Are Electric Field lines thought to be "in tension"? I had a thought the other day about how electric field lines and how they explain like charges repelling and opposites attracting. I read on another post that electric field lines are thought to be "in tension" hence when they connect positive and a negtive charge they move together and hence...force of attraction. And when you think about this, If you instead thought of a test charge for an electric field being "-q" instead of "+q", the direction of force arrows would change direction. Is my understanding of this correct? Please explain if i am wrong...cheers! 
 A: As a preliminary for the concept of 'stress in electromagnetism' there is the option of studying Maxwell's 1861 essay 'On physical lines of force'


In that essay Maxwell explores the ramification of a mechanical model of the medium of electric and magnetic phenomena. Maxwell was not necessarily committed to the notion of modeling the properties of the electromagnetic medium with a mechanical model. Rather, Maxwell used the model as an exploration tool. The mechanical model allowed Maxwell to arrive at an expression for the amount of stress that is involved.
The thing to recognize: the magnitude of the amount of stress that Maxwell arrived at is not dependent on the implementation details of the model. That is: the validity of the expression for the amount of stress extends beyond the particulars of the model.
Maxwell proposes: assume that there is a material vorticity around magnetic field lines. Now: a rotating sphere will bulge along its equator, and will contract along its axis of rotation. Maxwell proposes to attribute the tension and pressure associated with magnetic field lines so such a material vorticity.
Maxwell then proposes that all the areas of vortex motion are separated from each other by a layer of idle wheels.
These idle wheels are thought of as not having a fixed axle. Rather the idle wheels can transfer from vortex to (adjacent) vortex.
A flux of the idle wheels will tend to start vortex motion; when then manifests itself as a magnetic field.
These implementation details are extremely bizarre, of course. The thing is, by being methodical about it Maxwell is able to derive mathematical expressions for the properties of the mechanical model.

Wave propagation
In order for waves to propagate in a medium two properties must be present:

*

*There must be some form of elasticity. To have elasticity in some medium requires the following property: when the medium is deformed away from uniform state the medium tends to return to the uniform state. The stronger the tendency to return, the higher the corresponding modulus of elasticity.


*There must be a form of inertia. A form of inertia is present in the following circumstance: when the state of the medium is changing at some rate of change, the medium will tend to persist in that rate of change. In mechanics that is referred to as 'inertia', of course. Inertia makes things overshoot the equilibrium point, which tends to maintain oscillation.

Maxwell was able to correlate with experimental data obtained by Kohlrausch and Weber. (Experiments with capacitors, to find how many units of electric charge are needed to induce an electric field with 1 unit of field strength.)
Maxwell demonstrates:
If the medium of electromagnetism supports propagation of electromagnetic waves then these waves are expected to propagate at a velocity of 193088 miles per second
Maxwell proceeds to point out that Fizeau had obtained for the value of the speed of light 195647 miles per second
Maxwell comments:

[...] we can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.

