As discussed earlier, the force exerted by a dipole loop with moment m1 on another with moment m2 is
old F =
abla left(old m_2cdotold B_1
ight),
where B1 is the magnetic field due to moment m1. The result of calculating the gradient is[8][9]
old F(old r,old m_1,old m_2) = frac{3mu_0}{4pi|old r|^4}left(old m_2 (old m_1cdothatold r) + old m_1(old m_2cdothatold r) + hatold r(old m_1cdotold m_2) - 5hatold r(old m_1cdothatold r)(old m_2cdothatold r)
ight),
where r̂ is the unit vector pointing from magnet 1 to magnet 2 and r is the distance. An equivalent expression is[9]
old F = frac{3mu_0}{4pi|old r|^4} left((hatold r imesold m_1) imesold m _2 + (hatold r imesold m_2) imesold m_1 - 2hatold r(old m_1cdotold m_2) + 5hatold r(hatold r imesold m_1)cdot(hatold r imesold m_2)
ight).
The force acting on m1 is in the opposite direction.
The torque of magnet 1 on magnet 2 is
oldsymbol au = old m_2 imesold B_1.