CAPACITANCE

I. Capacitor - A charge storage device, usually two conductors separated by an insulator.

  A. Capacitance - C = Q/Vab     Units:[Coulomb/volt] = [farad]

    Q = magnitude of charge on either plate

    Vab  = potential difference between the conductors

II. Parallel plate capacitor

  A. For parallel plates, the electric field between the plates is constant and depends on the charge per unit area.

  B. What is the expression for the capacitance of a parallel plate capacitor?
 
 
 
 
 

III. Capacitor connections

  A. Series

   (1/Ceq) = 1/C1 + 1/C2

 
 
 
 
 
 
 

   1. What physical quantity do capacitors in series have in common?
 
 
 

  B. Parallel

   Ceq = C1 + C2
 
 
 
 
 
 
 

   2. What do capacitors in parallel have in common?
 
 
 

V. Energy stored in a charged capacitor - Energy = (1/2)(Q2/C) = (1/2)CV2 = (1/2)QV
 
 
 
 
 

  A. What is the energy density for a charged parallel plate capacitor?
 
 

VI. Dielectrics (non-conducting materials) can be used to increase the capacitance of capacitors.

  A. When inserted between the plates of a parallel plate capacitor, they effectively reduce the E-field between the plates. If E0 is the original field, then the new field is E and

   E = E0/k

where k is the dielectric constant.

   B. The new capacitance is now: C = kC0 , where k is greater than 1.
 
 

  C. Can you explain this, using an argument based on how electric fields affect atoms and molecules?