Today57

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Today5/7

• Questions?
• Circuits and Capacitors
• Course Evaluations

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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 FaradsEquation C
q/V
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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
Energy q?Vave 1/2q?V 1/2C?V2
C q/?V
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Kirchhoffs 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!! (/-)

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Kirchhoffs 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!! (/-)

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Capacitors with Resistors
Will current flow when the switch is closed?
Yes, but only for an instant until the capacitor
is charged.
Yes, but it will take longer to charge the
capacitor.
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Capacitors with Resistors
Describe current and voltage long after the
switch has been closed.
No current, 12 V across the capacitor.
Loop rule still applies!V IR for resistors
still applies!
No current, 12 V across the capacitor, zero V
across the resistor.
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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
Loop rule still applies!V IR for resistors
still applies!
6?F
4?
4 V across 2? and 8 V across the capacitor and 4?