An electret is similar to a magnet, but rather than being permanently magnetized, it has a permanent electric dipole moment. Suppose a small electret with electric dipole moment 1.0 × 10−7 Cm is 25 cm from a small ball charged to + 70 nC, with the ball on the axis of the electric dipole.
What is the magnitude of the electric force on the ball?
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The field along the axis of the dipole (moment p) is:
E = (1/(2πε₀)) p/r³ (e.g. see 1/3 of the way down the link)
Force on charge Q on the axis is F = QE
F = Q (1/(2πε₀)) p/r³
I’ll use scientific notation so, for example, 3x10⁸ is written as 3e8.
F = 70e-9 (1/(2π*8.854e-12)) 1.0e-7/0.25³
. .= 0.0081N to 2 sig. figs.
Anonymous
E = p / (4 π εo r^3)
F = q E
F = q p / (4 π εo r^3)
F = 7x10^-8 *10^-7 / (4*3.14*8.85x10^-12*0.25^3) = 4.032x10^-6 N