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JEE Main · 2007 · 29 Apr 2007 · Offline · Magnetic Effects of Current

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A charged particle moves through a magnetic field perpendicular to its direction. Then
  1. (A)the momentum changes but the kinetic energy is constant✓ Correct
  2. (B)both momentum and kinetic energy of the particle are not constant
  3. (C)both, momentum and kinetic energy of the particle are constant
  4. (D)kinetic energy changes but the momentum is constant

Answer: (A)

Solution

A magnetic force on a moving charge acts at right angles to the velocity, so it continually turns the particle without ever having a component along the motion. Being always perpendicular, it does no work, and the speed and hence the kinetic energy stay constant. The momentum, however, is a vector whose direction keeps changing, so momentum is not constant even though its magnitude is.

JEE Main · 2007 · 29 Apr 2007 · Offline · Single correct option

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