Which one among the following laws states that magnetic monopoles do not exist?

Correct — A, Gauss’ law. Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero, and that is precisely the statement that magnetic monopoles do not exist: every magnetic field line that enters a closed surface must leave it, because there is no isolated north or south pole inside for the lines to start or stop on. It is the magnetic counterpart of Gauss's law for electricity, where the flux through a closed surface equals the enclosed charge divided by the permittivity — and the fact that the magnetic version has zero on the right-hand side, with no source term at all, is the whole content of the no-monopole rule.

• (b)Newton's law — Newton's laws concern motion and gravitation. The law of universal gravitation does resemble Coulomb's law in form, but it says nothing about magnetic poles.
• (c)Pascal's law — Pascal's law is a statement of fluid mechanics: pressure applied to an enclosed fluid is transmitted undiminished in every direction. It is what makes a hydraulic lift work and has no connection to magnetism.
• (d)Ampere's law — Ampere's circuital law relates the magnetic field around a closed loop to the current threading it. It describes how currents produce magnetic fields; it is silent on whether isolated poles exist.

The four Maxwell equations each say something structural about electric and magnetic fields. Gauss's law for electricity says electric field lines begin and end on charges. Gauss's law for magnetism says the net magnetic flux through any closed surface is zero, which means magnetic field lines have no beginning and no end — they always form closed loops, so an isolated magnetic pole cannot exist. Faraday's law describes induction, and Ampere's law, extended by Maxwell, describes how currents and changing electric fields create magnetic fields.

The experimental version of this law is the familiar demonstration with a bar magnet: cut it in half and each half is a complete magnet with both a north and a south pole, and cutting further never yields a lone pole. The item's distractors are drawn from three different branches of physics, which makes it easier than it looks — only two options belong to electromagnetism at all, and of those, Ampere's law is about the field of a current rather than about the sources of the field. This card marks a different letter from the answer we hold on file for this question.

• Gauss's law for magnetism states that the net magnetic flux through any closed surface is zero.
• Magnetic field lines form closed loops; they have no starting or ending point.
• Breaking a bar magnet gives two smaller magnets, each with both poles.
• Gauss's law for electricity, by contrast, has the enclosed charge on its right-hand side, because electric monopoles do exist.
• Ampere's circuital law relates the field around a closed path to the current enclosed by it.

• Picking Ampere's law because it is the other magnetic law on the list; it concerns currents, not poles.
• Assuming Gauss's law is only about electricity — there is a magnetic version, and it is the one meant here.
• Confusing Pascal's law, a fluid statement, with anything electromagnetic.

A one-line law-recognition item that turns on knowing which of Maxwell's four equations carries the no-monopole statement.

• practice — not a real PYQ

  If a bar magnet is cut into two equal halves, each half will

  • (a)have only a north pole
  • (b)have only a south pole
  • (c)be a complete magnet with both poles
  • (d)lose its magnetism entirely

  Answer(c) be a complete magnet with both poles — isolated magnetic poles do not exist, which is the content of Gauss's law for magnetism.

• practice — not a real PYQ

  Pascal's law is applied in the working of a

  • (a)hydraulic lift
  • (b)transformer
  • (c)moving-coil galvanometer
  • (d)solenoid

  Answer(a) hydraulic lift — pressure applied to an enclosed fluid is transmitted undiminished throughout it.