Question 19M.3.SL.TZ2.5
| Date | May 2019 | Marks available | [Maximum mark: 4] | Reference code | 19M.3.SL.TZ2.5 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain, Outline | Question number | 5 | Adapted from | N/A |
Two protons are moving to the right with the same speed v with respect to an observer at rest in the laboratory frame.
Outline why there is an attractive magnetic force on each proton in the laboratory frame.
[1]
moving charges give rise to magnetic fields
OR
magnetic attraction between parallel currents ✔
Candidates usually realised that the magnetic field was due to the motion of the protons and that in the proton rest frame there could be no magnetic field. The answers were too often poorly worded and the candidates appeared to reword the question without providing a clear explanation.
Explain why there is no magnetic force on each proton in its own rest frame.
[1]
protons at rest produce no magnetic field
OR
mention of F = Bev where B and/or v =0 ✔
Candidates usually realised that the magnetic field was due to the motion of the protons and that in the proton rest frame there could be no magnetic field. The answers were too often poorly worded and the candidates appeared to reword the question without providing a clear explanation.
Explain why there must be a resultant repulsive force on the protons in all reference frames.
[2]
there is a repulsive electric/electrostatic force «in both frames» ✔
the attractive magnetic force «in the lab frame» is smaller than the repulsive electric force ✔
in all frames the net force is repulsive as all must agree that protons move apart
OR
mention of the first postulate of relativity ✔
A few candidates mentioned that there was an electrostatic repulsive force between the protons in both frames. However very few realised that there had to be an overall repulsive force in both frames because of the relativity postulate.
