I found that tension is an electromagnetic force. I suspect the cause is that it's due to the cohesion of elements of the wire which is due to dipolar attraction of wire which is due an electric field. But how does electromagnetic phenomena get in? I need a realistic explanation.
Let us consider the given cases first:
Imagine a mass-less negative charge attached to a fixed string which is also mass less. Now consider a test charge attached on another string kept close to it, but not close enough that the charges will be able to touch even if the strings are at their maximum extension.
Now the negative charge feels an attractive force from the test charge, so it bends in the direction of test charge and also the test charge bends towards it. This is, as we know, due to the electrostatic force of attraction, so we also know that the charges will try to cling to each other, but they can't. Why?
Yes, due to the tension in the string to which they are attached which acts as an equal and opposite force for the charges, so now we know that there is tension due to the electric field.
Imagine the exact above case by replacing the strings with current-carrying wires, in which both currents move in same direction. We know there will be a magnetic force of attraction pulling them together, and as a result they also exhibit the same property as that of strings, i.e, they put tension over strings in order to try to come close to each other. So now we know that the magnetic field is also responsible for tension.
Now in this final case let us combine the above two cases:
Consider a fixed mass-less current-carrying wire having a negative charge on its surface and also another with the same direction of current, but with positive charge on it. Now we observe an electromagnetic field, but again, the individual electric and magnetic force act in their own way and hence the net effect on the wires will be that the tension they experience will add up and hence finally producing the new tension as sum of individual tensions in each of the above cases.
So from the above case we can understand that the net tension is the sum of tensions due to both the electric and magnetic fields, and hence we can conclude that there exists tension even due to electromagnetic forces.
I hope the answer helps.