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When an electron in a hydrogen atom drops from an excited level to the ground level, which statement is correct?

1. Its kinetic energy rises, while its potential energy and total energy both fall.
2. Its kinetic energy falls, its potential energy rises, and its total energy stays unchanged.
3. Its kinetic energy and total energy both fall, and its potential energy rises.
4. Its kinetic, potential, and total energies all fall.

Correct answer: Its kinetic energy rises, while its potential energy and total energy both fall.

Solution

Dropping to a lower n makes total energy more negative (falls) and KE = -E_total increases (rises), while PE = 2*E_total becomes more negative (falls). So KE rises while PE and total energy both fall.

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