A beam of neutrons in a vacuum is passed between two charged metal plates. Which of the following diagrams shows the trajectory that the neutron beam will follow when it passes between the plates? The red plate is the positively charged plate, and the blue plate is the negatively charged plate. Ignore the effect of gravity.
Okay, so in this question, we’ve got a beam of neutrons in a vacuum passing between two charged plates. So here’s our beam of neutrons. And it’s moving this way into a region of space between the positively charged red plate and the negatively charged blue plate.
Now, because the beam of neutrons is in a vacuum, this means that there’re no particles of air, for example, in the region between the positive and the negatively charged plates. And as well as this, we’ve been told to ignore the effect of gravity. This means that the only thing that could potentially affect the motion of the neutron beam is the fact that these plates are charged.
In other words then, we’re being asked to consider whether the fact that the plates are charged will affect the motion of the neutron beam, for example, due to an electrostatic force.
However, we can recall that neutrons are not charged particles. They are neutral, hence the name neutron. And therefore, they will not interact with other charged particles or in this case charged metal plates, because they’re neutral and so their electrostatic interaction is zero.
If they were charged particles though, then they would be attracted to the plate with the opposite charge to themselves and repel from the plate that had the same charge. However, let’s not worry about that. Neutrons are not charged. And therefore, they will not interact with the charged plates.
So this diagram is incorrect, because in this case, the neutron beam is shown to be affected by the charged plates. The neutrons are shown to go around in circles which is incorrect.
Similarly, this diagram is also incorrect. This one shows the neutrons moving towards one of the plates as if being attracted to that plate. And the fourth diagram is incorrect for the same reason. It’s attracted to the other plate.
However, the correct diagram is this third diagram here, because it shows the neutron beam simply passing straight through the region between the plates. And that’s exactly what neutrons would do. They’re neutral.
Hence, this third diagram correctly shows the trajectory that the neutron beam will follow when it passes between the plates.