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Question 12.67: Two charges ±q approach the origin at constant velocity from...

Two charges ±q approach the origin at constant velocity from opposite directions along the x axis. They collide and stick together, forming a neutral particle at rest. Sketch the electric field before and shortly after the collision (remember that electromagnetic “news” travels at the speed of light). How would you interpret the field after the collision, physically?^{29}

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Just before: Field lines emanate from present position of particle.

Just after: Field lines outside sphere of radius ct emanate from position particle would have reached, had it kept going on its original “flight plan”. Inside the sphere E = 0. On the surface the lines connect up (since they cannot simply terminate in empty space), as suggested in the figure.

This produces a dense cluster of tangentially-directed field lines, which expand with the spherical shell. This is a pictorial way of understanding the generation of electromagnetic radiation.

a
b

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