IB Physics Does the Mass of an Atom Change when it emits Light?

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A tutorial sheet that explores the relationship between mass and energy when an atom emits light. Some explanatory notes are given from the article by Ralph Baierlein, “Teaching E=mc2, An Exploration of Some Issues” published in The Physics Teacher, March 1991, 170-173.

  1. When an atom emits a photon the atom's energy decreases.
  2. When the energy of the atom decreased so did its inertia (mass).
  3. Changes in mass and energy occur in parallel. There is no conversion of one into the other.
  4. As Einstein stated, if the energy changes by ΔE then the mass changes in the same sense by Δm/c2.
  5. An atom of mass m is at rest. The atom releases a photon of frequency f. Show that the mass of the atom after it releases the photon is m √( 1 - 2 hf/mc2 ). Explanation.Four momentum is conserved. The first component of the four momentum vector is energy. The second component of the four momentum vector is linear momentum. mc2= hf + √(p2c2+m12c2) and hf/c=p. Solve simultaneously to find m1 by eliminating p.
  6. The electrons, protons and neutrons in the atom do not change mass. The released energy comes from the electron in the atom making a transition to a lower energy state.