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An electric dipole moves with velocity v in a region of uniform magnetic field B directed into the plane of the page. Case I: The dipole moment P is horizontal, parallel to v, both directed to the right. Case II: The dipole moment P is vertical (upward) and v is horizontal (to the right). Which statement is correct regarding the effect of the external magnetic field on the dipole?

1. The dipole experiences a net force in Case I due to the external magnetic field
2. The dipole experiences a net force in Case II due to the external magnetic field
3. The dipole experiences a net torque in Case I due to the external magnetic field
4. The dipole experiences a net torque in Case II due to the external magnetic field

Correct answer: The dipole experiences a net torque in Case I due to the external magnetic field

Solution

In a uniform B field, both charges experience equal-and-opposite forces, so net force is zero in both cases. In Case I, the force couple has moment arms perpendicular to the force direction, producing a net torque; in Case II, the forces (+q and -q) are collinear with the dipole axis, so the torques cancel and net torque is also zero.

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