IIT JEE , AIEEE Forum - Physics , Electricity

the.sniper

True or False ?

Same amount of maximum charge can be given to a capacitor placed in vacuum and a capacitor placed in air because

q=CV
q=CV
q= $4 \pi \epsilon KR$ V

now for air and vaccum K=1

$\Rightarrow$ q= $4 \pi \epsilon R$ V $\Rightarrow$ maximum charge is also same for the two cases

(it is assumed that value of K for vacuum and air are exactly equal and therefore this reason that value of maximum charge is not equal for the two because their values of K are not equal is not valid )

kundan_leo

Interesting qs sniper yaar........

i think for the answer to this qs is TRUE......

bcoz the only thing thatcan vary was the dielectric constant.......K

but they have given it to be same.....

rest all

= permittivity of the material (dielectric)

and pot diff and resistance aren't likely to change either.....

BUT IT WAS INTRIGUING SO I LOOKED ......

AND U KNOW WHAT......

K DOES VARY FOR VACUUM AND AIR.....

u know free of the dust particles and stuff , the dielectric constant of VACUUM CAPACITORS ARE MORE .......they are in flurry rit now.....

hope it helps

cheers

newarnav

Electric Field between plates cannot exceed 3MV(megavolt)/metre

if it does air becomes conductor (dielectric breakdown) and pot of two plates becomes equal after the spark

hence for vaccum Q max is infinit , not for cap placed in air

(@uperwala as far as k is concerned NOwhere is perfect vaccume)

the.sniper

newarnav is right

newarnav

huh...

m i rit ?

eistien

i too feel the same way as you have to consider also the dielectric strength too where there is a varience between air and vacuum !!! under the normal conditions we then can say they can but consider the fact that air must not get ionised and we can keep the capacitor in a metal chamber under pressure to prevent this happening !!!!i hope i too am rite!!!

computer001

srini/eistien dielectric breakdown depends on the magnitude of electri field...how will metal chamber help?? do u mean something along the lines of electrostatic shielding??
if so then u r incorrect as the field mis not due to ne external source but it is produced between platec of the capacitor...cud u pls explain ur pt

trevoris

gokul check vandegraff generator working and construction for my explaination for your question!!!!

trevoris

hi da i am gaurav da gokul!!

computer001

pl post ur explanation...its not jus 4 me..its 4 every1 to c...and not every1 will be able to link the pt with van de graff etc

eistien

Oh cool da gaurav too online ah!!!

nways heres the explaination check even www.wikipedia.org and ncert text books

the whole vandegraff generator is placed inside a metallic chamber under pressure why???

the reason is that as the charge on the outer sphere there is increasing there is a possibilty for the air around to get ionised as air has a certain dielectric strength the metallic chamber prevents charge leakage and pressure prevents ionsiation

here in vacuum you dont have an molecules to get ionsied and thus dielectric strength no defined but air has a certain dielectric strength accepted ah gokul!!!

any contradictions???

computer001

ur explanation is not correct i think
a vandegraff generator is surrounded by a metallic chamber @ high pressure containing nitrogen or methane so as to minimise the leakage of current..only ..i dont see ne explanations regarding the ionisation part...if u c the explanation..it is given tht the van de graff works only upto the field of strength corresponding to dielectric breakdown of air..above which ionization takes place leading to leakage of charge which is minimised by nitrogen/methane @ hihgh pressure

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