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Physics 014 Capacitance.

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Presentation on theme: "Physics 014 Capacitance."— Presentation transcript:

1 Physics 014 Capacitance

2 F P

3 Topics Capacitors Determining capacitance
Capacitors in parallel and series Energy Dielectrics

4 Capacitors The capacitance is a measure of how much charge must be put on the plates to produce a certain potential difference between them.

5 Capacitors The charge q, and potential difference V, for a capacitor are proportional. C is called the capacitance.

6 Capacitors The SI unit of capacitance is the farad, F.

7 Capacitors Two conductors, isolated electrically from each other and from their surroundings, form a capacitor.

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11 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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13 Capacitors The circuit is incomplete because the switch is open
When the switch is closed, the circuit is complete and charge can flow

14 ??? 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?

15 Capacitors in parallel
When there is a combination of capacitors in a circuit, we can sometimes replace that combination with an equivalent circuit

16 Series Parallel

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18 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.

19 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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21 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.

22 Capacitors in series To get the equivalent capacitance for series capacitors, n capacitors in series

23 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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25 parallel series

26 Energy in the Field The potential energy of a charged capacitor may be viewed as being stored in the electric field between its plates

27 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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29 Energy in the Field Remember V=Ed for a parallel-plate

30 Dielectrics

31 Dielectrics

32 Dielectrics

33 Dielectrics

34 Dielectrics

35 Dielectrics

36 Quiz Friday 25-1 to 25-6

37 Determining Capacitance
Can we figure out C just by using the geometry of the capacitor and the charge on it?

38 Determining Capacitance
Assume q on the plates Calculate E using Gauss’ law Calculate V Calculate C

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40 E and A are in the same direction by symmetry E and s are in opposite directions and V=Vf-Vi

41 Determining Capacitance: Plates

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43 Determining Capacitance


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