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Objective Problems with Approach and Solutions (in Hindi)
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2 objective questions based on Coulomb's law

## Udit Gupta is teaching live on Unacademy Plus

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

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sir wo quad eqn wala part samaja nahi
Udit Gupta
3 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!
sir component bale 2 question me B samajha nahi aaya
Udit Gupta
10 months 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.
sir I don't understand the calculus part of Q. 2.
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
Tq for question discussion, will you do some more question discussion
Udit Gupta
a year ago
in some time
sir this realy increase my level
1. Important Questions Udit Gupta Alumnus MNNIT Allahabad Ex-VMC Faculty of physics

2. 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

3. 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.

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

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

6. 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

7. 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.

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

9. 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.