TBR Physics Chapter 8 - Eletrostatics

[knower214]

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Pg.154 - Passage 3 - Cathode Ray tube.

I do not understand this question. I think the answer should be A, but the answer is B.

Question - On occasion, the electron are made to pass through the plates undeflected. If the electrons enter the region between the plates with the same direction, but with an excessive velocity:

A) they will deflected upwards
B) they will deflected downwards
C) remain undeflected
D) turn back around

Btw, which way is the magnetic and electric fields in this question???

 

[jetsfan1234]

Look at the plates marked + and - in the diagram. The electric field goes from top to bottom. Therefore, the electric field will exert an UPWARDS force on the electron (remember you need to reverse the direction of the field when the particle has a negative charge).

The passage states that the magnetic field balances the force of the electric field. So if an electron is passing thru the plates (from left to right), the electric field pushes it UP, and the magnetic field pushes it DOWN. therefore, by the right hand rule, the B field is pointing into the page.

If a particle has "excessive velocity" it means the B force (=qvB) is overpowering the electric field force. The particle will be pushed downward (answer B).

 

[knower214]

That makes sense. I forgot that an electron would go against the electric field. Stupid mistake!

Thanks for your help!