Points to Ponder

Closed loops vs. open lines: Electric field lines start on positive charges and end on negative charges (or go to infinity). In contrast, magnetic field lines always form continuous closed loops — they have no starting or ending point.

Steady current assumption: All laws in this chapter (like Ampere’s law) apply only to steady currents ($\frac{dI}{dt} = 0$). For time-varying currents, Newton’s third law holds only if we include the momentum carried by the electromagnetic field itself.

Lorentz force and relativity: The Lorentz force $\mathbf{F} = q(\mathbf{v} \times \mathbf{B} + \mathbf{E})$ depends on velocity. If you switch to a frame moving with the particle’s instantaneous velocity $\mathbf{v}$, the magnetic term vanishes — the force is then explained by an electric field in that frame. This shows that electricity and magnetism are two aspects of the same phenomenon (electromagnetism), and the Lorentz force does not pick out a special, universal reference frame. …