Experiment on determination of charge mass ratio of the electron

Question

1. if the particles are switched from electrons to protons, how does the sign and mass change affect the motion?
2. Explain the influence of the Earth’s magnetic field (~0.05 mT) on the accuracy of the data obtained.

Answer 1

1.

The proton's charge is positive and equal in magnitude, which mean it will experience a force in the opposite direction (of that of the electron). The proton's mass is about 1800 times bigger that that of the electron, so the the acceleration (a=F/m) will be much smaller.

2.

Without the setup of the experiment you conducted it is hard to tell, but generally - the earth's magnetic field CAN interfere with the experiments.

A couple of ways to negate this effect are:

1. Use much stronger magnetic fields in your experimental setup. The stronger the fields you use (relative to the earth's) the more the effect of the earth can be negelected.
2. Construct the system in such a way that it is parallel to the earth's field (at your location on earth). That way, the partical will not feel any magnetic force from the earth (remember that a cross product between two parallel vectors is 0).