A student lights a match in a small glass cell, closes it, and looks at the smoke through a…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student lights a match in a small glass cell, closes it, and looks at the smoke through a microscope. The bright specks of smoke dance about all the time, even though the air is still. Explain what makes them move. Then say what happens (a) with larger specks and (b) if the cell is warmed.
Need a reminder? The key ideaShow
One mole holds particles, and . In a gas each molecule has a volume , and the average spacing is its cube root. In a liquid the volume for each molecule is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The cause. Air molecules hit each speck from every side, billions of times a second. At any instant the hits on one side do not exactly balance the hits on the other side. The speck is pushed one way, then another. This is Brownian motion.
Step 2. Larger specks. A larger speck is hit by many more molecules at once, so the imbalance is a smaller part of the total. It also has more mass. It moves less.
Step 3. A warmer cell. The air molecules move faster and hit harder. The specks move faster too.
Watch out. We do not see the air molecules. We see their effect on a speck that is large enough to see.
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