Past exam of the physics 2 paper of the University of Oxford / 2013 / 2 / 1 / Solution

The electric field is force per unit positive stationary test charge, in the limit that the test charge does not disturb the source:The magnetic flux density is defined by the velocity-dependent part of the Lorentz force, ${\mathbf{F}}_B=q\mathbf{v}\times \mathbf{B}$. Equivalently, a straight current element in a uniform field experienceswhich fixes its magnitude and right-hand-rule direction by force measurements with different current orientations.