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An electric dipole is placed in a non-uniform electric field that increases in magnitude along the positive x-direction. Which of the following correctly describes the motion of the dipole?
- Moves along +x direction and rotates clockwise
- Moves along -x direction and rotates clockwise
- Moves along +x direction and rotates anticlockwise
- Moves along -x direction and rotates anticlockwise
Correct answer: Moves along +x direction and rotates clockwise
Solution
In a non-uniform field increasing in the +x direction, the net translational force on the dipole is F = (p dot nabla)E. A component of p along +x gives a force in +x (toward stronger field). The torque tau = p cross E tends to align p with E. If p is at an angle above the +x axis, the torque is clockwise (into the page, -z direction). Hence the dipole moves in +x and rotates clockwise.
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