Particle size (general) facts for kids

Particle size helps us compare how big tiny things are. Bigger things are called rocks or stones. But a particle is super small, usually less than a centimeter.

Particles can be solid, like dust. They can also be liquid, like tiny water droplets, or gas, like small bubbles. We can measure the size of all these tiny things.

If a particle is perfectly round, like a tiny ball (a sphere), its size is just its diameter (the distance across its middle).

Measuring Irregular Particles

Most tiny things aren't perfect spheres. They have weird or uneven shapes. This makes it a bit tricky to say exactly what their "size" is.

To solve this, scientists imagine replacing the real particle with a perfect sphere. This imaginary sphere has one important feature that is the same as the real particle. Then, the diameter of that imaginary sphere becomes the "particle size."

Here are some ways scientists define particle size:

• Volume-based particle size: This is the diameter of a sphere that has the exact same amount of space (volume) as the real particle.
• Weight-based particle size: This is the diameter of a sphere that weighs the same as the real particle.
• Area-based particle size: This is the diameter of a sphere that has the same outside surface area as the real particle.
• Hydrodynamic particle size: This is the diameter of a sphere that moves through a liquid or gas in the same way (has the same drag coefficient) as the real particle.