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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 + 80 nC, with the ball on the axis of the electric dipole.

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In this question, we need to find the magnitude of the electric force on the ball; the answer is 9.21*10^{-3}N.

To resolve this question, we're going to need E=\frac{k(2p)}{r3 } that is the magnitude of an electric field in the axis of a dipole at some distance "r" .

where "p" is the magnitude of the electric dipole moment, so in this case p= 1.0*10^{-7} Cm or what is the same 10^{-7}cm; and k is a constant k value is = k=8.99*10^{9} \frac{Nm2}{C2}

now to get the force on the ball we need to use F=qE where E=\frac{k(2p)}{r3 }

so we have

F= q*\frac{k(2p)}{r3 }

for this, "q" is the charge on the ball, so we got 80nC or what si the same 80×10^{-9}C, and r is the distance between the ball and the dipole, r= 25cm= 0.25m (1m=100cm, so we need to divide 25cm into 100 to get it in meters)

Replacing we got:

F=(80×10^{-9})(8.99×10^{9})(2(10^{-7})) / (0.25)^{3}

and calculating this the answer is 9.21*10^{-3}N

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