Funny Egg Demonstration
Before we go on to the demonstration itself, let's look at some formal explanations and the equation related to pressure. Pressure measures the normal force per unit area exerted by a gas. If the pressure is uniform over an area, then we have
P = F/A
The SI unit of pressure is Newton per square meter, given the name Pascal(Pa) after the French mathematician, scientist Blaise Pascal (1623-1662). Another commonly used pressure unit is the atmosphere(atm), defined as Earth's normal atmospheric pressure at sea level and equal to 1.013*10^5 Pa (about 14.7 pounds per square inch).
The material of this demonstration are as follow:
- A shelled, hard-boiled egg
- A bottle with a neck just small enough that the egg won't fall in
- A little piece of paper
- Matches
Procedures:
First, light up a piece of paper and drop it into the bottle(Warning: Handle the flame carefully). While the paper is still burning inside, immediately put the egg onto the bottleneck and watch the egg dropped into the bottle slowly. You can view the demonstration
Explanation:
Assume the atmospheric pressure (as the top blue arrow) is constant everywhere initially, then pressure inside the bottle (the bottom red arrow) is equal to the outside. As the fire heats the air molecules inside the bottle, the air molecules gain extra energy.
These heated air molecules start to expand and move outside the bottle; therefore, the density of air inside the bottle is decreasing (the red arrow is getting relatively smaller and smaller than the blue arrow at the top). As soon as the egg seals the top, the air cools and pressure inside the bottle drops, so the egg is being pushed into the bottle by the atmospheric pressure.
Mini conclusion:
At the end of this demonstration, most students should be able to explain why the egg is being pushed into the bottle due to the difference between high and low pressure.