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In this question the answer is B but I don't understand how that could be possible considering the area has doubled.enter image description here

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Picture the current as a number of electrons moving along the wire in a given time, and you'll realise it has to stay the same throughout the wire, otherwise you would have to be creating extra electrons at the point where the diameter of the wire increases.

The doubled cross section is reflected in the fact that there are now twice as many moving electrons per unit volume of the wire, so they now need to move along at half the speed.

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  • $\begingroup$ What force causes the charges to move at half the speed they were moving at? I think this would be useful to include. $\endgroup$ – Aaron Stevens Oct 19 '19 at 6:44

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