Introduction to SAT II Physics - FreeExamPapers

capacitance is halved. The proportionality constant between C and A/d is
, called the
permittivity of free space, which we encountered in the previous chapter in relation to
Coulomb’s constant. In case you forgot, C 2 /N · m 2 .
The unit of capacitance is the farad (F). One farad is equal to one coulomb per volt. Most
capacitors have very small capacitances, which are usually given in microfarads, where 1
µF = 10 –6 F.
Energy
To move a small amount of negative charge from the positive plate to the negative plate of
a capacitor, an external agent must do work. This work is the origin of the energy stored
by the capacitor.
If the plates have a charge of magnitude q, the potential difference is . If q = 0,
and work is done to add charge until q = Q, the total work required is:
This is the energy stored by the capacitor. Manipulating this equation and the equation
for capacitance,
, we can derive a number of equivalent forms:
Equivalent Capacitance
Like resistors, capacitors can be arranged in series or in parallel. The rule for adding
capacitance is the reverse of adding resistance:
Capacitors in series add like resistors in parallel, and capacitors in parallel add like
resistors in series.
For two capacitors in series:
For two capacitors in parallel:
EXAMPLE
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