IIT JEE , AIEEE Forum - Physics , Electricity

kane

1.why the dielectric constant of an insulator can never be infinite whereas that of a conductor is infinite?(plz also tell me the physical significance of dielectric constant)

2.a positively charged rod is brought near an uncharged conductor.if the rod is then suddenly withdrawn the charge left on the conductor will be

a-positive b-zero c-negative d-none

(answer is b)

(plz tell me why the answer is not c)

3.which among the following is a sure test of electrification?

a-attraction b-induction c-repulsion d-conduction

(answer is c)

4.when a positively charged conductor is earth connected

a-electrons flow from conductor to earth

b-no charge flow occurs

c-protons flow from conductor to earth

d-electrons flow from earth to conductor

(answer is b)

(plz tell me why the answer is not c and also tell me exactly what is earthing)

zamuka

firstly, dielectric cons is a inverse measure of the tendency of a material to let the electric field through it.....since a conductor doesn't allow so it has infinite......

2 charging thru induction doesn't involve transfer of charges.....so net charge will remain 0 as before.....

3)repulsion is a sure test for electrification as attraction can b bet 1 charged and 1 uncharged body but repulsion is only bet 2 charged bodies(that too opp charged..)

4)when apositively charged conduc is connec to earth,electrons will flow frm earth to it....ur ans given is wrong....

kane

thnxs a lot for replying but only your 3rd answer convinces me.in 1st answer you have written conductors do not allow,yeh kaise ho sakta hain.or induction se bhi toh body charge hoti.e.g. if we bring a positively charged rod near a body,it gets negatively charged after earthing.

sid.shah.90

Dielectric constant of a medium is a factor by which forces between two charged particles at rest is decreased by as compared to force when in vacuum

for eg. If force is F in vacuum then in a medium of dielectric constt. k, force will now be F/k

sid.shah.90

(2)Look as the charge is conserved on the conductor( it is not connected or earthed), so as soon as the attractive force from the rod is removed, the electrons reorient themselves again so that the body becomes fully neutral

(3) If we bring an uncharged body near a charged one, they will attract each other, but won't repel in any situation...so repulsion can occur only if both bodies are charged.eg uncharged paper bits are attracted by charged comb

(4) Earthing is actually the process of making potential of body = zero

btw, answer should be (d) and not (b), plz check

because to make potential of conductor = zero, negative electrons must flow from earth into conductor

kane

still having problem in 1 and 4

sid.shah.90

Look, lemme put it this way--

(1) electric field in any conductor (in electrostats) is always 0.

Now dielecric constt. can also be said to be factorr by which elec field in a medium dec.

So As E=0 in conductor thus, k = infinity

(2) Potential will be zero if net charge = 0

So electrons will flow from earth to conductor to make charge = 0

Plz recheck answer for (4)

kane

why electric field of a conductor is zero?

and i have rechecked answer is b only and the explanation given in book is 'the conductor looses charge.'

sid.shah.90

See, as said in the book, the conductor loses its charge...

u know, protons don't move in conduction, its only the electrons(protons are like kernels with electrons as mobile with metallic bonds between them)

To prove electric field in a conductor is zero,

the conductor is an equipotential surface, so theres no component of field along the conductor's surface

As E=-dV/dr

now conductor is equipotential, dv/dr = 0

Thus, E = 0

suresan

soppose there is electric field in side a conductor then the free electrons within the conductor will re-arrange themselves because of this electric field till it becomes zero.

kane

alright 1st question ka abhi bhi kuch pata nahin chala par if you tell me the physical and not bookish meaning of dielectric constant i will be able to understand

sid.shah.90

This is the physical meaning itself dude!

lemme give u a formula which will help:

charge induced on a body in a medium of dielectric constt k by another body with charge Q is given by:

Qinduced = Q[1-1/k]

suresan

if we place a body in a uniform electric field some times the electric field lines will prefer to pass through it crowding in and some times they tend to move away avoiding it. Then the electric field line density will not be the same inside and out side. the ratio of the electric field line dinsity outside to the electric field line density inside is the dielectric constant.

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