PhysicsLAB AP Free Response Question
1999 B2

In a television set, electrons are first accelerated from rest through a potential difference in an electron gun. They then pass through deflecting plates before striking the screen.
(a) Determine the potential difference through which the electrons must be accelerated in the electron gun in order to have a speed of 6.0 x 107 m/sec when they enter the deflecting plates.

The pair of horizontal plates shown below is used to deflect electrons up or down in the television set by placing a potential difference across them. The plates have length 0.04 m and separation 0.012 m, and the right edge of the plates is 0.50 m from the screen. A potential difference of 200 V is applied across the plates, and the electrons are deflected toward the top of the screen. Assume that the electrons enter horizontally midway between the plates with a speed of 6.0 x 107 m/sec and that fringing effects at the edges of the plates and gravity are negligible.
(b) Which plate in the pair must be at the higher potential for the electrons to be deflected upward? Check the appropriate box below and then justify your answer.

(c) Considering only an electron's motion as it moves through the space between the plates, compute the following.
i. The time required for the electron to move through the plates

 ii. The vertical displacement of the electron while it is between the plates

 (d) Show why it is a reasonable assumption to neglect gravity in part (c).

 (e) Still neglecting gravity, describe the path of the electrons from the time they leave the plates until they strike the screen. State a reason for your answer.

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