A molecule has its centre P of positive charge situated a distance of 2.8 × 10–10 m from its centre N of negative charge, as illustrated in Fig.

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The molecule is situated in a uniform electric field of field strength 5.0 × 104 V m–1. The axis NP of the molecule is at an angle of 30° to this uniform applied electric field. The magnitude of the charge at P and at N is 1.6 × 10–19 C.

I need to draw the direction of the force on P and N due to the electric field. Is it correct force on P act on N and force of N act on P.
The next question asked to calculate Torgue, i get the result of perpendicular distance tan30 x Ef but in the answer sheet they use sin30 x Ef.

What am going wrong...


closed as off-topic by John Rennie, Brandon Enright, ACuriousMind, Neuneck, Kyle Kanos Nov 6 '14 at 14:15

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  • $\begingroup$ I only need to know how to tackle the question, that is to find the direction of the force and not to calculate the whole question. But i already get my answer below. $\endgroup$ – Arodi007 Nov 6 '14 at 15:04

this is basically a dipole type of problem. direction of dipole is from -ve to +ve charge in the direction of line joining the charge and it is p. torque is given by cross product of this p and electric field i.e. pXE. (please note that angle between dipole p and E). In this case it will be 150. had your p been N and N been P it would have been 30. So the equation now becomes PESin150=PEsin30 (not tan 30) and P by definition is Magnitude of charge (1.6X10^-19) times the seperation between charges. For direction of forces consider the on positive charge in the direction of electric field lines and opposite to that on negative charge.


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