Capacitance, OCR- Unit 5, module 2

Capacitance, OCR- Unit 5, module 2

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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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Sameer Jahabarali Physics
Unit 5, module 2 Capacitance
Charging capacitor:
Current decreases exponentially (goes less positive, as it is less attractive)
Charge increases, with the rate of increase, decreasing with time (plateaus)
Voltage increases, with the rate of increase, decreasing with time (plateaus)
Discharging capacitor:
Current increases (goes less negative), with the rate of increase, decreasing with time (plateaus)
Charge decreases exponentially
Voltage decreases exponentially
Time constant is the inverse coefficient of exponential discharge of a
capacitor each time, given by the product of resistance…read more

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