InterviewSolution
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InterviewSolution
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An electron moves in a uniform magnetic field and follows a spiral path as shown in Fig. 1.30. State the following statements as True or False. a. Angular velocity of the electron remains constant. b. Magnitude of velocity of the electron decreases continuously. c. Net force on the particle is always perpendicular to its direction of motion. d. Magnitude of net force on the electron decreases continuously. |
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Answer» (a) True, (b) True, (c) False, (d) True, Periode of revolution of a charged particle moving in a uniform magnetic field is given by `T=(2pim)/(qB)`. This period T does not depend upon speed of the particle. In this particular question, the moving particle is an electron. Hence, its mass and charge (q) both are constant. Magnetic field is also uni- form. Hence, its period of revolution remains constant. It means electron moves with a constant angular velocity. Hence (a) is correct. In previous question we have already discussed that if a charged particle experiences a resisting motion then it fol- lows a decreasing radius spiral path. In this question, electron is moving along a spiral path of decreasing radius. It means its speed is decreasing continuously. Hence (b) is correct. Since the speed of the electron is continuously decreasing. there- fore, it is experiencing a tangential retardation. It is possible only when the component of resultant force opposite to the direction of motion of electron has non-zero value. It means, net force on electron cannot be perpendicular to its direction of motion. Hence (c) is wrong. Since speed of the electron is decreasing continuously, therefore, the force exerted by the magnetic field `(F=qvB)` is also decreas- ing continuously. Hence, magnitude of net force acting on the electron is decreasing continuously. Hence (d) is correct. |
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