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An electron is moving round the nucleus of a hydrogen atom in a circular orbit of radius r. The Coulomb force F between the two is:

1. K e² / r³ r̂
2. K e² / r² r̂
3. -K e² / r³ r̂
4. -K e² / r² r̂ (where K = 1 / 4πε₀)

Correct answer: -K e² / r² r̂ (where K = 1 / 4πε₀)

Solution

The Coulomb force between the electron and the nucleus is attractive, so it is directed towards the nucleus. The magnitude of the force is given by Coulomb's law, F = K e² / r², and the negative sign indicates the direction of the force is opposite to the radial vector r̂.

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