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Unformatted text preview: L51CapacitorsTest Results246810121416181030507090110130#Test ScoreCapacitorsA capacitor= two conductors close together. In normal use there are equal but opposite charges+Q, Qon the two conductors. A capacitor can store energy.do workto charge a capacitor creating a potential difference VJargon:The conductors are often called plates.We speak of charging the capacitor even though the netcharge is zero.An old term for capacitor is condenser.The parallelplate capacitorExcept near the edges, the electric field is nearly constant in the gapAlso the charge on each plate is spread out nearly uniformly.Two metal plates of areaA, separated by a small distanced.Equal but opposite chargeson the plates+Q, Q.Recall:The field Ejust outside a conductor and the surface charge density are related byE= /V = Q(d/A) or Q = V(A/d).EWhat is the potential difference?The charge Qis proportional to the voltage difference VThe potential difference for a constant E field isV= Ed.Recall: The field Ejust outside a conductorand the surface charge density are related byE= /Here = Q/A. Therefore E = Q/A, and V = Q(d/A) or Q = V(A/d).EPotential differenceThe charge Qis proportional to the voltage difference VThe capacitance is a fixed property of capacitor size, shape, separation between plates .Capacitance:C=Q/V.The SI unit of capacitance is coulomb/volt, which is given the name farad (F): 1 F = 1 C/VReal capacitors are usually measured in F or pF.Example: parallelplate capacitor, C= 0 A/dDefinition of capacitance:In general:If all linear dimensions (length, width, height and plate separation) of a parallel plate capacitor are tripled while the shape of the plates remains the same, the capacitance is1. multiplied by 32. multiplied by 93. multiplied by 274. stays the same5. divided by 3The capacitance of a parallelplate capacitor can be increased by:1) increasing the charge2) decreasing the charge3) increasing the plate separation4) decreasing the plate separation5) decreasing the plate areaCombining More than One CapacitorIt is simpler to draw this as a schematic diagramusing standard symbols for capacitors and a battery (lines = wires)This diagram shows two capacitors C1, C2connected in parallel fixed potential difference:...
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This note was uploaded on 04/07/2008 for the course PHY 211 taught by Professor Souder during the Spring '08 term at Syracuse.
 Spring '08
 Souder
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