Consider the following : I. Gravitational field due to a spherical body II. Electrostatic field due to a point charge III. Strong nuclear force field due to a proton IV. Magnetic field due to a bar magnet How many of the above follow the inverse square law ?
- A.One
- B.Two
- C.Three
- D.All four
Correct answer
Explanation
The correct answer is B, Two. Only the gravitational field of a spherical body and the electrostatic field of a point charge fall off as 1/r².
Outside a spherical body, Newton's law gives a field g = GM/r², exactly as if all the mass sat at the centre. Coulomb's law gives the field of a point charge as E = kq/r². Both weaken four times when the distance doubles. The strong nuclear force, however, acts only over about a femtometre and dies away far faster than 1/r², while a bar magnet behaves as a magnetic dipole whose field at a distance falls as 1/r³.
A is wrong: two of the fields, not one, obey the inverse square law.
C is wrong: it wrongly adds either the short-range nuclear force or the dipole field of the magnet.
D is wrong: the nuclear and bar-magnet fields do not follow 1/r².
Exam tip: point mass and point charge give 1/r²; a dipole (electric or magnetic) gives 1/r³; nuclear forces are short-range.
Practice Questions
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