How do you determine E field direction in a velocity selector for a proton vs electron?

[josephcollins]

Hi ppl,
I'm having a little trouble determining the direction of the E field in a velocity selector question. I can easily find the direction of the magnetic B field and the direction it forces a particle in. Once I do this, I know that the E field must oppose the force of the B field. The problem I have is determining the direction of the E field for a proton or an electron ( what the difference is) and also how to assign the polarity of the plates of the E field again for a proton and an electron.

Any help would be welcome, thanks, joe

 

[member 553083]

In a velocity selector, the electric field (E-field) will always be perpendicular to the magnetic field (B-field). This means that for protons, the E-field will be directed towards the positive plate and away from the negative plate, while for electrons it will be in the opposite direction. The polarity of the plates will depend on the direction of the B-field. If the B-field is directed towards the positive plate then the E-field will also be directed towards the positive plate. Conversely, if the B-field is directed towards the negative plate then the E-field will also be directed towards the negative plate.

 

[wais]

Hi Joe,

The direction of the electric field in a velocity selector is dependent on the charge of the particle being selected. For a proton, which has a positive charge, the electric field must be directed towards the negative plate of the selector. This is because the electric force on a positive charge is in the direction of the electric field. Similarly, for an electron, which has a negative charge, the electric field must be directed towards the positive plate of the selector.

To determine the polarity of the plates, you can use the right-hand rule. Point your thumb in the direction of the magnetic field, and your fingers will curl in the direction of the electric field. For a proton, the fingers will point towards the negative plate, and for an electron, the fingers will point towards the positive plate.

I hope this helps! Let me know if you have any further questions.