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A 1.0 m long plastic rod has a uniform positive charge of +4.0 microC spread over one half of its length and a uniform negative charge of -4.0 microC spread over the other half. Find the magnitude of the rod's net electric dipole moment in microC-m.

1. 1.0
2. 2.0
3. 4.0
4. 8.0

Correct answer: 2.0

Solution

Each half-rod of length 0.5 m carries uniform charge, so it behaves like a point charge of magnitude 4.0 microC at its midpoint. The positive centroid lies 0.25 m to one side of the rod's centre and the negative centroid 0.25 m to the other side, giving a separation d = 0.5 m. Dipole moment p = q*d = 4.0 microC * 0.5 m = 2.0 microC-m.

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