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16 Nov 2011 11:40:10 IST

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Magnetic field work

Physics

can magnetic field do work???

1. it can never do on electric charges & i have not found an exception to it

2. what about magnetic charges???force on a bar magnet translating in a magnetic field is in the direction of its motion.

wikipedia says that it can never do work..

please explain

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Comments (5)

gokul naras

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1 Dec 2011 13:53:57 IST

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dude, there is no notion like manetic charges since - "magnetic monopoles do not exist". ur assumption is wrong.and coming to ur q., work can be done by mag. field only in when a component of velocity is in the direction of mag. field..! hope u got me!

santanu sinha

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1 Dec 2011 14:41:26 IST

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no magnetic field cannot do work..the force always remains perpendicular to the direction of propagation..this force is centripetal in nature..and if any component of velocity is in direction of the field,the velocity does not change..:) cheers

santanu sinha

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1 Dec 2011 14:50:54 IST

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oops..i dint read the question properly :Dwork can be done by magnetic fields for that case..just consider bringing a magnet near another.. :)

rishabh singh

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1 Dec 2011 18:38:56 IST

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@gokulnaras consider a bar magnet (which has 2 poles) kept at an angle to the field..as u do for finding torque on it...force is in dirn of diplacement for both poles ...so ur argument is valid only for moving real charges

edison

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26 Jan 2012 18:02:13 IST

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First, although static magnetic fields can't do work, time varying magnetic fields can. Second, whenever work is done under the influence of a static magnetic field, some external source will have to supply the energy require (as opposed to an electric field that can do work on its own).

Let's analyze the example, the bar magnetic picking up staples. We'll start with a simpler example: two dipole moments - current loops - pointing in the same way. Both have mass, but one is fixed and the other is directly under the the first one. Let's say the magnetic forces are greater than gravity so the fixed loop pulls the other up. So, obviously, someone did some work against the gravity, but where did the energy come from? the loop itself. While the loop is ascends, the current in both decreases, due to mutual inductance, so the loops serve as some sort of battery.

When you move a magnet near a staple, the staple feels a changing magnetic field that does work and changes the staple's internal energy - it makes many of the current loops in the metal point in the same direction as those in the magnet. After that both magnet and staple behave as magnetic dipoles. (actually some quantum effects such as ferromagnetism make this a bit more complicated, but the basic idea is similar)

Bottom line, time dependent magnetic field can do work, and static magnetic fields can take energy from other sources and use it to move things.

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