Physics 7B
Exit Handout
DLM 07
FNTs
Wrapup of Exponential Change and Capacitors
Finish the last activity of the day if you did not finish it in DL.
5.43)
You derive the equation 
When a capacitor is discharging, the circuit is simply
a capacitor and resistor hooked in series—endtoend.
From the definition of
capacitance, the voltage difference across the capacitor
V
is given by the
expression:
V
=
q
/
C
where q is the charge on the capacitor (i.e. +
q
on one plate
and 
q
on the other).
You know by definition that the current
I
is simply the time derivative of the
charge:
I
= d
q
/d
t
.
Apply the energydensity relation for a complete loop around this circuit and write the equation in
terms of
q
,
C
,
R
, and d
q
/d
t
.
Rearrange this expression to get it in the form d
q
/d
t
= 
q
.
What is
equal to in terms of the other electrical constants in this problem?
5.44)
You will probably have to think very hard and very clearly to be
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 Winter '09
 WEB
 Physics, Alternating Current, voltage difference, Galilean Space Time

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