# Is there mechanical strain on a wire along the direction of current flow?

The (attractive) magnetic force on two parallel wires can be seen by the two wires bending towards each other. This can be explained by the Lorentz force acting on the charges and the charges "pulling" transversally on the wires, because they cannot leave the wire because of the work needed to leave the wire.

Now the charges inside of a wire connected to the ends of a voltage source also experience a (generally much stronger) electric force along the main axis of the wire. As they collide with the atoms of the wire, shouldn't they "push" the wire in the direction of the positive cathode? Can this strain be measured?

• "As they collide with the atoms of the wire..." The electrons are part of the atomic structure and do not 'collide with' anything in the wire. Dec 17, 2015 at 0:35
• @Asher: the momentum of electrons changes as they propagate along the wire, and the momentum change will produce a force. It's an interesting question and one that will require some thought ... Dec 17, 2015 at 12:00

When the current is constant, the net force on the free electrons is zero, as they are not accelerating, the average speed of the free electrons is constant.

That zero force can be decomposed to two parts: The force caused by the collisions and the force caused by the electric field.

How about positive charges?

They feel a force caused by the collisions with the electrons and a force caused by the electric field. Net force is zero also here. (for simplicity's sake, let's say N electrons are colliding with N protons)

When the current is changing, the force caused by the collisions is either larger or smaller than the force caused by the electric field. Also in this case there is a symmetry between negative and positive charges.

What happens if there is a superconducting ring on a frictionless surface, and we induce a current to the ring? Some electrons start to revolve to one direction, while rest of the ring starts to revolve to the opposite direction.

• The wire may not move - but it could still experience stress. A spring-coil that I pull on from both sides will also experience a zero net force, but it will be stressed. I still don't see clear in case of the wire. Dec 19, 2015 at 12:22