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PHY213_Chapter16_Sec6to10

# PHY213_Chapter16_Sec6to10 - Chapter 16 Electrical Energy...

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Chapter 16 Electrical Energy And Capacitance

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General Physics Capacitance Sections 6 – 10
General Physics Capacitance A capacitor is a device used in a variety of electric circuits The capacitance , C, of a capacitor is defined as the ratio of the magnitude of the charge on either conductor (plate) to the magnitude of the potential difference between the conductors (plates) Units: Farad (F) 1 F = 1 C / V A Farad is very large Often will see μF or pF V Q C MU30T18

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General Physics Parallel-Plate Capacitor The capacitance of a device depends on the geometric arrangement of the conductors For a parallel-plate capacitor whose plates are separated by air: d A Q Q d Q Ed Q V Q C 0 0 / ε ε σ = = = = capacitor plate parallel a for d A C o ε =
General Physics Parallel-Plate Capacitor, Example The capacitor consists of two parallel plates Each have area A They are separated by a distance d The plates carry equal and opposite charges When connected to the battery, charge is pulled off one plate and transferred to the other plate Transfer stops when V cap = V battery Active Figure: Parallel Plate Capacitors EX16.6

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General Physics Electric Field in a Parallel-Plate Capacitor The electric field between the plates is uniform Near the center Nonuniform near the edges The field may be taken as constant throughout the region between the plates
General Physics Applications of Capacitors – Camera Flash The flash attachment on a camera uses a capacitor A battery is used to charge the capacitor The energy stored in the capacitor is released when the button is pushed to take a picture The charge is delivered very quickly, illuminating the subject when more light is needed

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General Physics Applications of Capacitors – Computers Computers use capacitors in many ways Some keyboards use capacitors at the bases of the keys When the key is pressed, the capacitor spacing decreases and the capacitance increases The key is recognized by the change in capacitance
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