20 - Current Electricity Questions Answers

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 = ∞]

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) 

you should take the formula in which one variable is constant and for a circuit current will be constant

H = W = (i x i) x R x t   [ H directly proportional to R ]

this one is correct for the heat generated

q= 2te^-kt 

on differentiating with respect to t, we get

dq/dt = 2[t(-k)e^-kt + e^-kt ]

so for V = 0, i will be 0

so t = 1/k  

for series R = R₁ + R₂ + R₃ + R₄

and R = V²/P { This is the general case to find the resistence of the device)

now use this and get the result 

to connect - and + of the cell together

emf generated between the extremes of equator line = vBl = RωBl

so E = RωBl/l = RωB = 2πRB/T 

now calculate

length are equal and metals are different 

but none of the two is a deciding factor for the thickness of wire, so question is irrelevent

This can be explain by using band theory.

According to this theory movable electrons are present in valence band but for conduction their transfer to conduction band (far from the nucleus in comparison to valence band) is compulsory. In insulators the gap between valence and conduction band is large thats why tranfer is not possible so no conduction