1true..some ans are not staisfactory..anyway thanks to iit2010 / arsh and kalyan..!
Pls explain the following VERY BASIC question with REASON / CONCEPT involved in it , the answers are given....
1. One brass plate is inserted b/w two charges.The force between the two charges will :
i. Remain the same
ii Increase
iii. Decrease
iv. Fluctuate
Ans given : ii [ the book can have mistakes ..so pls tell the ans which u think is rt..and of course its concept also ! ]
My doubt : [ How ? ]
2. Two negatively charged particles of unit magnitude and charge q are placed along a straight line.If charge q is placed at the midpoint b/w charges , then q will be in :
i. Stable Equlibrium
ii. Unstable Equlibrium
iii. Neutral Equlibrium
iv. None of the above
Ans given : ii
My doubt : [ Why not i ? ]
3. Four equal positive charges are placed at the respective vertices of a square having a point charge Q at the centre [ centre of the square ]..then Q :
i.Must NOT be in equlibrium
ii.Must be in stable equlibrium
iii..Must be in neutral equlibrium
iv..Must be in unstable equlibrium
Ans given : iv
My doubt : [ Why not ii ? ]
4. For a METALLIC Conductor,dielectric constant :
i.Must be zero
ii.Must not be infinity
iii.Must be infinity
iv.May be infinity
Ans given : iv
My doubt : [ Why not iii ? ]
5.A dimensionless body having a physical quantity varies as 1r2 , where r : distance from the body . The physical quantity may be :
i.Gravitational Potnetial
ii.Electric Field
iii.Gravitational Field
iv.None of the above
Ans given : ii
My doubt : [ Why not ii and iii ? ]
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17 Answers
15)
Electric Field and Grav Field R not physical quantities. They R just hypothetical values that make study of Electricity and gravitation easier!!!
There is no such thing as field physically
1q!....recall that in Coulomb's law...the force is medium dependent .....so if the permittivity changes.....the force changes!!!
1I've read in the NCERT textbook that electric fields are very real physical quantities and not just mathematical constructs . Otherwise EM waves wouldn't exist.
But gravitational fields were just invented by us to make studying gravity easier.
I think thats what they meant in question 5
11Sorry dude, I am not very good at this Dielctric stuff...........
I dont want to play wid ur knowledge by trying to answer!!
1am sorry my friend my little knowledge wiould allow me to think that may be implies that metas have cpacitance too.
there is no such thing on the earth wich allows 100% conductance os 100% resistance (except vaccum)
soo may be the limit of physical environment must be considered. that is what allows the explanation. even i am slightly dissapointed with my answer. it shud be [iii] as per our syllabus.
1of cource my friend electric field is a physical quantity. it will be very unwise to that it isnt. people in the 1990s subjected to that. but coming to the question.
we have given that the body is dimensionless. may i take the liberty to tell you dat and dimensionless body will either have zero mass or infinite mass (according to Einstein's special theory of relativity). but if you see logicall i dimentionless quantity cannot have mass. we have not defined it in any of our courses. so gravitation field moves out of question.
while a charge is considered it is well said that it is dimentionless and the mass f the cahrge is neglegted in our calculation. therefore we can presumebaly say within the limits of our syllabus that electric field is the answer.
in reality this answer may also be wrong. but we cannot dweell into higher prospects of physcs with such little amount of information that we have at this stage. thank you
1thanks to kalyan and iit2010...but CAN U PLS EXPLAIN QUE 1.and 4....pls ?
1let me explain you the same concept with a slightly different question. suppose on an inverted hemisphere a ball is kept as shown in the figure.
we define this as unstable equilibrium, it is one of the most famous examples of this. but what if we slightly displace it in the upward direction it will bounce back slowly on the midpoint and remain there. so this can also be argued as a case of stable equilibrum. but it is not so. so in these cases we see the potential energy. it will be maximum as compared to any other point on the hemisphere.
in the similar way if yew caclucate the potential enrgy by taking some rough measurement of the charge and distance you will see that it is highest at that point. this will proove that it is in unstable equilibrium. thank you
41) Increases because capacitance increases
3) I think i i coz F = 0
4) I think we can't say it must be infinity coz infinity is not a specific no. and there may be slight experimental variations too
5) Given dimensionless that means small (like a point charge)
But for gravitational field we have masses to be considered
1yeah thats right, but U need to consider all the possible ways, b4 deciding it is stable.
But NEway, wen U R moving it in the equitorial plane, the potential remains same...... U can check for stability of equilibrium only by slightly changing the potential..........
So, mere moving of charge without change in potential doesnt give NE idea of stability of eqilibrium!!
[339]
1but what if the charge is moved along the perpendicular bisector ..then equilibrium is STABLE rt..what explanation do u have for that ..pls ?[7]
12)
If you displace the charge slightly towards one side, the -ve charge closer to it attracts it more than the other -ve charge (inverse square law). So this q tends to move towards this -ve charge and not toward the equilibrium position. So the equilibrium is unstable!!!
Here is a rough diadram
A-._____a______q._________b__________.-B
b>a
The charge at A attracts q more than the charge at B, so it moves towards A and not equi position
[339]
1Also... the concept of infinity is clearly explained in Mathematics only...in physics we have to assume that it is infinity...we have no other way out..
1-5 :
HOW CAN WE ASSUME THAT POINT CHARGES ARE MASSLESS..THEN WE CANT HAVE KINETIC ENERGY FOR POINT MASSES..[ a lot of ques on KE of point charges are existing...]...???[7]
-4 : Sorry , but answer is not convincing..what can i do ?[7]
-1 : How will capacitance increase when a metal with dielectric onstant=infinity is inserted ??[11]