Electric dipole

Yep, definitely a touch of CDD on my part here!

Moment and Torque are essentially the same thing except that Torque is a movement force, while Moment is a static force.  Torque tends to be used when there is an axle or pivot to be turned around, while Moment tends to be used in essentially non-rotational situations, such as analysis of forces on a beam.

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Anyone ?! 

If you have found the force in part (a), resolve it into horizontal and vertical components.  The horizontal component will not give rise to a torque, but the vertical component will.

The magnitude of the torque will be given by the magnitude of the vertical component multiplied by the horizontal distance from the isolated charge to the centre of the dipole.  

The direction will depend on the direction in which the vertical component of the force is pointing.  If it is pointing up then there will be an anti-clockwise moment, which means the direction of the torque is pointing out of the Page towards the viewer.  If it is pointing down there will be a clockwise moment so the direction of the torque will be into the page away from the viewer.

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but why the horizontal component of the force will not give rise to a torque ! 

I mean waht is the relation ship between force and torque ?

Torque is force multiplied by perpendicular distance from the force to the object it is acting on.  

Here the horizontal component of the force will simply try to move the object in a straight line (left or right depending on which way it is pointing).  Only the vertical force will tend to rotate the object.

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But in the solution manual of the booke ! why they said that the x component produce torque ? 

Oops!  Yes, the book is correct.  I answered without thinking it through carefully enough.  I should have drawn a picture!

I've taken a point off myself!

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well but what do you mean by anti-clockwise moment 

I just know that moment is $$P = qd$$ .. 

and P is a vector from the negative charge of the dipole do the positive charge . 

and torque = $$P.E sin$$θ

and we use the right hand rule to determine the direction of the torque . 

Now we didn't use the electric dipole moment (  P ) 

so how we determined the direction of the tourqe 

I'am sorry for askin you alot of question 

but really I want to make it clear for me 

Is this a touch of CDD I see here Alan?  There seems to be an epidemic a presen.   (woops, no t)

My computer is refusing to type t's half the time.  I think that it has CDD too!     

Very clear,  Thank you Alan