الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Lectures in General Physics
6.3 Calculation of capacitance
The most common type of capacitors are:-
• Parallel-plate capacitor
• Cylindrical capacitor
• Spherical capacitor
We are going to calculate the capacitance of parallel plate capacitor using
the information we learned in the previous chapters and make use of the
equation (6.1).
6.3.1 Parallel plate capacitor
Two parallel plates of equal area A are separated by distance d as shown in
figure 6.4 bellow. One plate charged with +q, the other -q.
d
The capacitance is given by C =
Figure 6.4
q
V
1+q
1-q
Gaussian
surface
First we need to evaluate the electric field E to workout the potential V.
Using gauss law to find E, the charge per unit area on either plate is
σ = q/A. (6.2)
σ q
∴ E = =
ε ο
ε ο
A
(6.3), (4.9)
The potential difference between the plates is equal to Ed, therefore
V = Ed =
qd
A
(6.4)
ε ο
The capacitance is given by
Dr. Hazem Falah Sakeek