Propagation speed of the electrostatic field

[jeremyfiennes]

I have a proton and an electron at a certain distance from it. The proton exerts an electrostatic force on the electron. I then neutralize the proton's charge by firing another electron at it from behind. How long does it take for the first electron to sense the change?

 

[anorlunda]

EM in a vacuum propagates at c, the speed of light.

However :

I have a proton and an electron at a certain distance from it. The proton exerts an electrostatic force on the electron. I then neutralize the proton's charge by firing another electron at it from behind.

is a faulty description, the fields of the three particles effect each other at all distances and all directions. You can not suddenly make the net charge of the multi particle system vanish.

 

[jeremyfiennes]

That makes sense. Thanks.

 

[vanhees71]

Also note that if a field changes, it's not static anymore. Then you have to use the retarded propagator to solve for the Maxwell equations, and that makes everything causal going with a speed less than or equal to the speed of light in a vacuum.

 

[jeremyfiennes]

Ok. My doubt, however, is that this thought experiment refers to a propagating electrostatic, and not an electromagnetic field.

 

[Ibix]

Ok. My doubt, however, is that this thought experiment refers to a propagating electrostatic, and not an electromagnetic field.

If the electric field is changing ($\partial\vec E/\partial t\neq 0$) then the magnetic field is non-zero by the fourth Maxwell's equation.

 

[vanhees71]

There's an electromagnetic field, which usually is split up into electric and magnetic components, but this split depends on the chosen reference frame.

 

[jeremyfiennes]

I had suspected something like that. Thanks.