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Physics 014 Capacitance
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F P
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Topics Capacitors Determining capacitance
Capacitors in parallel and series Energy Dielectrics
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Capacitors The capacitance is a measure of how much charge must be put on the plates to produce a certain potential difference between them.
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Capacitors The charge q, and potential difference V, for a capacitor are proportional. C is called the capacitance.
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Capacitors The SI unit of capacitance is the farad, F.
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Capacitors Two conductors, isolated electrically from each other and from their surroundings, form a capacitor.
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Capacitors An electric circuit is a path through which charge can flow
A battery is a device which maintains a certain potential difference between its terminals
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Capacitors The circuit is incomplete because the switch is open
When the switch is closed, the circuit is complete and charge can flow
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??? Does the capacitance C of a capacitor increase, decrease, or remain the same (a) when the charge q on it is doubled and (b) when the potential difference V across it is tripled?
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Capacitors in parallel
When there is a combination of capacitors in a circuit, we can sometimes replace that combination with an equivalent circuit
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Series Parallel
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Capacitors in parallel
Capacitors in parallel can be replaced with an equivalent capacitor that has the same TOTAL charge q and the same potential difference V as the actual capacitors.
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Capacitors in parallel
To find the equivalent capacitance of a parallel combination, we add the individual capacitances. n capacitors in parallel
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Capacitors in series When in series, capacitors can be replaced with an equivalent capacitor that has the same charge q and the same total potential difference V as the actual series capacitors.
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Capacitors in series To get the equivalent capacitance for series capacitors, n capacitors in series
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Capacitors in ??? Example: Find the equivalent capacitance for potential V if C1=12.0μF, C2=5.30μF, and C3=4.50μF.
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parallel series
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Energy in the Field The potential energy of a charged capacitor may be viewed as being stored in the electric field between its plates
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Energy in the Field In a parallel-plate capacitor, the E field has the same value everywhere between the plates The energy density u should also be uniform
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Energy in the Field Remember V=Ed for a parallel-plate
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Dielectrics
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Dielectrics
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Dielectrics
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Dielectrics
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Dielectrics
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Dielectrics
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Quiz Friday 25-1 to 25-6
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Determining Capacitance
Can we figure out C just by using the geometry of the capacitor and the charge on it?
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Determining Capacitance
Assume q on the plates Calculate E using Gauss’ law Calculate V Calculate C
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E and A are in the same direction by symmetry E and s are in opposite directions and V=Vf-Vi
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Determining Capacitance: Plates
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Determining Capacitance
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