PhysicsLAB AP Free Response Question
2007 Form B - B2

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A beam of particles of charge q = +3.2 x 10-19 C and mass m = 6.68 x 10-26 kg enters region I with a range of velocities all in the direction shown in the diagram above. There is a magnetic field in region I directed into the page with magnitude B = 0.12 T.
(a) Charged metal plates are placed in appropriate locations to create a uniform electric field of magnitude E = 4800 N/C in region I. As a result, some of the charged particles pass straight through region I undeflected. Gravitational effects are negligible.
 i. On the diagram above, sketch electric field lines in region I.

 ii. Calculate the speed of the particles that pass straight through region I.

The particles that pass straight through enter region II in which there is no electric field and the magnetic field has the same magnitude and direction as in region I. The path of the particles in region II is a circular arc of radius R.
 (b) Calculate the radius R.

(c) Within the beam there are particles moving slower than the speed you calculated in (a)ii. In what direction is the net initial force on these particles as they enter region I?
Justify your answer.

(d) A particle of the same mass and the same speed as in (a)ii but with charge q = —3.2 x 10-19 C enters region I. On the following diagram, sketch the complete resulting path of the particle.

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