Interaction force, two dipoles

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SUMMARY

The discussion focuses on calculating the force between two dipoles in an electric field, specifically how to determine the factor by which the distance \( r \) must increase to reduce the net force by a factor of 64. The relevant formula for the force on a dipole \( p \) in an electric field \( E \) is given by \( F=(p\cdot\nabla)E \). The electric field of a dipole is expressed as \( E=\frac{p}{4\pi\epsilon{r^3}}(2\cos\theta\hat{\theta}+\sin\theta\hat{r}) \). The derived force equation is \( F_{(r,\theta)}=-3\frac{\cos\theta}{4\pi\epsilon{r^4}}(2p_\theta+p_r) \).

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  • Familiarity with electric fields and forces
  • Knowledge of vector calculus, specifically the gradient operator
  • Basic principles of electrostatics, including Coulomb's law
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mathman44
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Homework Statement



A dipole p is directed in the positive z direction at the origin.

The force on a dipole p in an electric field E is F=(p\cdot\nabla)E

If a second dipole is placed at (r, \theta), by what factor must r increase – in terms
of p, \theta and r – so that the net force between the two dipoles decreases by a factor of 64?

---

I don't know how to start this... any hints please? This isn't homework, just a practice test question.
 
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mathman44 said:

Homework Statement



A dipole p is directed in the positive z direction at the origin.

The force on a dipole p in an electric field E is F=(p\cdot\nabla)E

If a second dipole is placed at (r, \theta), by what factor must r increase – in terms
of p, \theta and r – so that the net force between the two dipoles decreases by a factor of 64?

---

I don't know how to start this... any hints please? This isn't homework, just a practice test question.

Well, you have a situation where one dipole is exerting a force on another...you already know how to determine the force exerted on a dipole from an external electric field, so...what is the electric field of a dipole? What force does that field exert on a dipole a distance r away from it (do not assume that the dipoles are aligned)?
 
The electric field of a dipole is

E=\frac{p}{4\pi\epsilon{r^3}}(2cos\theta\hat{\theta}+sin\theta\hat{r})

If I break p apart into p_r and p_\theta, dot it with del, then multiply by E, I get a mess of an answer.
 
This is what I got, by the way:

F_(r,\theta)=-3\frac{cos\theta}{4\pi\epsilon{r^4}}(2p_\theta+p_r)

Any help please?
 

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