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conantee
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« on: December 31, 2009, 12:54:15 PM »

25. A charged particle is released from rest in a region where there is a constant electric field and a constant  magnetic field. If the two fields are parallel to each other, the path of the particle is
(A) circle
(B) parabola
(C) helix
(D) cycloid
(E) straight line
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conantee
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« Reply #1 on: December 31, 2009, 01:01:04 PM »

The key here is that the particle is released from rest. Hence, the only force exerted on the charge at the beginning is only from E-field. So, the force is parallel to electric field. According to Newton's law of motion, and the fact that the particle is released from the rest. The velocity is also parallel to E-field. Now, when we consider magnetic force, velocity of particle is parallel to magnetic field (since both fields are parallel to each other). That makes magnetic field always zero since it comes from cross product of velocity and magnetic field. The particle is then driven by E-field only. The path of particle is therefore straight line.

The correct answer is (E) and 57 of 100 people got it correctly.
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