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**dipole torque problem.. Help please!!!**

Hi, the problem is:

Show that if the force action on a dipole p placed in an Non uniform electric field is [tex]p\cdot \nabla E_{ext}[/tex], the torque acting on the dipole in this field is

[tex]\tau =r \times (p\cdot\nabla E_{ext})+p\times E_{ext}[/tex]

where r is the vector distance to the dipole from the point about which the torque is to be measured.

How can i do this??? it's urgent.. please!!!