Transcript Capacitor - GTU e

VADODARA INSTITUTE OF ENGINEERING
NAME:PATHAN M.AASHIQ(13ELEE134)
SHIVANI RATHOD(13ELEE134)
PATEL DHAVAL(13ELEE135)
(13ELEE131)
BRANCH:EE-2
CHRGING AND DISCHRGING OF CAPACITOR
CHARGING AND
DISCHARGING OF
CAPACITOR
Structure of capacitor

Capacitor
= conductor (metal plate)
+
insulator (dielectric)
+
conductor (metal plate)
Type of capacitor

Non-polar capacitors

Polarized capacitors
Charging up a capacitor

After connected to the battery, the
capacitor is charging up. More and
more charges are on the plates of the
capacitor.
++ +
++ +++
-- -
- - ---
Current Flow (I) when charging up

Current flows from the positive terminal
of the supply to the negative terminal.
++ +
++ +++
-- -
- - ---
Current I is decreasing and then comes to be zero.
Voltage of capacitor (Vc) when charging up

A voltage Vc is built up when charging.
The capacitor is charged up.
e.m.f.
Vc =0
e.m.f.
++ +
++ +++
-- -
- - ---
Vc ↑
e.m.f.
Vc =e.m.f.
i.e.: I=0
E.m.f. of the battery is constant while voltage across capacitor (Vc) is
increasing until it is equals to e.m.f. of the battery.
Discharging

If the p.d. across the capacitor is equal to that
of the battery, discharging doesn’t occur.
 If a charged capacitor is connected to a
resistor, current flows from its positive terminal.
++ +++
++ +
- - ---
-- -
Current I is decreasing and flows at the direction which is opposite as before.
Also, Vc is decreasing (expotentially)
Capacitance

Which one is at a higher potential?
+
+
+
+
B
A
+
+
+
+
+
Capacitance (C)

The potential of a conductor is
proportional to the charge stored on it.
V  Q
∵ V = kQ/R
i.e.: Q/V = constant = C (capacitance)
Capacitance

If VA=VB, which one has higher capacitance?
+
+
+
+
B
A
+
+
+
+
+
VA = 1000V
QA = 6mC
C = 6F
VB=1000V
ANSWERQ = 3mC
B
C = 3 F
Unit of capacitance
C = Q/V
 Unit of capacitance = 1C V-1
 Or Farad (F)
 1F = 1C V-1

Capacitance of a conducting sphere

A conducting sphere can store charges.
 Voltage at the surface of a conducting sphere
=kQ/R
 V=kQ/R
 C = Q / V = R/k = 4oR

So, the capacitance of a conducting sphere is
proportional to the radius of the sphere.
Capacitance of a parallel plate capacitor

Recall
E=kQ/r2 = Q/(4or2)=Q/(4r2o)= / o
= Q / A
E= -dV/dr = V/d (∵E is uniform between plates)

E= / o = Q / o A = V / d

C = Q / V = o A / d
Capacitance I
a)
b)
c)
Area A
d
r
C1 =
C2=
C3=
Capacitance II
A = effective area
= overlapping area
Function of dielectric
Plates induced charges at dielectric
 Dielectric formed an opposite E-field
which reduced both E-field and V of
metal plates.
 By C=Q/V
=> C=Q/V’ where V’ < V
=> C 
 E’ = E/ Also V’ = V/ 

THANK YOU