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# 7and8 - Phys102 Lecture 7/8 Capacitors Key Points...

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Phys102 Lecture 7/8 Capacitors Key Points Capacitors Determination of Capacitance Capacitors in Series and Parallel Electric Energy Storage Dielectrics References SFU Ed: 24-1,2,3,4,5,6. 6 th Ed: 17-7,8,9,10+.

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A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.
Parallel-plate capacitor connected to battery. (b) is a circuit diagram.

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When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage: The quantity C is called the capacitance. Unit of capacitance: the farad ( F ): 1 F = 1 C/V. 24-1 Capacitors
Determination of Capacitance For a parallel-plate capacitor as shown, the field between the plates is E = Q / ε 0 A. The potential difference: V ba = Ed = Qd / ε 0 A. This gives the capacitance:

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Example 24-1: Capacitor calculations. (a) Calculate the capacitance of a parallel-plate capacitor whose plates are 20 cm × 3.0 cm and are separated by a 1.0-mm air gap. (b) What is the charge on each plate if a 12-V battery is connected across the two plates? (c) What is the electric field between the plates? (d) Estimate the area of the plates needed to achieve a capacitance of 1 F, given the same air gap d .
Capacitors are now made with capacitances of 1 farad or more, but they are not parallel- plate capacitors. Instead, they are activated carbon, which acts as a capacitor on a very small scale. The capacitance of 0.1 g of activated carbon is about 1 farad.

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