Why is magnetic force perpendicular

Viewed 17k times. Improve this question. Stephen Jennings. Stephen Jennings Stephen Jennings 1 1 gold badge 5 5 silver badges 5 5 bronze badges. Add a comment. Active Oldest Votes.

Improve this answer. More physical restatement of the argument The above is sort-of formal sounding, but it is just saying this: the magnetic field doesn't change sign under reversing the coordinates of space. Hamiltonian argument and gauge invariance The best argument is from the concept of momentum potential or vector potential. Ron Maimon Ron Maimon 1. Intuitive doesn't mean not precise. The force is perpendicular to "v" and to "B", and one must argue this without relativity since it predates relativity, and inspires it.

It only becomes non-arbitrary when you have relativity. Perhaps the right thing is to say "relativity" right at the start, but it won't justify why 19th century folks were sure they understood it before Einstein. If you think this is a bad answer, perhaps I'll agree the reflection issue is no longer considered fundamental, and the Hamilton formulation might be arbitrary too but I don't think so.

No one should expect technical writing to be easy, and no one should demand to understand everything at first reading - one wouldn't learn anything if one weren't challenged. Not all answers work for everybody - so what. Indeed your answers sometimes lose me - so what - I'm damn glad people like you put the effort you do into your posts. Show 4 more comments. For the Lorentz transformation of the electromagnetic field, see en.

Unfortunately this is a bit beyond my knowledge, but I will be trying my best to understand it. Thank you. The first three paragraphs just restate that fact without explaining why it's true. The fourth, fifth, and sixth paragraphs are again just a statement of the fact, but now in fancier mathematical dress.

Art Brown Art Brown 5, 1 1 gold badge 24 24 silver badges 38 38 bronze badges. Though I start with a charged particle at rest net to a current carrying wire - no force. Then look at the fields in the moving frame - there is both magnetic and electric fields. The particle must not move towards the wire in the moving frame, so there must be a force qvB and it must be perpendicular to v and B. As you say - just an example, but a powerful one. Thanks for the info. Feynman's argument is very profound because it allows the derivation of the whole Maxwell theory together with the Lorentz force equation starting from very simple and basic assumptions: The canonical Posisson brackets of the position and velocity.

At an abstract level it IS exactly the kind of thing the OP is asking for: deeper, intuitive motivation. Thanks David for posting this - I learnt something.

And it isn't too different in level from the ideas in Ben's book which I also think are damned interesting. Thank you! Couldn't edit the last comment so had to remove it and entered a new one. Glorfindel 1, 1 1 gold badge 14 14 silver badges 20 20 bronze badges. Paulo Buchsbaum Paulo Buchsbaum 4 4 silver badges 13 13 bronze badges. Field lines aren't physical objects that attract and repel each other, and the charge doesn't experience forces from fields that occur at some distance from it.

However, it's an useful abstraction tpub. If you follow carefully the above explanation, you notice that it matches with hand rule convention. Use the magnetic field lines analogy it's useful to draw the right force direction. See the magnets and parallel wires examples. This doesn't really make sense. They do exist, but they aren't physical objects. The powerpoint page you linked to actually doesn't support the analysis in your answer.

It describes a rule for telling whether physical objects attract or repel. Your answer talks about field lines attracting or repelling. Supposes two parallel wires A and B with positive current to the right. How does distance affect magnetic force? How do you measure magnetic force? How do lines of magnetic force travel? How do magnetic forces pass through objects?

How can two magnets demonstrate magnetic force? A straight section of the wire is 0. If the current is in the positive x-direction, what is the magnetic force on the section of wire. Link: The Lorentz force on a wire Youtube. A demo: We pass a current through a wire a section of which passes between the poles of two magnets, as shown below. Make a prediction. When we connect the power supply, which way the wire will move?

Discuss this with your fellow students in the discussion forum! Review with them the Lorentz force and the right-hand rule. In this video clip a hand crank generator is used to produce a voltage across a thin rod which can move in a magnetic field produced by a set of magnets.

The north pole of the magnets points up. When the crank is turned in the direction of the arrow shown, the red lead is positive, and a current flows from the left to the right through the rod.

If the crank is turned the other way, a a current flows from the right to the left through the rod. Look at a positively charged particle up a bit from the wire, standing still in the wire frame. It sees no force, since the wire is neutral. If it's set in motion in any direction perpendicular to the wire, it sees no contraction of either the positive or negative line of charges.

What if it's moving a bit parallel to the wire, say to the right? The negative charge line is more contracted in its frame, since it's moving to the left, and the positive charge line is less contracted. So our charged particle sees a more concentrated line of negative charges.