A measurement is only complete when a unit is given: a quantity is a number times a unit. Every unit is built from seven base units, so you can derive a unit from the formula instead of memorising it. A prefix like kilo or milli is a power of ten.
One power bank says 20,000 on the box. The other says 10,000 and costs half as much. Yet you cannot say which one charges your phone more often, because those two numbers do not belong to the same quantity.
In seven minutes you learn how a measurement is written down in physics. You learn to tell quantity, symbol, number and unit apart, you see the seven base quantities the SI system rests on and how every other unit is built from them, and you learn that a prefix like milli, kilo or mega is not a letter but a power of ten.
For upper secondary physics. Twice the video asks a question and then waits: pause it there and think for yourself first, because the answer only comes afterwards.
In this video
- 0:00Twenty thousand or ten thousand: which power bank is better?
- 0:37Measuring turns it into a number
- 1:09A number on its own is not a measurement
- 2:10Who is taller: 180 or 1.79?
- 2:27Seven quantities the rest follows from
- 3:21Everything else is built from these
- 3:56A prefix is a power of ten
- 4:59Do mAh and Wh measure the same thing?
The full explanation, in writing
The full written transcript is on the Dutch page of this video. Nederlands
Frequently asked questions
What is the difference between a quantity and a unit?
A quantity is what you measure, for example length. A unit is what you express it in, for example the metre. Together they form the measurement: a quantity is a number times a unit.
What are the seven SI base units?
The metre, kilogram, second, ampere, kelvin, mole and candela. Every other unit is built from these, so you can derive them instead of memorising them.
Why is the kilogram the odd one out among the base units?
Because it is the only one that already carries its prefix in its name. The metre, second, ampere, kelvin, mole and candela have none; the kilogram has the kilo built in.
How many millimetres are there in a kilometre?
One million. Kilo is ten to the third and milli is ten to the minus third, so there are six between them: ten to the sixth millimetres.
Do mAh and Wh measure the same thing?
No. The mA in mAh is milliampere, a current, and multiplied by a time that gives a charge. The W in Wh is watt, a power, and multiplied by a time that gives an energy. Charge and energy are different quantities, so you cannot put those numbers side by side.
The power bank question is settled in the next video, where we work out how much is really in there.
Physics with Thomas makes physics explainers for upper secondary school. No words on screen, so what you see works in any language.
In this series
Quantities and unitswhy 20,000 is not more than 10,0007:21
Wetenschappelijke notatiemachten van tien en orde van grootte5:21
Eenheden afleiden uit een formuleomrekenen naar grondeenheden7:47
Significante cijfers en meetonzekerheidhoeveel cijfers schrijf je op6:24
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.