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    Question

    A dipole placed in uniform electric field experiences:

    A Net force Correct Answer Incorrect Answer
    B No torque Correct Answer Incorrect Answer
    C Torque but no net force Correct Answer Incorrect Answer
    D Acceleration Correct Answer Incorrect Answer

    Solution

    An electric dipole consists of two point charges, +q and –q, separated by a small fixed distance 2a, with dipole moment p = q × 2a directed from –q to +q. When this dipole is placed in a uniform electric field E, each charge experiences an electric force: ●      The positive charge experiences a force F = +qE in the direction of the field. ●      The negative charge experiences a force F = –qE in the opposite direction. Since the electric field is uniform, the magnitudes of the forces on +q and –q are equal and opposite, and they act along parallel lines. Therefore: ●      Net force on the dipole = 0
      ⇒ No translational motion (i.e., no linear acceleration). However, the two equal and opposite forces are separated by a distance, and hence form a couple, resulting in a torque τ on the dipole. The torque experienced by the dipole is given by:
      τ = p × E
      Or in scalar form:
      τ = pE sinθ,
      where θ is the angle between the dipole moment p and the electric field E. This torque tends to align the dipole along the direction of the electric field.

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