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It is written in our course book that the magnetic field outside a solenoid is negligible and almost zero. Why is it negligible? Is it negligible compared to the field inside solenoid or is it nearly equal to zero?

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A simple model to help you visualize: The magnetic flux forms loops, you cannot have a magnetic field line terminate on nothing in free space. That means that all those flux lines inside the solenoid heading (for example) to the right. The flux line must continue outside the solenoid and loop around to join the other end of the flux loop. In doing so it heads to the left. No external flux lines -> no flux.

So what could be the issue?

1) In an infinite solenoid, there is no external magnetic field. One example of this is a Tokamak or toroidal solenoid. Like a big doughnut. The flux lines form loops within the doughnut and have no need to go outside. - of course a mathematically ideal infinite solenoid could have been what they were discussing, in which case the field lines loop around infinity. Hard to test though.

2) The flux density outside of the linear finite solenoid is much less than inside. So while there may be X flux across the throat of the solenoid that same amount of flux (X) is spread out over a much much larger cross sectional area. The density is much less. [here the area is the cross sectional area across the solenoid.]

3) Perhaps they are talking about a shielded solenoid? This would involve having a high permittivity material external to the coils to trap the flux lines.

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  • \$\begingroup\$ Since it's a textbook, my vote goes for option 2, since that was what was taught in our textbooks a couple of decades ago, and there were no mentions of toroidals. \$\endgroup\$ – Anindo Ghosh Mar 5 '13 at 16:17

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