Electronics
What is electric current?
Electric current is the rate at which charge flows past a point in a circuit. Its symbol is I and its unit is the ampere, or amp (A): one amp is one coulomb of charge going past every second. Voltage is the push; current is the flow it causes.
Amps and milliamps
Small currents use milliamps: 1 mA = 0.001 A. An indicator LED runs at around 5–20 mA, while a phone charging draws around 1–3 A.
Charge, current and time are linked by Q = I × t. A battery rated 2000 mAh can deliver 2 A for an hour, moving 2 × 3600 = 7200 C of charge.
Live experiment
Count the charge going past
Pick a flow, from a trickle to a rush, and watch two counting windows tally the charge passing the top wire and the bottom wire. However fast it flows, A and B always agree, and more flow makes Pip brighter.
“Current: the flow” is lesson 3 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
The same all the way round
In a single loop the current is the same everywhere. Think of a bike chain: every link moves together, and none can pile up or go missing. A meter put anywhere in the loop reads the same.
So parts don't "use up" current. They use energy, which shows up as a voltage drop across each part. Open a switch anywhere and the current stops everywhere at once, not just near the switch.
Which way does it flow?
Engineers draw current leaving the battery's + end, going round the loop and coming back in at −. This is conventional current, settled long before anyone knew about electrons.
In a metal wire the moving charges are electrons. They're negative, so they actually drift from − to +. Both pictures predict exactly the same circuit behaviour, and circuit symbols and diagrams are drawn for conventional current.
What sets the current
The current depends on two things: the push behind it and the squeeze in its way. More voltage, more current; more resistance, less. That's Ohm's law, I = V ÷ R. With an LED in the loop, a resistor is what keeps the current at a safe 20 mA or so.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Current: the flow” is lesson 3 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: How much charge flows past a point each second. Current is charge on the move. Stand at one point on the wire and count what goes past each second. That count is the current.
- Why not “How thick the wire is”? A wire's thickness changes its resistance: a thicker wire squeezes the flow less. The current is how much charge actually flows.
- Why not “How warm the wire gets”? Warmth is a result of current flowing through resistance. Current itself is the rate charge flows, measured in amps.
- Why not “The push between two points”? The push between two points is voltage. Current is the flow that the push drives.
Show the answer
Answer: 0.25 A. Milli means one-thousandth, so 250 mA = 250 ÷ 1000 = 0.25 A.
- Why not “0.025 A”? 0.025 A is 25 mA, ten times too small. 250 ÷ 1000 = 0.25.
- Why not “2.5 A”? 2.5 A would be 2,500 mA, ten times too much. To go from milliamps to amps, divide by 1000.
- Why not “25 A”? 25 A is 25,000 mA, a hundred times too much. Milli means a thousandth, so the number in amps is smaller.
Show the answer
Answer: From + to − through the circuit. By convention, current flows out of the + terminal, through the circuit, and back into the − terminal. Electrons actually drift the opposite way.
- Why not “From − to + through the circuit”? That's the way electrons drift in a metal wire. Conventional current was defined the other way, + to −, before anyone knew about electrons.
- Why not “Both ways at once”? In a battery circuit the current flows one way round the loop. Only AC changes direction, and even then it flows one way at a time.
“Current: the flow” is lesson 3 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 2 before it first.
Questions people ask
What is current in simple terms?
Current is how much electric charge flows past a point each second. Its symbol is I and its unit is the ampere, or amp (A). One amp is one coulomb of charge per second.
Which way does current flow?
By convention, current flows out of a battery's + terminal, round the circuit, and back in at −. In a metal wire the moving charges are electrons, which are negative, so they actually drift the other way. Both descriptions predict the same circuit behaviour.
Is current used up in a circuit?
No. In a single loop the current is the same everywhere, before and after every part. Parts use energy, which shows up as voltage drops, not current.
How much current does an LED need?
A typical indicator LED runs at around 5–20 mA, which is 0.005–0.02 A. A series resistor sets that current, because nothing inside the LED limits it.
Related topics
- What is voltage?The push that makes charge flow: what a volt is, and why stacked cells add up.
- What is resistance?The squeeze on the flow, in ohms: how resistors set the current and protect an LED.
- Kirchhoff's laws explained: KCL and KVLWhat flows into a junction flows out, and the drops round a loop add up to the rise.