(37) Capacitance

(37) Capacitance - Lecture 37 Capacitance Capacitors A...

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Lecture 37 Capacitance
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Capacitors Capacitors A capacitor is a device whose purpose is to store electric energy which can be released in a controlled manner. Usually made of two conductors with charges +Q and –Q… +Q -Q … separated by an insulator (dielectric). Can be air or vacuum .
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Capacitance Capacitance +Q -Q Q C V = Capacitance: Q C V = Usually written as V + V Δ V = V + V Potential difference between the conductors Units: Farad 1 F = 1 C/V Symbol:
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Example: Parallel plate capacitor Example: Parallel plate capacitor Determine the capacitance of two parallel conducting large plates of area A separated by a small distance d. A d Can be considered infinite planes
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Let the plates have charges +Q and –Q. Q - Q 0 2 E E σ ε + - = = r r Sur f ace char ge densit y: Q A σ= E + r E - r 0 0 t ot al E r t ot al 0 E = r Between the plates
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Potential difference between the plates: t ot al 0 0 Q d V E d d A σ ε = = = r Capacitance of a parallel plate capacitor: 0 Q Q C Q d V A = = 0 A C d = The charge cancels out…
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The capacitance of a system: Does not depend on: charge Q, potential difference Δ V. Depends on: the geometry of the system, the dielectric between the conductors (because Δ V is always proportional to Q)
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How can I “imagine” capacitance? Device Capacitor Balloon Use to store… Charge Air “Price” Potential difference (electric field) Pressure, tension in the walls If you store too much… E is too strong, dielectric breaks down (= burnt capacitor) Tension is too strong, walls rip, explosion. What makes a
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(37) Capacitance - Lecture 37 Capacitance Capacitors A...

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