# 18 - Current Electricity Questions Answers

Sir, in the formula for current , in the ncert it first talks about steady state current. Then it writes i=q/t. but then it generalises and writes the formual i=(delta)q/(delta)t

sir my doubt is that how do we define flow of charge in steady state across a cross section. and why is there (delta)q in formula since the charge is not changing with time.

is it the amount of charge passing per unit time or the change in charge per unit time?

pls help

Joshi sir comment

It is the amount of charge passing through a particular cross section of wire in unit time

Joshi sir comment

Current comes out from the surface of one sphere symmetrically

and enters to the surface of other symmetrically.

Resistance of the medium around one sphere is

Joshi sir comment

By superposition principle, it is solved in minimum time and effort, other method is very complex. So use superposition principle.

Divide current in point 1 and 2, considering symmetry throughout, we get the equivalent resistance of the whole network except resistor R between 1 and 2 is 2R

Now 2R and R in parallel between 1 and 2

Now solve

if an area of isoscale right angle triangle(made of uniform wire of unit resistivity) is 8 unit. a battery of emf 10 unit is connected with across the longest branch then the current drawn from battery is approx                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     ans is  3A

Joshi sir comment

area = l2/2 = 8 so l = 4

ρ = 1

so  resistance of the 3 legs of the triangle = 4/A, 4/A, 4√2/A

battery is connected across the longest branch so eq. resistance = 4/A+4/A+4√2/A = 13.6/A

i think area of cross section will be given then calculate current

Can You explain some easy way to find potential between A and B?

Joshi sir comment

use kirchhoff laws

4-10i+1-30x = 0

1-30x+20(i-x) = 0

solve it for i and x

potential across A and B = 4-10i+1 = 30x

Joshi sir comment

since the circuit is symmetrical with respect to the line given so V, the potentials at the given points will be same

now calculate the eq. for the half circuit and double it for getting the eq. of the whole circuit

Solution by Joshi sir

since the circuit is symmetrical with respect to the line given so V, the potentials at the given points will be same

now calculate the eq. for the half circuit and double it for getting the eq. of the whole circuit

for one side eq. of 2 and 4 = 4/3

and then 4 in series of it so 4/3 + 4 = 16/3

finally 16/3||4/3

eq = 16/15

final answer = 16/15 * 2 = 32/15

Two voltameters one of copper and another of silver, are joined in parallel. When a
total charge q flows through the voltameters, equal amount of metals are deposited.
If the electrochemical equivalents of copper and silver are z1 and z2 respectively the
charge
which flows through the silver voltameter is

Joshi sir comment

m = z1q1 = z2q2 (1)

and q = q1 + q2

now solve

Can you please explain me to find potential difference between two plates of capacitor like this shown in figure. Can you Give anymore examples if possible? Thank You! (Please omit time)

Joshi sir comment

after time t charge on plates of capacitor q = CE(1-e-t/RC)

then potential difference between the plates of capacitor = q/C

after a long time this potential difference = E      [this can be obtained by putting t = ∞]

Joshi sir comment

fig 1 and fig 2 are explaining symmetry method.

divide the circuit in two parts by a line symmetry, then calculate eq. resistance between A and O by series parellel rules then for getting eq. resisitance between A and B double the answer between A and O

Delta star is the method to convert the circuit into an equivalent circuit in which series parallel rules can be applied

By this method a set of three resistances R1, R2 and R3 can be converted to 3 new resistances 1, 2 and 3

where 1 = R1R2/(R1+R2+R3), 2 = R1R3/(R1+R2+R3) and 3 = R2R3/(R1+R2+R3)