IB Physics HLCurrent & CircuitsPaper 1 & 2Rate of Flow of Charge~9 min read
Electric Current
Flick a switch and a light comes on almost instantly. What’s actually happening inside the wire? Tiny charged particles — electrons — start to shuffle along. Electric current is simply how fast that charge flows past a point. On this page we’ll define current in plain words, meet the little equation that links it to charge and time, and sort out the age-old muddle of which way it “really” flows.
📘 What you need to know
Electric current is the rate of flow of charge — how much charge passes a point each second
Current is measured in amperes (A), often just called amps
Charge is measured in coulombs (C) and can be positive or negative
The equation is I = Δq / Δt
Conventional current flows from + to −; electrons flow the opposite way, from − to +
Direct current (d.c.) flows in one direction only — the kind you get from cells and batteries
What is electric current, really?
Picture a wire as a pipe, and the charge inside as water. Current is like asking “how much water rushes past this point every second?” The more charge that flows by each second, the bigger the current.
So current isn’t the charge itself — it’s the rate at which charge flows. That’s the whole idea in one sentence:
Electric current — in words
Electric current is the rate of flow of charge
We give current the symbol I, and we measure it in amperes (A) — “amps” for short. A current of 1 amp means 1 coulomb of charge flows past every second.
Stand at the dashed line and count how much charge sails past each second — that count is the current.
Here’s the trick to never mixing things up: charge is the “stuff” (measured in coulombs), and current is how fast that stuff moves past you (measured in amps). Charge is the water; current is the flow rate. Keep those two roles separate and this whole topic stays simple.
The equation for current
Turning that sentence into maths is easy. If an amount of charge Δq flows past a point in a time Δt, then the current is:
Current equationI = Δq / Δt
Where:
I = current, in amperes (A)
Δq = charge that flows, in coulombs (C)
Δt = time taken, in seconds (s)
Read it out loud and it says exactly what we defined: current equals charge divided by time — the charge per second. From this one equation you can find any of the three if you know the other two.
WE 1
A charge of 6.0 C flows through a lamp in 2.0 s. Calculate the current in the lamp.
Step 1 — write the equationI = Δq / Δt
Step 2 — put the numbers inI = 6.0 ÷ 2.0I = 3.0 A3 coulombs sail past every second — that’s what “3 amps” means.
WE 2
A current of 0.25 A flows through a wire for 40 s. Calculate the charge that passes through the wire.
Step 1 — rearrange for chargeI = Δq / Δt, so Δq = I × Δt
Step 2 — substituteΔq = 0.25 × 40Δq = 10 CMultiply the flow rate by how long it flowed — that gives the total charge, just like speed × time gives distance.
Which way does current flow?
Now the famous muddle. Inside a metal wire, the things that actually move are electrons. They’re negatively charged, so they’re pushed away from the negative terminal of the cell and travel round to the positive one. That’s the true, physical flow.
But here’s the history: scientists picked a direction for current before they even knew electrons existed. They chose positive to negative, and that choice stuck. We call it conventional current, and it’s still what every rule and equation in physics uses.
Two arrows, always opposite. Orange (conventional current) leaves the + terminal; blue (electrons) leaves the – terminal.
Don’t let this trip you up in the exam. Unless a question says the words “electron flow”, you always use conventional current: plus to minus. It’s the language the whole subject is written in — diodes, motors, circuit rules, all of it. The electrons quietly go the other way, but we rarely need to mention them.
Direct current (d.c.)
The current from a cell or battery is direct current, or d.c. for short. “Direct” means it always flows in one direction only, and it holds a steady value. If you drew a graph of this current against time, you’d get a flat, straight line — the current just stays put.
Direct current: one direction, one steady value. A flat line on a current–time graph.
WE 3
A steady current of 8 mA flows in a circuit. How long does it take for a charge of 4 C to pass a point? (Remember: 8 mA = 8 × 10−3 A.)
Step 1 — convert mA into A8 mA = 8 × 10⁻³ A = 0.008 AStep 2 — rearrange for timeI = Δq / Δt, so Δt = Δq / IStep 3 — substituteΔt = 4 ÷ 0.008Δt = 500 sAlways change mA into A before you divide — forgetting this is the number-one slip here.
Charge coulombs, C
÷ time (per second)
Current amperes, A
💡 Top tips
Current is charge per second — if you can say that sentence, you understand the topic.
Convert units first: mA → A (×10−3) before using the equation. Same for µA.
Conventional current is + to −; electrons go the opposite way. Default to conventional unless told otherwise.
1 A = 1 C per second — the amp is really “coulombs per second” in disguise.
Current can be positive or negative, but it’s a scalar — the sign just tells you direction, not a vector.
⚠ Common mistakes
Mixing up charge (coulombs) and current (amps) — current is the rate, not the charge
Forgetting to convert mA into A before dividing
Saying electrons flow from + to − — that’s conventional current; electrons go the opposite way
Thinking direct current changes direction — d.c. is one steady direction only
Treating current as a vector — it’s a scalar; the sign only marks direction
Quick recap: Electric current is the rate of flow of charge, I = Δq / Δt, measured in amperes (coulombs per second). Conventional current runs + to −, electrons the opposite way. Cells and batteries give direct current — one steady direction.
You now know how fast charge flows. But what makes it flow in the first place? Something has to push it round the loop — and that push is potential difference, or voltage. In the next page, Potential Difference, we’ll see how a cell gives each coulomb of charge a shove of energy, meet the equation V = W/q, and even discover a handy new energy unit, the electronvolt.
Current still feels fuzzy?
Book a free meeting and we’ll go through it together, at your pace, in plain language.
