How this calculator works
Current counts the charge crossing a section of conductor per second. With uniform density and one type of carrier, a slab of length vd dt contains n A vd dt carriers. Their signed charge gives the current relation. The model uses average drift along the wire, not random thermal speed or signal propagation speed. A carrier charge of -e describes electrons and reverses their drift relative to conventional current.
Formula
I = n q A vd. vd = I/(nqA), A = I/(nqvd), n = I/(qAvd) and q = I/(nAvd).
Worked example
With I = 12 A, n = 2 m^-3, q = 3 C and A = 4 m², drift velocity is 0.5 m/s. Carriers with n = 3 m^-3, q = 4 C, A = 5 m² and vd = 2 m/s carry 120 A.
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Frequently asked questions
Is number density mass density?
No. It counts mobile carriers per cubic metre, rather than kilograms per cubic metre.
Why is electron drift negative?
For positive conventional current, negative electron charge requires motion in the opposite direction.
Does this predict how fast a light switches on?
No. Electrical signals propagate through the electromagnetic field much faster than the mean carrier drift.