db_lec05_post

db_lec05_post - Phys 221 Last Lecture*indepe Electric force...

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1/24/12 1 Phys 221 Last Lecture •Electric force is conservative •Electric potential energy •Potential PE elect = W elect = qE Δ x V = PE elect q E = Δ V Δ x D. Bortoletto PHYS221 f i Distance if = Δ x Δ x parallel to E Many medical applications: EKG and EEG measures Δ V between electrodes E
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1/24/12 2 Lecture 5: Capacitors &Capacitance A d Parallel Plates Cylindrical Capacitor D. Bortoletto PHYS221
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1/24/12 3 Capacitor • Capacitor is a device that stores electrostatic potential energy. • A capacitor consists of 2 spatially separated conductors which can be charged to +Q and - Q . D. Bortoletto PHYS221 Equipotential surfaces
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1/24/12 4 Capacitance • The capacitance is defined as the ratio of the charge on one conductor of the capacitor to the potential difference between the conductors. Capacitance is measured in Faradays represented by the letter F which is equal to (coulomb/volt) Q C V = Δ D. Bortoletto PHYS221
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1/24/12 5 Capacitance between Parallel Plates Calculate the capacitance. We assume + σ , - charge densities on each plate with potential difference V : - - - - - + + + + A d Need Q : Need V : from definition Use Gauss Law to find E QA = V E x Ed Δ = Δ = Φ = EA = Q ε o E = Q o A C = Q Δ V = Q Q 0 A d = 0 A d C depends only on the geometry of the capacitor (A,d) D. Bortoletto PHYS221
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1/24/12 6 Demo Demo: Capacitance versus d D. Bortoletto PHYS221
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1/24/12 7 Example What is the capacitance of a parallel plate capacitor made out of two square plates 10 x10 cm 2 separated by 1 mm wide gap? d=1 mm A=0.01 m 2 C = ε o A d C = 8.85 x 10 12 ( C 2 N 2 m 2 ) 0.01 m 2 10 3 m = 88.5 x 10 12 F = 88 pF D. Bortoletto PHYS221
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1/24/12 8 ILQ 1 • Does the capacitance C of a capacitor increase, decrease or remain the same when the charge q on it is doubled?
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db_lec05_post - Phys 221 Last Lecture*indepe Electric force...

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