Topic 11.1 Electric Current: define electric current as the rate of charge flow and relate it to drift of charge carriers.

What this topic is asking

The College Board (Topic 11.1) wants you to define electric current as the rate of flow of charge, $I = \dfrac{\Delta q}{\Delta t}$, identify the conventional-current direction, and describe how charge carriers drift through a conductor.

What electric current is

Current measures how much charge flows per second, much as a river's flow rate measures how much water passes per second. The defining relation $I = \Delta q / \Delta t$ also runs backward: the charge that passes in a time $t$ is $\Delta q = I t$. Because charge is conserved and does not accumulate in a wire, the current is the same at every point along a single unbranched path, a fact used constantly in circuit analysis.

Conventional current and drift

The convention dates from before the electron was discovered, and AP keeps it: current arrows point the way positive charge would move, opposite to the electrons in a metal. The two-speed picture resolves a common puzzle. The electrons crawl, yet circuits respond instantly because the field propagates near light speed, pushing all the electrons in the wire into motion together the moment the circuit closes. So the signal is fast even though the carriers are slow.

Carriers, conservation and the strategic role

Current needs mobile charge carriers: free electrons in a metal, ions in a solution or gas. The conservation of charge means current cannot vanish or appear, so in a single loop the current is uniform, and at any junction the total current in equals the total current out (the junction rule of Topic 11.7). The strategic point of this topic is that current is the quantity that ties the whole unit together: voltage (from Unit 10) drives it, resistance opposes it (Topic 11.3), power is the rate it delivers energy (Topic 11.4), and Kirchhoff's rules account for it around any circuit. Getting the definition $I = \Delta q / \Delta t$ and the conventional direction right is the foundation everything else is built on.

Try this

Q1. A charge of $24$ C passes a point in $8.0$ s. Calculate the current. [2 points]

• Cue. $I = \Delta q / \Delta t = 24/8.0 = 3.0$ A.

Q2. State the direction of conventional current relative to electron flow in a metal wire. [1 point]

• Cue. Opposite to the electron flow (conventional current is the direction positive charge would move).