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Problems from Electricity

  1. The parallel plates of an air filled capacitor with area of area 1.1 x 10^8 m^2 . What must be the width of the plate be if the Capacitance is 1 Farad?
  2. Four capacitor each of 2microfarad are connected in such a way that total capacitance is also 2microfarad. Show what combination gives this value. Also try to illustrate the diagram.

(Fari posted these)


1.Using the formula: C= A\varepsilon /d

and substituting , C= 1 F, A = 1.1 x 108, \varepsilon = 8.8 x 10-12,

we get  , d =9.68×10-4 metres


Difference between Dielectric Break Down and Corona Discharge


Asked Deepika

  • Corona (egrejeen.wordpress.com)

Electric field inside and outside the plates of a capacitor

How do you find the direction of electric field on the outer or inner surface of  2 parallel plate of a capacitor ?



The direction of electric field is from the positive plate to negative plate.

The electric field outside the plates is zero as the electric field due to each plate (E=Q/2ε0A) cancel out being equal in magnitude and opposite in direction.

In between the plates the two electric fields add up as they are in same direction. (From positive plate to negative plate).

The magnitude of electric field between the plates is twice the electric field due to either; i.e; E=2 x Q/2ε0A = Q/ε0A

The mechanism of operation in a capacitor

The DC doesnt flow through capacitor. I thought it was because the electrons jus get saturated on one side and are lost on other side, such that it can’t take or give up anymore or plausibly due to voltage developed across it whereas in AC as directions keep changing, saturation doesnt occur. But it if it were so then AC would flow through two plates seperated by large distances or switches which can be considered capacitors.So please guide me as to what’s actually happening during current flow through capacitor and how it happens? (The question was posted by Thushar Misra)


Your logic is almost correct. A capacitor is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field.

A current is not “flowing” through a capacitor, the “current” that exists between the plates of a capacitor is called “Displacement current” which is actually caused by the time varying electric flux. This causes the circuit to be continuous though there is no electrical contact or flow of charges between the plates of the capacitor.

When a capacitor is connected to DC, there exists a change in electric flux when the switch is just turned on but soon it reaches steady state and there is no “current”. A capacitor can “conduct” only when there is a “change” in electric flux, which is continuously taking place in an AC.


Electric Field due to Polarisation

Sreeram asks:

“do charges produced due to polarization in a dielectric produce electric field outside?”


When a dielectric is kept in an electric field, it is polarized. The polarized ends are just like the two plates of a parallel plate capacitor. The electric field is confined between the two surfaces (except for some edge effect). So, in a macroscopic point of view, the answer is no.

(However, I agree that there is scope for disagreement and debate when we think and look at the microscopic level. Users of the site are requested to post comments)

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