Answer to Question #102024 in Electricity and Magnetism for kishwer

Question

how to like currents attract each other and how to unlike currents repull each other?

Answer 1

With two currents flowing in opposite directions you can determine that the magnetic fields are in the same direction and will therefore repel. When the currents flow in the same direction the magnetic field will be opposite and the wires will attract.

In the frame of reference of the moving charges they will see a length contraction of the universe along the direction of travel. If electrons in both wires are moving in the same direction they see the same number of electrons in the other wire (because they are moving at the same speed.) But they see more protons. The difference in electric charges attract each other. It's a very tiny length difference, but there are lots and lots of charges.

If the currents flow in opposite directions, the electrons will "see" a higher density of electrons in the other wire due to relativistic length contraction. And the wires will repel.