CBSE Class 12 · 2016 · All India · Magnetic Effects of CurrentShort Answer

\textbf{Ampere's law:} the line integral of $\vec{B}$ around any closed loop equals ${\mu }_0$ times the current threading it: $\oint \vec{B}\cdot d\vec{\ell }={\mu }_{0}I$. For an infinite straight wire, take a circular amperian loop of radius $r$ coaxial with the field lines. By symmetry $B$ is constant along it and tangential, so Magnetic field lines form continuous closed loops (no magnetic monopoles), whereas electrostatic field lines are open, starting on positive and ending on negative charges.