Magnetostatic

Given a charge q in the presence of a charge Q, both fixed, we have seen that Q is the source of an electrostatic force on q (and vice versa).

Now, if Q moves, while keeping q fixed, the latter charge is still subject to the static field seen previously and independent of Q's displacement, but also to a contribution depending on Q's speed. By generalizing, this contribution gives us the Lorentz force generated by currents on external circuits

Given a rigid circuit with current flowing through it, the circuit does not move spontaneously, so the resultant of the forces applied to the circuit can be said to be zero. To break this equilibrium, we need to generate forces from outside the circuit.

The...