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A charged particle moves through a magnetic field perpendicular to its direction. Then

Asked in JEE Main 2007 · Lorentz force and work done

• (1) kinetic energy changes but the momentum is constant
• (2) the momentum changes but the kinetic energy is constant
• (3) both momentum and kinetic energy of the particle are not constant
• (4) both momentum and kinetic energy of the particle are constant.

Step-by-step solution

Why the other options are wrong

• (1) Reverses the roles: the force changes direction of momentum but does no work.
• (3) Forgets that a force perpendicular to velocity does no work, so kinetic energy stays constant.
• (4) Forgets that the force changes the direction, and hence the vector momentum.

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