Question

# As an illustration of the point made in Footnote 13 in Section 4.7, consider a black...

As an illustration of the point made in Footnote 13 in Section 4.7, consider a black box that is approximately a 10 cm cube with two binding posts. Each of these terminals is connected by a wire to some external circuits. Otherwise, the box is well insulated from everything. A current of approximately 1 A flows through this cir- cuit element. Suppose now that the current in and the current out differ by one part in a million. About how long would it take, unless something else happens, for the box to rise in potential by 1000 volts?

Footnote

It is perfectly possible to have 4 A flowing into one terminal of a two-terminal object with 3 A flowing out at the other terminal. But then the object is accumulating positive charge at the rate of 1 coulomb/second. Its potential must be changing very rapidly – and that can’t go on for long. Hence this cannot be a steady, or time-independent, current.

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