chapter24--PHY131 JAB

# chapter24--PHY131 JAB - Chapter 24 Capacitance and...

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Unformatted text preview: Chapter 24: Capacitance and dielectrics §24.1 Capacitors and capacitance A capacitor is a device that stores electric potential energy by storing separated positive and negative charges. Work must be done to separate the charges. Parallel plate capacitor + + + + + + +------- V Q V E Q E Δ ∝ ∴ Δ ∝ ∝ Written as an equality: Q = C Δ V, where the proportionality constant C is called the capacitance. For a parallel plate capacitor: . where d A C V C V d A Q d A Q d Ed V ε ε ε ε σ = Δ = Δ = ∴ = = = Δ Note: C depends only on constants and geometrical factors. The unit of capacitance is the farad (F). 1 F = 1 C 2 /J = 1 C/V What is the capacitance for a parallel plate capacitor? Besides parallel plate capacitors, cylindrical and spherical capacitors are also common. In all cases V Q C Δ = where ∫ ⋅- = Δ b a d V l E See examples 24.3 & 24.4. Example (text problem 17.42): A parallel plate capacitor has a capacitance of 1.20 nF. There is a charge of magnitude 0.800 μ C on each plate. (a) What is the potential difference between the plates? Volts 667 nF 20 . 1 C 800 . = = = Δ Δ = μ C Q V V C Q (b) If the plate separation is doubled, while the charge is kept constant, what will happen to the potential difference? d V A Qd C Q V ∝ Δ = = Δ ε If d is doubled so is the potential difference. Example continued: Example (text problem 17.86): A parallel plate capacitor has a charge of 0.020 μ C on each plate with a potential difference of 240 volts. The parallel plates are separated by 0.40 mm of air....
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chapter24--PHY131 JAB - Chapter 24 Capacitance and...

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