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.

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

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

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

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.

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

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

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

43. Determining Capacitance