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Electrostatics question

2025 · 23 Jan · Shift 1 · Q74
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  5. /2025 · 23 Jan · Shift 1 · Q74

Electrostatics question

2025 · 23 Jan · Shift 1 · Q74

JEE MainPhysicsElectrostaticsNumerical+4 / −1
A positive ion AAA and a negative ion BBB has charges 6.67×10−19C6.67 \times 10^{-19} \mathrm{C}6.67×10−19C and 9.6×10−10C9.6 \times 10^{-10} \mathrm{C}9.6×10−10C, and masses 19.2×10−27 kg19.2 \times 10^{-27} \mathrm{~kg}19.2×10−27 kg and 9×10−27 kg9 \times 10^{-27} \mathrm{~kg}9×10−27 kg respectively. At an instant, the ions are separated by a certain distance rrr. At that instant the ratio of the magnitudes of electrostatic force to gravitational force is P×10−13\mathrm{P} \times 10^{-13}P×10−13, where the value of P is ‾\underline{\hspace{2cm}}​. (Take 14πε0=9×109Nm2C−1\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \mathrm{Nm}^2 \mathrm{C}^{-1}4πε0​1​=9×109Nm2C−1 and universal gravitational constant as 6.67×10−11Nm2 kg−26.67 \times 10^{-11} \mathrm{Nm}^2 \mathrm{~kg}^{-2}6.67×10−11Nm2 kg−2 )
Numerical answer
View written solutionFree

Correct answer: BONUS

  1. Write the forces

For two particles separated by distance rrr:

  • Electrostatic force magnitude: Fe=14πε0∣q1q2∣r2F_e = \frac{1}{4\pi\varepsilon_0}\frac{|q_1 q_2|}{r^2}Fe​=4πε0​1​r2∣q1​q2​∣​

  • Gravitational force magnitude: Fg=Gm1m2r2F_g = G\frac{m_1 m_2}{r^2}Fg​=Gr2m1​m2​​

So their ratio is FeFg=14πε0∣q1q2∣Gm1m2\frac{F_e}{F_g} = \frac{\frac{1}{4\pi\varepsilon_0}|q_1q_2|}{Gm_1m_2}Fg​Fe​​=Gm1​m2​4πε0​1​∣q1​q2​∣​

Notice that r2r^2r2 cancels, so the ratio is independent of distance.


  1. Substitute the given values

Given: q1=6.67×10−19 C,q2=9.6×10−10 Cq_1 = 6.67\times 10^{-19}\,\text{C}, \qquad q_2 = 9.6\times 10^{-10}\,\text{C}q1​=6.67×10−19C,q2​=9.6×10−10C m1=19.2×10−27 kg,m2=9×10−27 kgm_1 = 19.2\times 10^{-27}\,\text{kg}, \qquad m_2 = 9\times 10^{-27}\,\text{kg}m1​=19.2×10−27kg,m2​=9×10−27kg 14πε0=9×109,G=6.67×10−11\frac{1}{4\pi\varepsilon_0}=9\times 10^9, \qquad G=6.67\times 10^{-11}4πε0​1​=9×109,G=6.67×10−11

Thus FeFg=(9×109)(6.67×10−19)(9.6×10−10)(6.67×10−11)(19.2×10−27)(9×10−27)\frac{F_e}{F_g}=\frac{(9\times 10^9)(6.67\times 10^{-19})(9.6\times 10^{-10})}{(6.67\times 10^{-11})(19.2\times 10^{-27})(9\times 10^{-27})}Fg​Fe​​=(6.67×10−11)(19.2×10−27)(9×10−27)(9×109)(6.67×10−19)(9.6×10−10)​


  1. Simplify carefully

First, cancel common factors 6.676.676.67 and 999 from numerator and denominator: FeFg=109⋅9.6×10−19×10−1010−11⋅19.2×10−27×10−27\frac{F_e}{F_g}=\frac{10^9\cdot 9.6\times 10^{-19}\times 10^{-10}}{10^{-11}\cdot 19.2\times 10^{-27}\times 10^{-27}}Fg​Fe​​=10−11⋅19.2×10−27×10−27109⋅9.6×10−19×10−10​

Now, 9.619.2=12\frac{9.6}{19.2}=\frac{1}{2}19.29.6​=21​

So FeFg=12×109−19−10+11+27+27\frac{F_e}{F_g}=\frac{1}{2}\times 10^{9-19-10+11+27+27}Fg​Fe​​=21​×109−19−10+11+27+27

Add exponents: 9−19−10+11+27+27=459-19-10+11+27+27 = 459−19−10+11+27+27=45

Hence FeFg=12×1045=5×1044\frac{F_e}{F_g}=\frac{1}{2}\times 10^{45} = 5\times 10^{44}Fg​Fe​​=21​×1045=5×1044


  1. Match with the given form

The question states the ratio is P×10−13P\times 10^{-13}P×10−13.

So, P×10−13=5×1044P\times 10^{-13} = 5\times 10^{44}P×10−13=5×1044

Therefore, P=5×1057P = 5\times 10^{57}P=5×1057


  1. Interpretation

Since this is an integer-type question, PPP is expected to be an integer. But here, P=5×1057P = 5\times 10^{57}P=5×1057 which is not a usual finite integer entry format for such exams.

This strongly indicates a misprint in the question statement, most likely in the power of 101010 attached to one of the charges, or in the form P×10−13P\times 10^{-13}P×10−13.

If the intended form had been P×1044P\times 10^{44}P×1044, then P=5P=5P=5.

So the given data and asked format are inconsistent, which justifies the stored answer being BONUS.

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