Current Electricity

A steady current in a metallic conductor is the net directed drift of free electrons moving opposite to an applied electric field. The electrons are already in constant random motion; what the field adds is a small drift superposed on that randomness, and it is the drift, not the random motion, that carries the current. This is why the definition of current is written as the rate of charge transfer, $$I = \frac{Q}{t}$$ where $I$ is in amperes, $Q$ in coulombs and $t$ in seconds. One ampere is one coulomb per second, so amp-second is simply the coulomb, and the charge moved over an interval is $Q = I t$.

Because the ampere is defined from charge and time, the unit questions here tend to be one step deep. The ampere is the SI unit of conventional current, the coulomb is the unit of charge, and the two are linked by the second. The chapter uses this repeatedly: a stated current and a stated time give a charge, or a stated charge and time give a current, and the examiner expects the conversion to be done in one line.

A bird perched on a high-voltage line is the standard thought experiment that separates current from potential. The bird is safe because its whole body sits at the same potential as the wire, so no potential difference exists across it and no current flows through it. A student who reads "high voltage" and refuses to believe the bird is unharmed has missed the point of the chapter: it is difference in potential, not potential itself, that drives charge.