0
$\begingroup$

The left bar chart in the figure above is complete and represents the initial electric potential energy UE, i of the atom and the initial kinetic energy K i of the electron before the photon is absorbed. In the space provided on the right, draw a bar chart to represent a possible final electric potential energy of the atom and final kinetic energy of the electron

Answer: Potential and kinetic energy get smaller in magnitude, but not zero.

This is a question from the AP Physics 2 2022 FRQ. Why does the potential energy of the atom and the kinetic energy of the electron decrease after a photon is absorbed? Shouldn't both of them increase?

Improve this question
$\endgroup$

1 Answer 1

Reset to default
2
$\begingroup$

In the answer key, especially the picture, it clearly showed that the potential energy increased.

The wording of it being

Smaller in magnitude but still negative

is a positive change.

As for why the kinetic energy decreased, that is a consequence of a detail in the calculation. You can get the correct behaviour from Virial theorem—since the magnitude of the potential energy decreased, so must the magnitude of the kinetic energy.

If you want a more physical understanding, the approximate idea is that, if you have a circular orbit of greater radius at the same angular momentum, then the orbit goes slower. This is the case even in the orbits of the planets in our Solar system. Orbits that are farther away simply has to be slower. The gravitational force gets weaker, and this is the physical interaction causing the centripetal acceleration, and so the orbits must be slower.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Thank you! That makes a lot of sense. I realized later that you could also see that the kinetic energy decreases from the equation they make you derive earlier in the equation. $\endgroup$
    – Frog
    Commented May 24, 2023 at 22:19

Not the answer you're looking for? Browse other questions tagged or ask your own question.