# Homework Help: Torque on/due to dipoles.

1. Jan 24, 2010

### minimark1234

1. The problem statement, all variables and given/known data
Two dipoles as in pic below a distance r apart, find the torque applied to each dipole due to the other.
$$\uparrow1 ------------2\rightarrow$$

2. Relevant equations
$$\itshape \vec{p}=p(cos\:\theta \hat{r}-sin\:\theta\hat{\theta})$$
$$\itshape \vec{E}=\frac{p}{4\pi\epsilon_{0}r^{3}}\: (2cos\:\theta\hat{r}+sin\:\theta\hat{\theta})$$
$$\itshape \vec{N}=\vec{p}\:X\:\vec{E}$$

3. The attempt at a solution
well i only am concerning myself with the non constants in the cross product.
$$\itshape \vec{N}=\vec{p_{2}}\:X\:\vec{E_{1}}$$
$$\itshape (cos\:\theta\hat{r}-sin\:\theta\hat{\theta})X(2cos\:\theta\hat{r}+sin\:\theta\hat{\theta})$$
ok this is where i am confused... do i use \theta as angle under the z axis for the direction the dipole and electric field are pointing, or do i use them for angles of separation or what? if i use them for angle under the z axis for the vector it is pertaining to i get the torque will be $$-3cos\:\theta sin\:\theta sin \:\phi\hat{i}+3cos\:\theta sin\:\theta cos \:\phi\hat{j}$$, which applying angles under z axis is 0. But that isn't what the answer should be.