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QPages 229-230: Q9: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Pages 229-230: Q9 Solution: An electron is moving along the positive -axis. You want to…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often think a magnetic field can directly reverse a charged particle’s motion, but the magnetic force $\mathbf{F}=q\,\mathbf{v}\times\mathbf{B}$ is always perpendicular to the velocity, so it cannot have a component along $\pm\mathbf{v}$ and cannot do work or change the speed. To reverse direction you need an electric field or another mechanism, not a magnetic field alone.

An electron is moving along the positive xx-axis. You want to apply a magnetic field for a short time so that the electron may reverse its direction and move parallel to the negative xx-axis. This can be done by applying the magnetic field along

Multiple Choice Options

  • A yy-axis
  • B zz-axis
  • C yy-axis only
  • D zz-axis only

Step-by-Step Verified Solution

**Answer:** (a), (c). **Step-by-Step Explanation:** To reverse the electron's direction, the magnetic force must be opposite to its velocity. This requires B along y-axis (a) or y-axis only (c).

All Questions in this Chapter

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