Current Of Electricity — study notes

Stand at a turnstile and count how many people click through in one second — that number, not the size of the crowd waiting behind it, is what tells you whether the queue is moving fast or slow. Electric current works the same way, and once that single idea clicks, most of this topic is just arithmetic built on top of it.

Current Is a Rate, Not a Count

Inside a wire, tiny charged particles called electrons are constantly on the move. What we call current isn't a count of how many electrons exist in the wire — it's how much charge squeezes past any one point every second. (Careful: it isn't the electrons' speed either — a slow but heavy flow of charge can still be a big current; a trickle is a small one.) We measure current in amperes (A) using an ammeter, and charge itself in coulombs (C). Because current is fundamentally a rate, it links directly to time: if you know the rate charge is flowing and how long it flows for, multiplying the two together tells you the total charge that has gone past. That relationship — charge equals current multiplied by time — is one of the first formulas worth memorising cold, because so much of the rest of the topic builds straight on top of it.

Worked Example — Charge Through a Desk Lamp

1. A small desk lamp draws a steady current of 1.2 A and stays on for 5 minutes. Find the total charge that flows through it.
2. Before touching the calculation, get the time into seconds — one ampere means one coulomb of charge passing a point each second, so the seconds are what the rate is counted in: 5 × 60 = 300 s.
3. Now multiply the current by the time in seconds: charge = 1.2 × 300 = 360 coulombs (C).

That's the pattern behind every charge calculation in this topic — get the time into seconds, then multiply. From here the full lesson below walks through conventional current versus electron flow, e.m.f. and potential difference, how a wire's length and thickness change its resistance, and reading I–V graphs — complete with an audio walkthrough and a worksheet to practise on.