# Capacitance, OCR- Unit 5, module 2

Capacitance, OCR- Unit 5, module 2

- Created by: Sameer
- Created on: 13-05-12 18:34

First 380 words of the document:

Sameer Jahabarali Physics

Unit 5, module 2 Capacitance

Capacitance (C) is charged stored per unit p.d. (voltage). It is measured in farads (F), where 1F = 1 Q

V-1. Capacitance is created by capacitors, they are:

Two metal plates, place close together, so they can `experience'

one and others `activity', without touching

This is joined to a circuit, with a cell/ battery

The cell or battery, push electrons from the negative terminal to

the neutral plate (relatively positive) of the capacitor (notice

electron flow without complete circuit)

electrons build up in the plate

the plate becomes more negatives, so less electron are attracted

The negativity is `experienced' by the other plate which pushes a

few more electrons on that place

This makes one plate negative and the other positive, hence a p.d

of the cell is created between the two plates (capacitors)

As charge (Q) builds up (electors), the voltage(V) builds up

proportionally (as each electron are pushed by the same cell with

same energy), hence the proportionality constant is given by

capacitance(C).

The overall capacitance for a circuit with multiple capacitors is given by:

Series- is the sum of the inverse of each capacitor, because the current in each

capacitor is the same but the voltage is different. As:

V=Q/C

QT=Q1=Q2=Q3

VT = V1+V2+V3 Q/CT = Q/C1+ Q/C2+ Q/C3 (divide both side by Q)

Parallel- is the sum of each capacitors, because the voltage stays the same on different junctions but current

changes. As a result:

Q=VC

VT=V1=V2=V3

QT = Q1+Q2+Q3 VCT= VC1+VC2+VC3 (divide both side by V)

The energy stored in a capacitor is given by the area under the graph. This is

because V=W/Q, and the total energy stored is sum of all energy held by all

charges (i.e. the area of the graph). This gives us the formula for work (energy),

which can also be manipulated by substituting Q=CV,if we have an unknown.

As time increases in a:

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