Practice: Magnetism Going Deeper Solution
1976B4.
An ion of mass m and charge of known magnitude q is observed to move in a straight line through a region of space
in which a uniform magnetic field B points out of the paper and a uniform electric field E points toward the top edge
of the paper, as shown in region I above. The particle travels into region II in which the same magnetic field is
present, but the electric field is zero. In region II the ion moves in a circular path as shown.
(a) Indicate on the diagram below the direction of the force on the ion at point P 2 in region II.
(b) Is the ion positively or negatively charged? Explain clearly the reasoning on which you base your conclusion.
(c) Indicate and label clearly on the diagram below the forces which act on the ion at point P 1 in region I.
(d) Find an expression for the ion’s speed v at point P 1 in terms of E and B.
Solution
1984B4: Cathode Ray accelerating particle into B field, with an electric field
balancing in one region
Solution
1990B2: Charged Particle between two parallel plates (old problem)- path without b
field. Then calculate B field to balance
1990B2. A pair of square parallel conducting plates, having sides of length 0.05 meter, are 0.01 meter apart and are
connected to a 200-volt power supply, as shown above. An electron is moving horizontally with a speed of 3 x 10 7
a. Determine the magnitude of the electric field in the region between the plates and indicate its direction on the
figure above.
meters per second when it enters the region between the plates. Neglect gravitation and the distortion of the electric
field around the edges of the plates.
b. Determine the magnitude and direction of the acceleration of the electron in the region between the plates.
c. Determine the magnitude of the vertical displacement of the electron for the time interval during which it moves
through the region between the plates.
d. On the diagram below, sketch the path of the electron as it moves through and after it emerges from the region
between the plates. The dashed lines in the diagram have been added for reference only.
e. A magnetic field could be placed in the region between the plates which would cause the electron to continue to
travel horizontally in a straight line through the region between the plates. Determine both the magnitude and
the direction of this magnetic field.
1990B2: Solution