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hello, ive just gone through a derivation using the vector potential A, and using the equation derived (a wave equation) which arises from using the lorentz gauge, it is applied to the problem of a current sheet in the xy plane, located at z=0 with current in the x direction only. the current varies with time so we are looking for wave solutions propagating in the + and  z directions.
Ive attached the relevant page (its not long at all), and basically my problem is simply: i understand pretty much everything up until equation 6.46, however, i cannot for the life of me understand how he gets to 6.47.
i know its probably something really simple, probably like a nifty application of the chain rule, but Id appreciate if someone could point out the elephant to me becuase i really cant see it and its annoying the hell out of me.
cheers
(im sure the answer will make me feel like an Rtard)
Ive attached the relevant page (its not long at all), and basically my problem is simply: i understand pretty much everything up until equation 6.46, however, i cannot for the life of me understand how he gets to 6.47.
i know its probably something really simple, probably like a nifty application of the chain rule, but Id appreciate if someone could point out the elephant to me becuase i really cant see it and its annoying the hell out of me.
cheers
(im sure the answer will make me feel like an Rtard)
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