I just never understand how it works
In: Biology
I think about this a lot. A lot depends on understanding the physics of diffusion through the air.
Imagine a campfire. Crackling logs on fire, smoke pouring up.
If you observe the smoke, you’ll notice that the smoke close to the fire moves fast. As the smoke drifts away from the fire, it slows down, curls around chaotically, and gets pushed around by the wind.
You’re watching two kinds of diffusion:
1) Molecular diffusion – smoke particles and molecules spread out. The hotter and closer together they are, the faster they spread. When they cool down and spread out, they continue to spread but more slowly.
2) Eddy diffusion – Air currents, large and small, grab the smoke and move it around.
Molecular diffusion moves away from the source in all directions, at different speeds for different molecules, but it’s generally slow moving at great distances. Eddy diffusion is faster, and the direction just depends on which way the wind is blowing and how hard.
But even with a lot of diffusion spreading out molecules over great distances, the molecules will remain relatively near each other for a long time, like a cloud that gets inflated and stretched out for miles. That means if you smell one molecule, you’re likely to smell one of its neighbors nearby.
Let’s say you’re trying to find your friends at a camp ground. It’s getting dark and you’re walking through a forest.
Suddenly, you detect a slight smell of smoke.
You turn to the left, and sniff. No smoke.
You turn to the right, and sniff. Slight smoke.
You take a step right and sniff again, turning and stepping this way and that in an intuitive process of elimination until the smell of smoke is strong and you can hear your friends.
The combined action of molecular and eddy diffusion brought enough smoke particles into your nose that you caught that first whiff. You happened to be downwind of the campfire — that was good luck.
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