molecules. This reduces the overall electric field between the parallel plates, reducing the potential difference needed to transfer a given charge to the capacitor. Therefore, the capacitance increases.
• Capacitance is the amount of charge an object is able to store per unit of potential difference across it.
• Charging and discharging graphs:

  
• When a capacitor is connected to d.c. :

  • Electrons flow from negative terminal to capacitor and so a negative charge builds up on one plate.
  • Electrons on the opposite plate are repelled and so a positive charge of equal magnitude builds up.
  • Current eventually stops as oncoming electrons are repelled away by the negatively charged plate.

  Current continues to flow until the p.d. across the capacitor is equal to the emf of the supply.