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NEETPhysicsMagnetic Effects of Current

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): When an electron is projected in the direction of parallel uniform electric and magnetic fields, its kinetic energy decreases but its path remains a straight line. Reason (R): The magnetic force on the electron is zero, and the electric force acts in the direction of its velocity. In the light of t

Options

  1. ABoth Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A)
  2. BAssertion (A) is true but Reason (R) is false
  3. CBoth Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A)
  4. DAssertion (A) is false but Reason (R) is true

Correct answer

B. Assertion (A) is true but Reason (R) is false

Step-by-step solution

Assertion (A) is true: When an electron is projected parallel to the magnetic field, the angle between its velocity and the magnetic field is zero. The magnetic force is F_m = qvB (0^ ) = 0 , so the magnetic field does not deflect it, and the path remains a straight line. However, the electron is negatively charged, so the electric force F _e = -e E acts in the direction opposite to the electric field (and thus opposite to its initial velocity). This opposing force causes the electron to decelerate, decreasing its

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