Physics · Magnetism · chapter 4 of 14

Class 12 Physics Chapter 4: Moving Charges and Magnetism Assertion-reason questions with answers

2 assertion-reason questions on Moving Charges and Magnetism, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): A charged particle moving in a uniform magnetic field keeps a constant speed. Reason (R): The magnetic force on a moving charge is always perpendicular to its velocity.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. A force perpendicular to the velocity does no work, so the kinetic energy and the speed cannot change; R explains A.

Question 2
Assertion (A): In a uniform magnetic field, a faster proton describes a larger circle than a slower one. Reason (R): A proton moving parallel to a uniform magnetic field experiences no magnetic force.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

A is true: for v perpendicular to B, r = mv/qB grows with v. R is true: q(v × B) is zero when v is parallel to B. But R concerns motion along the field, where there is no circle at all; the larger circle follows from r = mv/qB, not from R.

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