Capacitor Parallel Plates Charged plates each store charge

Capacitor

Parallel Plates ] Charged plates each store + - charge. • Positive charge at higher potential • Negative charge at lower potential ] The charge storing device is called a capacitor. • Old term condenser + -

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/V Farads are a large amount of capacitance Microfarads (m. F) and picofarads (p. F) are common

Isolated Charge ] A metal sphere with a 0. 25 m ] The potential of the sphere is radius is in a vacuum. Find the capacitance. ] Note there is only one surface, but the formula still works. ] The capacitance is ] For the sphere • C= (0. 25 m) / (8. 99 x 109 Nm 2/C 2) • C = 28 x 10 -12 F = 28 p. F.

Parallel Plate Capacitor ] A capacitor is made from two + - plates. • Area A • Distance d ] The field is due to the charge on the plates • E = Q/Ae ] The voltage is V = Ed. ] The capacitance is + -

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

Two Capacitors ] Two capacitor charged by the C 1 + - same potential are in parallel. ] The voltage is the same across each capacitor. ] The charge will be equal to the total from both. C 2 ] The total capacitance is the sum of the individual capacitances. + -

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 C 1 + C 2 -

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