20 - Current Electricity Questions Answers

Sir/Madam, 

                    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)

Asked By: BONEY HAVELIWALA
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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 = ∞]

Mam/Sir,  Please Help Me to Understand The Symmetry method And Star-Delta Method To find Eqivalent Resistance?

 

 

Asked By: GAUTAM SAGAR
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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) 

In an electrical circuit...

H = W = i2Rt   [ H directly proportional to R ]

But i2Rt can also be written as (V/R) x (V/R) x R x t  =  V2t/R...

H = W =  V2t/R .  [ H inversely proportional to R ]

 

therefore my question is how can heat be both directly and inversely proportional to the resistance in the circuit??

Asked By: AAYUSH TANEJA
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Joshi sir comment

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

In an electrical circuit...

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

H = W =  (V x V) x t /R .  [ H inversely proportional to R ]

So is Heat Generated directly proportional or inversely proportional to the resistance in the circuit ??

Asked By: AAYUSH TANEJA
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Joshi sir comment

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

this one is correct for the heat generated

charge flowing thru a xsection of wire varies with time as q= 2te-kt , where k is const . the voltmeter reads zero at t=??

Asked By: SARIKA
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Joshi sir comment

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  

the effective power of 3 lamps A (100W , 200V)  , B(60W,200V) ,C (40W,200V) connected in series is equal to 600/31W . how??

Asked By: SARIKA Solved By: ONLY NAME IS ENOUGH
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Joshi sir comment

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 

 what is a short circuit?

 

Asked By: SWARNENDU MANNA
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Joshi sir comment

to connect - and + of the cell together

why is electric field of earth is in order of 100v/m any reason by formulae

Asked By: ABHISHEK JAIN
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Joshi sir comment

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

so E = RωBl/l = RωB = 2πRB/T 

now calculate

we taken a equal length of wire like copper and  maganim which one is more thicker ?

Asked By: VIKRANT KUMAR
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Joshi sir comment

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

why current is not flow in insultor  give me proper reasion and its funtions ?

Asked By: VIKRANT KUMAR
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Joshi sir comment

 

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

 

 

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