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Electricity
13 Mar 2008 19:58:31 IST
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A Few Conceptual Problems
None
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2.a positively charged rod is brought near an uncharged conductor.if the rod is then suddenly withdrawn the charge left on the conductor will be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)
Comments (17)
zamuka chen
Scorching goIITian
Joined: 10 Mar 2007
Posts: 216
13 Mar 2008 20:04:55 IST
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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....
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13 Mar 2008 20:22:29 IST
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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.
13 Mar 2008 20:28:35 IST
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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
13 Mar 2008 20:35:36 IST
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(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
13 Mar 2008 20:44:43 IST
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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)
13 Mar 2008 21:00:58 IST
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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
13 Mar 2008 21:11:08 IST
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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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