Atomic Physics and Quantum Effects for AP Physics B - Free Response Question for Practice

Today I’ll give you a free response practice question from the section, ‘atomic physics and quantum effects’ on the hope that you will try to answer it and will become aware of the use of mastering fundamental concepts in this section. Here is the question:

An electron and its antiparticle positron, moving with the same speed, undergo a head-on collision and get annihilated to produce two photons of equal wave length. This process is called pair-annihilation. Positron has the same mass as the electron, but its charge is positive.

(a) Determine the rest energy of a positron in electron volt.

(b) Determine the maximum wave length of the photons generated in the electron-positron pair annihilation.

(c) Which one of the following statements [(i), (ii) and (iii)] is correct regarding the direction of motion of the two photons? Put a tick (√) mark against the correct choice.

(i) The photons move along the same direction……..

(ii) The photons move along opposite directions……..

(iii) The photons move along perpendicular directions……..

Justify your answer to part (c) above.

(d) Explain why two photons (and not a single one) have to be produced in pair annihilation.

(e) If the particles undergoing pair annihilation were proton and its antiparticle antiproton (instead of electron and positron), what can you say about the maximum wave length (λ₁) of the photons generated, in comparison with the value obtained in the case of electron-positron pair annihilation? Put a tick (√) mark against the correct answer choosing from the options (i), (ii) and (iii) given below:

(i) λ₁ is increased…….

(ii) λ₁ is the same…….

(iii) λ₁ is decreased…….

Justify your answer.

I’ll post a model answer for your benefit soon. Meanwhile, answer the above question yourself so that you will be aware of your strength and weakness (if any)!

The above question carries 10 points and you may take 11 minutes for answering it.