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In this, F (the force set by the field) is perpendicular to the speed of the particle. In which case does F change the absolute value of V? Why or why not?

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closed as too localized by user1504, Emilio Pisanty, akhmeteli, Waffle's Crazy Peanut, Manishearth Apr 26 '13 at 9:20

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up vote 3 down vote accepted

Magnetic field never does any work , as it acts perpendicular to the velocity and thus doesn't change Kinetic energy , however electric field is able to change the direction as well as magnitude of velocity.

Hence answer is electric field.

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The direction of the electric force on a charged particle is constant, independent of the direction the charge is moving in. The magnetic force on the other hand is always at right angles to the direction the charge is moving in.

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