Topic B.5 — Current & CircuitsPaper 1 & 2I = ∆q / ∆t~6 min read
Electric Current
Every circuit relies on charge being pushed around a loop. Electric current is simply a measure of how much of that charge is passing a point per second — and the direction we conventionally draw it in isn’t quite the direction the electrons actually travel.
📘 What You Need to Know
Current is the rate of flow of charge, measured in amperes (A)
I = ∆q / ∆t, where charge ∆q is measured in coulombs (C)
Conventional current flows from the positive terminal to the negative terminal of a source
Electrons are negatively charged, so they physically drift the opposite way — from negative to positive
Direct current (dc) flows in one direction only and is produced by cells and batteries
Current is measured with an ammeter, connected in series in the circuit
Defining Electric Current
Inside a conducting wire, huge numbers of charge carriers (usually electrons in a metal) are jostling about. When a potential difference is applied, they gain a slow net drift in one direction on top of their random motion. Current is a measure of how much charge streams past a fixed point every second — the faster that charge moves through, the larger the current.
Electric currentI = ∆q / ∆t
where I is current in amperes (A), ∆q is the charge transferred in coulombs (C), and ∆t is the time interval in seconds (s). Rearranged, this also tells you that 1 ampere is 1 coulomb of charge passing a point every second.
Conventional Current vs Electron Flow
Current was defined long before anyone knew electrons carried the charge in a metal wire — scientists simply picked a direction and called it “positive to negative”. When electrons were later discovered to be the actual charge carriers, it turned out they travel the opposite way. Both descriptions are used in physics, so it’s worth being able to picture them side by side.
Conventional current (teal) is drawn leaving the positive terminal and flowing round to the negative terminal. The electrons actually doing the moving (grey) drift the opposite way, leaving the negative terminal.
Direct Current (dc)
Cells and batteries produce direct current — current that flows in a single, unchanging direction. Plotted against time, a dc supply gives a flat horizontal line: the current has one constant value and never reverses.
Direct current has a single, constant value that never crosses the time axis — it always flows the same way.
Quick recap:I = ∆q/∆t; conventional current runs + → −, electrons drift − → +; dc stays constant in one direction.
WE 1
A charge-logging sensor records that 9.0 mC of charge passes through a wire every 1.5 ms. What current does this correspond to?
