Physics 201- Introduction to Electricity & Magnetism
October 4- Capacitors

Capacitance

Defined by:

The unit of capacitance is farad= C/ V.  Typically we see mF and pF, not farads.  Just like when we had mC, we must remember to convert to the proper units.

How do we compute the capacitance of an object?

• If the objects are uncharged, give them a fictional charge.  Now compute the potential difference between them, usually do this by first finding the electric field by Gauss' law (or some other simple technique).  Then divide Q by V to get C.
• Once you know the capacitance of an object you can then use that and voltage to find the charge on the capacitor.
• Weíll later look more at their energy storing abilities, signal processing (how capacitors alter current).

Capacitance is non-zero whenever there are two objects- can talk about the capacitance of a wrench and a hacksaw.  (Also, a single object can be said to have a capacitance.  Here we just find the potential difference from V=0 (at infinity).)

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Fall 2002