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Unformatted text preview: Physics 2Ba quiz 1 solutions (Dated: January 15, 2009 0:19) Problem 1 A charged particle moves on a straight trajectory. No forces other than, possibly, electric act on the particle. We can deduce that the electric field in the region of the particle trajectory must be parallel to the trajectory . If the particle is moving along a straight line path, this implies that the acceleration (if non-zero) must point in the same direction as the velocity, else the particle would deviate from straight line motion. By the second law, F = m a , the force must also point in the same direction as the velocity. Finally, since F = q E , the electric field must point in the same (or opposite) direction of the force, and thus the velocity. So why is E = 0 not a valid answer? The statement that We can deduce that the electric field in the region of the particle trajectory must vanish is clearly false since the electric could be non-zero. If it is non-zero, it must be parallel to the trajectory. Problem 2 An electron is placed at the orgin. In the the next situations P stands for a point 1 m away from the origin: 1. A proton is placed at P 2. A positron (charge e, same mass as the electron) is placed at P 3. A Helium nuclei (charge 2e, four times the mass of the proton) is placed at P Rate from larger to smaller the electric field at P due to the electron in each of these situations. Denote by E1 the field in situation 1, etc....
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