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In an electric dipole, the distance between the two charges is 5 mm and the charges have equal magnitude 9×10⁻⁹ C. Find the electric dipole moment of this dipole.

Asked in GSEB Board July 2023 · Dipole moment and dipole field

• (1) 0.45 μC m
• (2) 0.0045 nC m
• (3) 0.045 μC m
• (4) 0.045 nC m

Step-by-step solution

Why the other options are wrong

• (1) No single slip gives 0.45 μC m = 4.5×10⁻⁷ C m, which is 10⁴ times too large. Sanity check: p = q(2a) = (9×10⁻⁹)(5×10⁻³) = 4.5×10⁻¹¹ C m.
• (2) This comes from converting 5 mm as 5×10⁻⁴ m: (9×10⁻⁹)(5×10⁻⁴) = 4.5×10⁻¹² C m = 0.0045 nC m, ten times too small.
• (3) Right digits, wrong prefix: 4.5×10⁻¹¹ C m is 0.045×10⁻⁹ C m (nano), not 0.045×10⁻⁶ C m (micro). This would be the answer only if each charge were 9 μC.

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