When a wire carrying an electrical current is placed in a magnetic field, each of the moving charges, which comprise the current, experiences the Lorentz force, and together they can create a macroscopic force on the wire (sometimes called the Laplace force). By combining the Lorentz force law above with the definition of electrical current, the following equation results, in the case of a straight, stationary wire:
\mathbf{F} = I \mathbf{L} \times \mathbf{B} \,
where
F = Force, measured in newtons
I = current in wire, measured in amperes
B = magnetic field vector, measured in teslas
\times = vector cross product
L = a vector, whose magnitude is the length of wire (measured in metres), and whose direction is along the wire, aligned with the direction of conventional current flow.
Alternatively, some authors write
\mathbf{F} = L \mathbf{I} \times \mathbf{B}
where the vector direction is now associated with the current variable, instead of the length variable. The two forms are equivalent.
If the wire is not straight but curved, the force on it can be computed by applying this formula to each infinitesimal segment of wire dℓ, then adding up all these forces via integration. Formally, the net force on a stationary, rigid wire carrying a current I is
\mathbf{F} = I\oint d\boldsymbol{\ell}\times \mathbf{B}(\boldsymbol{\ell}\ )
(This is the net force. In addition, there will usually be torque, plus other effects if the wire is not perfectly rigid.)
One application of this is Ampère's force law, which describes how two current-carrying wires can attract or repel each other, since each experiences a Lorentz force from the other's magnetic field. For more information, see the article: Ampère's force law
