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Today5/7
Questions?
Circuits and Capacitors
Course Evaluations
Capacitors
Capacitance tells me how many coulombs of charge are
stored in a capacitor when it has 1 Volt across it.
A 25F capacitor will have 25C of charge on it when
it has 1 volt across it. Think of capacitance as
“coulombs per volt”.
Units:
coulombs per volt or “Farads”
Equation: C = q/V
Capacitors and Energy
How many Joules of energy are stored in the capacitor?
As the battery moves charge from one plate to another, the
potential difference increases from zero to 12 V. The average is
6 V and the energy stored is q times 6 V (qVave).
12 V
6F
Energy = qVave = 1/2qV = 1/2CV2
C = q/V
Kirchhoff’s Rules, Loop
The sum of all voltages around any
closed loop is zero. (what goes up must
come down)
or VA,A = 0
!!must keep track of ups and downs!! (+/-)
Kirchhoff’s Rules, Junction
The sum of all currents at any junction
is zero. (what goes in must come out)
!!must keep track of ins and outs!! (+/-)
Capacitors with Resistors
Will current flow when the switch is closed?
Yes, but only for an instant until
the capacitor is charged.
12 V
6F
12 V
6
6F
Yes, but it will take longer to
charge the capacitor.
Capacitors with Resistors
Describe current and voltage long after the switch has
been closed.
No current, 12 V across the capacitor.
12 V
12 V
6F
Loop rule still applies!
V = IR for resistors still applies!
6
No current, 12 V across the capacitor,
zero V across the resistor.
6F
Capacitors with Resistors
Describe current and voltage long after the switch has
been closed.
2 Amps through the battery
and both resistors.
2
12 V
6F
4
Loop rule still applies!
V = IR for resistors still
applies!
4 V across 2 and 8 V across the
capacitor and 4