Electric field of a moving charge that's abruptly stopped

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• vish22
In summary, the conversation discusses the behavior of the field outside of a sphere with a radius of ct, in relation to the motion of an electron. The author explains that the field outside of the sphere acts as though the electron had continued its motion, based on Maxwell's equations. The field does not assume an "apparent position" of the charge and does not think that the charge is at x = v*t. The speaker shares their opinion that it would make more sense for the field to act as though the particle is in a state of motion at x=0, but the expert reminds them that physics is based on experimental evidence and not personal opinions. The conversation concludes with the speaker accepting the explanation and continuing their readings.
vish22
Hello everyone,

This is in reference to fig 5.19 (screen shot attached - please read the paragraph which says "Figure 5.19 shows the...").
I don't get why the field outside of the sphere of radius ct acts as though the particle would have continued its motion. Author's words : "The field outside the sphere of radius R = ct must be that which would have prevailed if the electron had kept on moving at its original speed. That is why we see the “brush”
of field lines on the right in Fig. 5.19 pointing precisely down to the posi-
tion where the electron would be if it hadn’t stopped."

Why does it act this way? Why does the field outside R=ct assume an "apparent position" of the charge? Why does it think the charge is at x = v*t and that it remains in its state of motion?

In my opinion, it would make more sense if the field (outside R=ct) acted as though the particle is in a state of motion (with uniform velocity v) at x=0, ie. The field outside R=ct must belong to that of a charge moving (with a uniform velocity v along the x-axis and situated at x=0), before transforming into an electrostatic field belonging to a charge that's stationary at x=0. The field will transform once it knows the particle has stopped. But before such a transformation occurs, the field should reflect the actual state of the particle right before it's abruptly stopped, no (neglect any deceleration effects)? I'm not sure why the field assumes an "apparent position" in this case.This just doesn't seem right to me - I'm not sure what to make of it.

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vish22 said:
Why does it act this way?
Because that is what comes out of Maxwell’s equations. What is outside of the sphere is not causally connected to any part of the particle worldline after the change in motion, only parts before the change. The field will therefore be the same as if the particle motion did not change because the change simply does not have enough time to propagate.

vish22 said:
Why does the field outside R=ct assume an "apparent position" of the charge?
It doesn’t. It depends on where the particle was and how it was moving on the past lightcone of the event.

vish22 said:
Why does it think the charge is at x = v*t and that it remains in its state of motion?
It doesn’t. See above.

vish22 said:
In my opinion, it would make more sense if the field (outside R=ct) acted as though the particle is in a state of motion (with uniform velocity v) at x=0, ie.
Physics is not required to adhere to what you think would make sense. The theory of electromagnetism is based on Maxwell’s equations - a relativistic field theory. It behaves this way and has been very well tested experimentally.

vish22 said:
This just doesn't seem right to me - I'm not sure what to make of it.
See above. Physics depends on experimental evidence, not on what you think seems right.

vanhees71
See above. Physics depends on experimental evidence, not on what you think seems right.

Yes, yes absolutely.

For now I shall consider it as being experimentally proven and carry on my readings, thank you!

1. What is the electric field of a moving charge that's abruptly stopped?

The electric field of a moving charge that's abruptly stopped is the force field that surrounds the charge and determines the direction and strength of the force that would be exerted on other charges in the vicinity.

2. How is the electric field affected when a charge suddenly stops moving?

When a charge abruptly stops moving, its electric field does not immediately disappear. Instead, the field will continue to exist and propagate outward at the speed of light until it dissipates.

3. Does the electric field of a moving charge change direction when it stops?

Yes, the electric field of a moving charge will change direction when it stops. This is because the direction of the electric field is determined by the direction of the charge's motion.

4. What happens to the electric field of a moving charge when it collides with another charge?

When a moving charge collides with another charge, the electric field of the moving charge will interact with the electric field of the other charge. This can result in a change in the direction and strength of the electric field for both charges.

5. Is the electric field of a moving charge affected by its speed?

Yes, the speed of a moving charge can affect the strength of its electric field. As the speed of the charge increases, the strength of its electric field also increases.

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