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Electrostatics question

2021 · 25 Jul · Shift 2 · Q56
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  5. /2021 · 25 Jul · Shift 2 · Q56

Electrostatics question

2021 · 25 Jul · Shift 2 · Q56

JEE MainPhysicsElectrostaticsMCQ+4 / −1
Two ideal electric dipoles A and B, having their dipole moment p1 and p2 respectively are placed on a plane with their centres at O as shown in the figure. At point C on the axis of dipole A, the resultant electric field is making an angle of 37 ∘^\circ∘ with the axis. The ratio of the dipole moment of A and B, p1p2{{{p_1}} \over {{p_2}}}p2​p1​​ is : (take sin⁡37∘=35\sin 37^\circ = {3 \over 5}sin37∘=53​) JEE Main 2021 (Online) 25th July Evening Shift Physics - Electrostatics Question 141 English
  1. A
    38{3 \over 8}83​
  2. B
    32{3 \over 2}23​
  3. C
    23{2 \over 3}32​
  4. D
    43{4 \over 3}34​
View written solutionFree

Correct answer: C

  1. Electric field due to a dipole on axial and equatorial lines

For an ideal dipole of moment ppp at a point at distance rrr from its centre:

  • On the axial line, Eaxial=14πε02pr3E_{\text{axial}}=\frac{1}{4\pi\varepsilon_0}\frac{2p}{r^3}Eaxial​=4πε0​1​r32p​
  • On the equatorial line, Eequatorial=14πε0pr3E_{\text{equatorial}}=\frac{1}{4\pi\varepsilon_0}\frac{p}{r^3}Eequatorial​=4πε0​1​r3p​
  1. Interpret the figure

Since point CCC lies on the axis of dipole AAA, the field due to dipole AAA at CCC is along the axis of dipole AAA.

From the usual configuration implied by the question, dipole BBB is perpendicular to dipole AAA, and point CCC lies on the equatorial line of dipole BBB.

Let the distance OC=rOC=rOC=r.

Then,

  • Field at CCC due to dipole AAA: EA=14πε02p1r3E_A=\frac{1}{4\pi\varepsilon_0}\frac{2p_1}{r^3}EA​=4πε0​1​r32p1​​
  • Field at CCC due to dipole BBB: EB=14πε0p2r3E_B=\frac{1}{4\pi\varepsilon_0}\frac{p_2}{r^3}EB​=4πε0​1​r3p2​​

These two fields are perpendicular.

  1. Use the given angle

The resultant electric field makes an angle 37∘37^\circ37∘ with the axis of dipole AAA.

So, tan⁡37∘=EBEA\tan 37^\circ=\frac{E_B}{E_A}tan37∘=EA​EB​​

Given, sin⁡37∘=35\sin 37^\circ=\frac{3}{5}sin37∘=53​ which implies cos⁡37∘=45\cos 37^\circ=\frac{4}{5}cos37∘=54​ Hence, tan⁡37∘=34\tan 37^\circ=\frac{3}{4}tan37∘=43​

Therefore, EBEA=34\frac{E_B}{E_A}=\frac{3}{4}EA​EB​​=43​

Substitute the expressions: 14πε0p2r314πε02p1r3=34\frac{\frac{1}{4\pi\varepsilon_0}\frac{p_2}{r^3}}{\frac{1}{4\pi\varepsilon_0}\frac{2p_1}{r^3}}=\frac{3}{4}4πε0​1​r32p1​​4πε0​1​r3p2​​​=43​

p22p1=34\frac{p_2}{2p_1}=\frac{3}{4}2p1​p2​​=43​

4p2=6p14p_2=6p_14p2​=6p1​

p1p2=23\frac{p_1}{p_2}=\frac{2}{3}p2​p1​​=32​

  1. Match with options

p1p2=23\boxed{\frac{p_1}{p_2}=\frac{2}{3}}p2​p1​​=32​​

So the correct option is C.

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