20.19. Set Up: In the acceleration process the electric potential energy decrease, equals the increase in kinetic energy, Use this to find the speed v of the protons after they have been accelerated. In the magnetic field, Protons have charge and mass Electrons have charge and mass Solve: (a) and (b) In only m changes. and Reflect: For lighter particles a smaller force and hence smaller B is needed to produce the same path. To produce electrons of the same speed as the protons a smaller accelerating voltage would be required. 20.20. Set Up: The earth has radius If the dust speck moves in orbit just outside the earth’s surface, its orbit radius equals the radius of the earth. Solve: (a) (b) The gravity force is much larger and should not be neglected. 20.21. Set Up: An electron has charge and mass When it moves in a circular path of radius R, its acceleration is When a particle of charge is accelerated through a potential difference of magnitude V, it gains kinetic energy eV. Solve:
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