An ion with a charge of+ is trapped in the earth's magnetic field. It suddenly picks up 2 electrons, and its charge becomes -1. How does its motion change? Assume the gain of the 2 electrons does not involve a mechanical force.
A. It reverses direction along the same spiral path.
B. It travels along the same spiral path at a greater speed.
C. It travels along the same spiral path at a lesser speed.
D. The rotation of its spiral path reverses direction.

D. The rotation of its spiral path reverses direction.

Explanation:

Magnetic force on a free moving charge is perpendicular to the velocity of the charge and the magnetic field

F = qvB (eq. 1)

where,

q = charge

v = velocity of the particle

B = strength of the magnetic field

direction of the force is perpendicular to the motion of the particlethe particle moves in a circular motion under the action of this magnetic force. So the force is equal to the centripetal force

 (eq. 2)

Where,

m = mass of the particle

r = radius of the orbit

An ion with initial charge +1 picks up 2 electrons and its charge becomes -1. From eq.(1), when the sign of q changes, the direction of the force changes So, the ion rotates in the opposite direction From eq.(2),  the value of q does not change, so the radius of the orbit does not change.Therefore, the rotation of its spiral path reverses direction