1. Capacitor

2. Parallel Plates  Charged plates each store charge. Positive charge at higher potentialPositive charge at higher potential Negative charge at lower potentialNegative charge at lower potential  The charge storing device is called a capacitor. Old term condenserOld term condenser  

3. Capacitance  The measure of a device to store charge at a given voltage is its capacitance.  The unit of capacitance is the farad (F). 1 F = 1 C / V1 F = 1 C / V Farads are a large amount of capacitanceFarads are a large amount of capacitance Microfarads (  F) and picofarads (pF) are commonMicrofarads (  F) and picofarads (pF) are common

4. Isolated Charge  A metal sphere with a 0.25 m radius is in a vacuum. Find the capacitance.  Note there is only one surface, but the formula still works.  The potential of the sphere is  The capacitance is  For the sphere C = (0.25 m) / (8.99 x 10 9 Nm 2 /C 2 ) C = 28 x 10 -12 F = 28 pF.

5. Parallel Plate Capacitor  A capacitor is made from two plates. Area AArea A Distance dDistance d  The field is due to the charge on the plates E = Q/A E = Q/A   The voltage is V = Ed.  The capacitance is  

6. Keyboarding  A key on a keyboard is a parallel plate capacitor.  A springy insulator separates the plates.  A circuit measures the capacitance from each key. If pressed d decreases The capacitance increases The circuit records the key

7. Two Capacitors  Two capacitor charged by the same potential are in parallel.  The voltage is the same across each capacitor.  The charge will be equal to the total from both.  The total capacitance is the sum of the individual capacitances.   C1C1 C2C2

8. Series Capacitors  Two capacitors connected is sequence with the ends separated by a potential are in series.  The total charge on the middle section must be neutral. Same charge on capacitors Voltages sum to the total   C1C1 C2C2

9. More and Less  Two capacitors 1mF and 3 mF are placed in parallel and in series.  Find the combined capacitance in both situations.  In parallel the capacitances add directly.  C = 1 μ F + 3 μ F = 4 μ F  In series the inverses are summed, then inverted.  1 ( μ F) -1 + 1/3 ( μ F) -1 = 4/3 ( μ F) -1  C = 3/4 μ F next