Force Acting on Moving Charges: Practice Problem & Solution

Short Solution (Best for NEET):

To solve this problem, we apply the core principles of Force Acting on Moving Charges. Understanding the underlying formula is key to arriving at the correct answer below:

Assertion (A): An electron moving with velocity \(\vec{v}\) in a magnetic field \(\vec{B}\) experiences a magnetic force \(\vec{F}_B = q(\vec{v} \times \vec{B})\). If the electron moves parallel or anti-parallel to the magnetic field (i.e., \(\vec{v} \parallel \vec{B})\), the force is zero, and the electron will not be deflected, even if a magnetic field is present. Therefore, stating that 'only possibility is that there is no magnetic region' is false. So, (A) is false. Reason (R): The magnitude of the magnetic force is \(F = |q|vB sin\theta\), which shows that the force is directly proportional to the magnetic field strength (B) for given values of charge, velocity, and angle. So, (R) is true. Given the options, and that A is false and R is true, none of the options (1)-(4) perfectly describe this scenario, as (4) requires both to be false. If forced to select one, (A) is definitively false, ruling out (1), (2), (3).