Gauss’s Law in Magnetism — quick, friendly reminder
What it says (one line): No magnetic monopoles ⇒ total magnetic flux through any closed surface is zero. (Magnetic field lines never start or end; they always form closed loops.)
Picture to keep in mind: Imagine rubber-band loops (field lines) threading space. Draw any closed bubble (sphere, cube, weird potato). For every line that enters the bubble, it must leave somewhere else. Enters = leaves ⇒ net flux = 0.
Bar magnet trap: Even if your bubble surrounds the “north” end of a bar magnet, lines that enter from the sides/inside also leave—you can’t trap an isolated pole with a closed surface.
Solenoid reminder: A surface capping the bore has strong inward/outward flux, but the small, spread-out outside flux + edge effects exactly cancel it → net zero.
Time-varying fields: Current changing, fields changing… still no magnetic charge. Net flux through a closed surface remains zero.
5-second checklist
Closed surface? → 0 net flux
Open surface? → use dot with the area direction and symmetry
Watch end effects and sign conventions
Beware “isolated pole” language
Revisit the theory short notes: field lines = closed loops, no monopoles, net flux through any closed surface = 0. Once this is burned in, most Gauss-law-in-magnetism MCQs collapse to one-step answers.
QUESTION
A closed surface S is drawn in each situation below. Decide which single claim about the net magnetic flux through
S is correct.
A) S cuts a bar magnet so that only the “north” face lies inside while the “south” face is outside. Therefore the net flux through S is positive.
B) S is threaded by a long straight wire carrying steady current. Since field lines circle the wire and pierce S , the net flux through S is non-zero.
C) S encloses one circular end-cap inside the bore of a very long solenoid, while the rest of S lies outside where the field is tiny. The net flux is zero.
D) In case (B), if the current is ramped with time, “displacement current” acts like magnetic charge, so the net flux through S becomes non-zero.
CRACKNEETPhysics
Gauss’s Law in Magnetism — quick, friendly reminder
What it says (one line):
No magnetic monopoles ⇒ total magnetic flux through any closed surface is zero.
(Magnetic field lines never start or end; they always form closed loops.)
Picture to keep in mind:
Imagine rubber-band loops (field lines) threading space. Draw any closed bubble (sphere, cube, weird potato). For every line that enters the bubble, it must leave somewhere else. Enters = leaves ⇒ net flux = 0.
Bar magnet trap:
Even if your bubble surrounds the “north” end of a bar magnet, lines that enter from the sides/inside also leave—you can’t trap an isolated pole with a closed surface.
Solenoid reminder:
A surface capping the bore has strong inward/outward flux, but the small, spread-out outside flux + edge effects exactly cancel it → net zero.
Time-varying fields:
Current changing, fields changing… still no magnetic charge. Net flux through a closed surface remains zero.
5-second checklist
Closed surface? → 0 net flux
Open surface? → use dot with the area direction and symmetry
Watch end effects and sign conventions
Beware “isolated pole” language
Revisit the theory short notes: field lines = closed loops, no monopoles, net flux through any closed surface = 0. Once this is burned in, most Gauss-law-in-magnetism MCQs collapse to one-step answers.
QUESTION
A closed surface S is drawn in each situation below. Decide which single claim about the net magnetic flux through
S is correct.
A) S cuts a bar magnet so that only the “north” face lies inside while the “south” face is outside. Therefore the net flux through S is positive.
B) S is threaded by a long straight wire carrying steady current. Since field lines circle the wire and pierce
S , the net flux through S is non-zero.
C) S encloses one circular end-cap inside the bore of a very long solenoid, while the rest of S lies outside where the field is tiny. The net flux is zero.
D) In case (B), if the current is ramped with time, “displacement current” acts like magnetic charge, so the net flux through S becomes non-zero.
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