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-Capacitors and Capacitance

-Capacitors and Capacitance. AP Physics C Mrs. Coyle. Capacitors: devices that store electric charge. Consist of two isolated conductors (plates) with equal and opposite charges +Q and −Q; the charge on the capacitor is referred to as "Q". Ex:Parallel Plate Capacitor.

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-Capacitors and Capacitance

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  1. -Capacitors and Capacitance AP Physics C Mrs. Coyle

  2. Capacitors: devices that store electric charge • Consist of two isolated conductors (plates) with equal and opposite charges +Q and −Q; the charge on the capacitor is referred to as "Q". Ex:Parallel Plate Capacitor

  3. Applications of Capacitors • Tune the frequency of radio receivers • Used as filters in power supplies • Used as energy-storing devices in electronic flashes (ex: cameras)

  4. Charging a Parallel Plate Capacitor • The battery establishes a field on the plates. • This forces the electrons from the wire to move on to the plate that will become the negative plate. • This continues until equilibrium is achieved(the plate, the wire and the terminal are all at the same potential) and the movement of the electrons ceases. • At the other plate, electrons move away from the plate, leaving it positively charged. • Finally, the potential difference across the capacitor plates is the same as that between the terminals of the battery.

  5. Capacitor Animation • http://phet.colorado.edu/en/simulation/capacitor-lab

  6. Capacitance: a measure of the capacitor’s ability to store charge • Ratio of the magnitude of the charge on either conductor to the potential difference between the conductors. • The SI unit of capacitance is the farad (F) • 1 F = 1 Coulomb/Volt • Also see the units pF (10-12) or mF (10-6)

  7. Factors that affect capacitance • Size (Area, distance between plates) • Geometric arrangement • Plates • Cylinders • Spheres • Material between plates (dielectric) • Air • Paper • Wax

  8. Note: • Capacitance is always positive • The capacitance of a given capacitor is constant. If the voltage changes the charge will change not the capacitance.

  9. Note: • The electric field is uniform in the central region, but not at the ends of the plates. It is zero elsewhere. • If the separation between the plates is small compared with the length of the plates, the effect of the non-uniform field can be ignored.

  10. Capacitance of a Parallel Plate Capacitor • A is the area of each plate • Q is the charge on each plate, equal with opposite signs • The capacitance is proportional to the area of its plates and inversely proportional to the distance between the plates

  11. A single conductor can have a capacitance. • Example: Isolated charged sphere can be thought of being surrounded by a concentric shell of infinite radius carrying a charge of the same magnitude but opposite sign.

  12. Capacitance of an Isolated Charged Sphere, Cont’d • Assume V = 0 at infinity • Note, the capacitance is independent of the charge and the potential difference.

  13. Capacitance of a Cylindrical Capacitor • From Gauss’s Law, the field between the cylinders is E = 2kel/ r, l=Q/L • DV = -2ke l ln (b/a)

  14. Capacitance of a Spherical Capacitor • Potential difference: • Capacitance:

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