Important Questions Udit Gupta Alumnus MNNIT Allahabad Ex-VMC Faculty of physics

Question 1 Two identical charged spheres are suspended in air from the same point by strings of equal length. The strings make an angle of 300 with vertical When suspended in a liquid of density 800 kg/m3, the angle remains same. If the density of material of sphere is 1600 kg/m3, the dielectric constant of the liquid is? (a) 1 (b) 2 (c) 3 (d) 4

Approach I hope you can imagine these two figures by reading the question. The difference in both is that in one case they are immersed in air, and in other, a liquid. But the angle remains same. This brings us to the point that electric force would change, hence if the angle has to remain same there must be change in some other force. 'IMMERSED'='BUO YANCY, FBD should do the job now.

Solution In ai Tsine F Tcose mg F mgtane In liquid Tsing F B +T'cose mg ir mg 2 mg Liquid

Solution F mg mg - B dVg 1600 2 d_ 1600-800

Question2 Two positive point charges a distance b apart have a total charge Q. For what values of charges, is the Coulomb force between them maximum? (a) Q,0 2 Q (b) 2'2 Q 30 (d) Need more information

Approach We can assume some charge (x) on one and remaining on the other (Q-x) Coulomb's force can be given as x(Q-x) This force is dependent on x. To maximize this force we can use either quadratic equation or calculus Mathematics. Physics here is done.

Solution x(Q-x) for maximum FO 0 'dx 2 Correct option is (b).

Solution By quadratic equation method, Assume q1 and q2 to be the charges on the bodies (q1+q2)2-(q1-q2)2 Q2-(q1-q2)2 1 + q2 Q. To maximize q1q2- Or,q1 = q2 = 2.

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Udit Gupta

A Mechanical Engineer from NIT Allahabad here to help learners with physics. Also a Star ⭐ educator.

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Sarath G

2 years ago

Hi,
I have one doubt regarding the seating arrangement question. In the question it is mentioned as "A, B, C,D,E is sitting along with their wives" implies there are 10 persons in which A, B,C,D,E are men's so as per question C's wife should come to the left side of A not C
Please correct me if I am wrong

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Venkatesan S

2 years ago

Hi Sarath.. Yes, you are correct. We have 10 persons (5 Pairs. Ex: Mr & Mrs. A). And the location of Mrs. C is also right. All the best. Happy Learning :)

Dr

Dipeeka rathod

a year ago

hello sir
I have no problem in quant,VRC n DI but LR is fully blanked so what can I do in this section please help me

Dr

Dipeeka rathod

a year ago

oh sry

Sc

Sayani chowdhury

6 months ago

sir wo quad eqn wala part samaja nahi

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Udit Gupta

6 months ago

kewal differentiation wala bhi yaad rakhoge toh kaam ho jayega.. no need for quadratic part if you are not getting it.. waise just ask your maths teacher to solve it for you. I hope he/she would find it easy. since idhar maths nhi likh sakte toh it's gonna be difficult for me. hope you understand!

Saxena Satyam

a year ago

sir component bale 2 question me B samajha nahi aaya

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Udit Gupta

a year ago

i hope.ki aapko maths mei differentiation wala topic aata hoga. All you need to.differentiate is x(Q-x) and put it equal to zero (Q-2x). It will give the option b.

Ananya

a year ago

sir I don't understand the calculus part of Q. 2.

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Udit Gupta

a year ago

Assume the denominator to be k. if you differentiate kx(Q-x) and put it equal to zero this should do the job. on differentiating, you get, kQ-2kx. Putting it equal to zero, kQ-2kx=0, x=Q/2

Sachin kumar

a year ago

Tq for question discussion, will you do some more question discussion

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Udit Gupta

a year ago

in some time

Samar Kumar

a year ago

sir kya,itna hi numerical kafi h electrostatic ka iit jee agr hcv solve nhi krte h toh

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Udit Gupta

a year ago

you need to do more, this is just the polishing from zero to something