Electric current is moving charge. Without a source, free electrons in a metal move in all directions, so nothing moves on balance; with a voltage source more of them go one way. The current is I equals Q divided by t, in amperes, and an ampere is one coulomb per second. The electrons crawl; what travels fast is the push.
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You press the switch and the light is on straight away. There are metres of wire in between, so you think: those electrons must race through. They do the opposite. In a wire connected to a battery they shuffle along about a quarter of a millimetre per second, one and a half centimetres in a minute. What does travel almost as fast as light is the push.
In this video you learn what charge is and what moves in a metal. An atom is neutral because the charge of the nucleus is exactly as large as that of all its electrons together. The charge of one electron is the smallest piece that exists on its own: the elementary charge, 1.602 times ten to the minus nineteen coulombs. One coulomb holds just over six times ten to the eighteenth of them.
In a metal the atoms sit in a lattice, and from each atom about one electron moves freely between the ions, in all directions. Without a source nothing moves on balance. Connect a voltage source and more of them go one way than the other, and that is electric current. How big that current is you state with the current: I equals Q divided by t, in amperes, and an ampere is nothing but one coulomb per second.
Then you calculate with it. An e-bike charger delivers 2.0 amperes and you charge for three hours: that is 10.8 kiloseconds, so just over twenty thousand coulombs through one plug. If you get 6.0, you have not converted the hours to seconds.
And finally the convention everyone trips over. By convention the electric current flows from positive to negative, while the electrons go from negative to positive. That is a historical accident: the direction was fixed before anyone knew electrons existed. For the calculations it makes no difference.
For upper secondary physics, in electricity.
In this video
The full explanation, in writing
Almost as fast as light
0:00You press the switch and the light is on straight away. There are metres of wire in between, so those electrons race through that copper, almost as fast as light.
Charge, and the smallest piece of it
0:14To understand what goes through that wire, you first need to know what charge is. An atom has a nucleus with electrons around it. The nucleus is positive, the electrons are negative, and in an ordinary atom those two are exactly equal. Net charge zero: the atom is neutral.
0:34Charge is a quantity with symbol Q, and the unit is the coulomb. And the charge of one electron is the smallest piece of charge that exists on its own. That is called the elementary charge, symbol e, and it is one point six zero two times ten to the minus nineteen coulombs.
0:55That is so small it is hard to imagine, so turn it around: one coulomb holds just over six times ten to the eighteenth electrons. A six with eighteen zeros after it. And because charge only comes in whole electrons: Q equals n times e, where n is a whole number.
What moves in a metal
1:17In a metal the atoms are fixed in a lattice, but each atom has about one electron that is no longer bound to its own nucleus. These are called free electrons, and they move between the ions, in all directions, even when nothing is connected.
1:34Then nothing moves on balance. Connect a voltage source, and more electrons go one way than the other. Net charge is moved, and that is what you call electric current. How big that current is, you state with the quantity current. Symbol I, unit ampere.
1:55And it is nothing but charge per second: I equals Q divided by t. Take the charger of an e-bike. It delivers two point zero amperes, and you charge for three hours. Three hours is ten point eight kiloseconds. Q is then I times t: two point zero times ten point eight thousand, is two point one six times ten to the fourth coulombs.
2:20Just over twenty thousand coulombs, through one plug. And mind the unit: ampere times second is coulomb. If you get six point zero, you have not converted the hours to seconds. In the wire, negative electrons move from negative to positive. Which way does the electric current flow by convention?
The convention, and why it is the wrong way round
2:45The convention is: the current flows in the direction positive charge would move, so from the source's positive terminal, through the circuit, to the negative terminal. The electrons do exactly the opposite. They go from negative to positive.
3:00That is confusing, and it is a historical accident: the direction was agreed before anyone knew electrons existed. But it has never been changed, because for the calculations it makes no difference. All the formulas simply work. So remember two arrows: the electron flow from minus to plus, and the electric current, which you draw, from plus to minus.
How fast does an electron really go
3:27And now that speed from the start. Connect a bulb to a battery, and the electrons in that wire shuffle along about a quarter of a millimetre per second. One and a half centimetres in a minute. On the mains it is even stranger. There the voltage goes back and forth fifty times per second, so the electrons get nowhere: they vibrate a thousandth of a millimetre around their place.
3:53Yet your lamp is on straight away. Think of a garden hose already full of water. You turn on the tap and water comes out of the end straight away. That is not the water now leaving the tap, because that is still at the start. It is the water that was already waiting at the end.
4:14The water does move, along the whole hose at once, only slowly. What travels through quickly is the push. In the wire it is the same. It is already full of free electrons, and the push travels through almost at the speed of light, so everywhere they start to move at the same time.
4:33Only they themselves move painfully slowly.
Three things to remember
4:38Three things. Charge comes in whole electrons: Q equals n times e, and e is one point six zero two times ten to the minus nineteen coulombs. Current is charge per second: I equals Q divided by t, in amperes. And the conventional current direction is from positive to negative, while the electrons go from negative to positive. The electrons crawl; the push goes almost as fast as light.
Two questions
5:05Two questions. One. Through a wire, for half a second, a current of four point zero amperes flows. How much charge has passed? Two. You extend the cable of your desk lamp by five metres. Does it then take noticeably longer before the lamp comes on?
The answers
5:25The first: Q equals I times t, so four point zero times zero point five is two point zero coulombs. Ampere times second is coulomb, so the unit is right. The second: no. The electrons that make your lamp light up were already in that last bit of wire; they do not first have to travel from the socket to the lamp.
5:46What does have to cross those five metres is the push, and it takes a few hundred-millionths of a second.
Current, charge and the two directions
5:55Now you know what moves and how fast it goes. Where the energy comes from that sets those electrons moving, and why that is expressed in volts, there is a separate video about that on the channel.
Frequently asked questions
How fast do electrons move in a wire?
Much more slowly than you think. In a wire connected to a battery they shuffle along about a quarter of a millimetre per second, so one and a half centimetres in a minute. On the mains they get nowhere at all: there the voltage changes direction fifty times per second, so they vibrate a thousandth of a millimetre around their place. Yet your lamp is on straight away, because the wire is already full of free electrons and the push travels through at almost the speed of light.
Why does the current flow from positive to negative while the electrons go the other way?
Because the direction was agreed before anyone knew electrons existed. The convention is: the current flows in the direction positive charge would move, so from the source's positive terminal through the circuit to the negative terminal. The electrons are negative and do exactly the opposite. The convention has never been changed because nothing in the calculations depends on it: all the formulas simply work.
How do you calculate how much charge has passed?
With Q equals I times t. Mind the unit of time: it has to be in seconds. A charger of 2.0 amperes charging for three hours delivers 2.0 times 10.8 kiloseconds, so 2.16 times ten to the fourth coulombs. If you calculate with three instead of 10800, you get 6.0, and that is a factor of 3600 out.
Next comes voltage: what a volt actually tells you, and why two batteries with the same number on them last very different lengths of time.
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Physics with Thomas: physics explained with pictures where they add something, and without them where they distract.
In this series
Electric currentthe electrons crawl, your lamp is on at once6:12
Voltage and capacitytwo batteries, the same number4:43
Connecting ammeter and voltmeterswapping them wrecks your meter4:10
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.