21- Capacitors


21.1 - Capacitors

- Designed to store charge, similar to a cell

- Two metal plates seperated by a insulator, i.e. ceramic, plastic

-When connected to a battery/cell, electrons are moved from the left plate and deposited on the right plate.

- Plate charges become +Q and -Q

- Full charge is achieved when capacitor p.d.=cell e.m.f.

- Q on a plate is directly proportional to p.d. across it, so Q=VC (where C is capacitance)

- Capacitance (measured in Farads, F) is:- The charge stored per unit p.d.

- 1 farad = 1 coulomb per unit volt

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21.2 - Capacitors in circuits

- Parallel:- p.d. is constant                                                                                                                                  - Qtotal = sum of individual charges (Q1+Q2+...)                                                                                - Ctotal = sum of capacitances (C1+C2+...)

- Series:- Q is constant                                                                                                                                      - Vtotal sum of individual voltages (V1+V2+...)                                                                                  - 1/Ctotal = sum of 1/capacitances (1/C1+1/C2+...)

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21.3 Energy in capacitors

- During charging, work is done to move the elctrons away from the +ve plate and towards the -ve plate

- Work done = area under a p.d.-charge graph (W=1/2QV)

- Using Q=VC, we also get W=(Q^2)/2C and W=1/2(V^2)C

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