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(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 devicewhosepurposeis to storeelectric energy which can be released in a controlled manner. Usually madeof two conductors with charges +Q and –Q… +Q -Q … separated by an insulator (dielectric). Can be air or vacuum .
Capacitance Capacitance +Q -Q Q C V = Capacitance: Q C V = Usually written as V + V Δ V = V + V Potential difference between theconductors Units: Farad 1 F = 1 C/V Symbol:

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Example: Parallel plate capacitor Example: Parallel plate capacitor Determinethecapacitanceof two parallel conducting large plates of area A separated by a small distanced. A d Can be considered infinite planes
Let theplates have charges +Q and –Q. Q - Q 0 2 E E σ ε + - = = r r Surf ace charge densit y: Q A σ = E + r E - r 0 0 t ot al E r t ot al 0 E σ ε = r Between the plates

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Potential differencebetween the plates: t ot al 0 0 Q d V E d d A σ ε ε = = = r Capacitanceof a parallel platecapacitor: 0 Q Q C Q d V A ε = = 0 A C d ε = The chargecancels out…
Thecapacitanceof a system: Does not depend on: chargeQ, potential difference Δ V. Depends on: the geometry of thesystem, the dielectric between theconductors (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.
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