I have a quick question about the retarded time when dealing with moving charges.(adsbygoogle = window.adsbygoogle || []).push({});

The retarded time is:

[itex] t' = t - \frac{r}{c}[/itex]

where [itex] r [/itex] is the distance between the point of observation and the position of the charge.

My question is very simple, is [itex] r [/itex] a function of the normal time [itex] t [/itex], or the retarded time [itex] t' [/itex]?

That is, which equation is correct?

1. [itex] t' = t - \frac{r(t)}{c}[/itex]

2. [itex] t' = t - \frac{r(t')}{c}[/itex]

Thanks.

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# Retarded time

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