# 18 - Capacitors Questions Answers i was trying to solve by differentiating total charge equation but was getting confused sir when taking capacitors charge seperatly  q1 and q2 and adding do i have to use tou as 2rc or rc in chargingingh of capacitor equation and
Joshi sir comment
Joshi sir comment
Physics >> Electricity >> Capacitors Medical Exam

distance b/w plate of capacitor C &q charge is increased to double . work done=??

as distance d is double thus C= Ae/2d  thus capacitance will be halved so wd shld be (1/2 )q2/(C/2)   ie 1/4 q2/C

BUT Sir the ans given is=( 1/2)q2/C Plzz do me a little favour by solving this query

Joshi sir comment

(1/2 )q2/(C/2) = q2/C not 1/4 q2/C

so work done = new energy - previous energy

Physics >> Electricity >> Capacitors Medical Exam

a II plate capacitor has an electric field of 105 V/m b/w d plates . if charge on capacitor plate is 1uC d force on each capacitor is??

ans 0.05N

Joshi sir comment

F = σq/2ε0 = Eq/2 = 105*10-6/2 = 0.05

A charge of +2.0 x 10^-8 C is placed on the positive plate and a charge of -1.0 x 10^-8 C on the negative plate of a parallel plate capacitor of capacitance 1.2 x 10^-3 uF. Calculate the potential difference developed between the plates. Please add explanation!

Joshi sir comment

due to induction the charges on the four faces will be so V = q/C = 1.5 * 10-8/1.2 * 10-9

solve now

One end of a 10cm long silk thread is fixed to a large vertical surface of a charged nonconducting plate having a mass 10g and a charge of 4*10^-6 c . In equlibrium the thread makes an angle of 60 deg. With he vertical.find the surface charge density on plate.
Joshi sir comment according to the diagram

tanθ = qE/mg = q(σ/2ε0)/mg

solve this eq. for σ.

Remember that length of the thread is not applicable in the question.