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# Alvl P2: capacitance (ch6) Cheat Sheet by MostAncientDream

aqa A-level physics Year 2 (yr13) capacitance: chapter 6

### Defini­tions

 Capaci­tance the charge stored per unit pd capacitor component that stores charge. composed of two parallel conducting plates with a dielectric between them dielectric an insulating material placed between the two plates of a capacitor in order to increase the amount of charge it can store time constant time taken for a capacitor to dishcarge 37% of its inital charge it is equal to the product of capaci­tance and the resistance of a fixed resistor (that the capcitor is being discharged through)

### intro

 - the positive side of the battery attracts electrons from one side of the initally uncharged plate --> this causes the left plate to become positively charged - the negative side of the battery repels these new electrons to the right of the parallel plates --> causing the right to become positively charged it can also be used to recity ac current by bring the current down gradually instead or sharply (? refer to graph) these plates store these electrons as charge so if the circuit breaks connection with the battery a separate circuit connected to the capacitor can continue to operate with the capacitor acting as a battery until it runs out of charge maximum stored charge = pd across the battery

### Equations

 capaci­tance Q/V (farad) V EQ energy stored in a capacitor 1/2 QV (area under graph) 1/2 CV2 V/Vo (V over V inital) e -t/RC C Er Eo A / d Er -relative permit­ivity of a dielec­tri­c/i­nsu­lator between plates Eo -relative permit­ivity of free space A area of plates d distance between plates - -
in a capcitor half the energy is always lost to heat either in resistor or wires etc
(refer to energy = 1/2 QV - this is where the missing energy is going)

### Capacitors in series and parallel

 capacitor series parallel charge same 2x pd split same C total 1/Ct = 1/C1+1/C2 Ct = C1+C2

### Decay and Time constant

 compared to the V-t graph of a charging capacitor (similar to x1/2) which changes to a curved expone­ntial decreasing graph of I-t the V and I-t graphs for a discha­rging capacitor are the same (same shape as charging I-t) V/Vo = e-t/RC r- resistance c- capaci­tance when t = RC we get e-1 which is 0.37 (37%) therefore when the time equals the resist­ance, we get 37% of the original voltage this is why RC is our time constant - tc

### Capaci­tance and dielec­trics

 if you were to increase the distance between the two plates the capaci­tance would decrese two things can follow from that: 1. if the battery was connected - you have a constant v as E = 1/2CV2 the energy would decrease 2. if the battery was discon­nected - you have a constant Q as E = 1/2Q2/C energy increases (2 makes sense as if you try to separate two charged plates that are attracting each other then you are putting energy into the system to do this)

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