Embedded Files
Introduction:
Demonstrate how surface tension can make water appear to be "floating".
Materials:
Mason Jar (pint size) with twist-on lid
Water
Card
Circular Plastic Screen Insert
Procedure:
1. Place the plastic screen material over the opening of the jar and secure it by screwing on the lid.
2. Fill the jar with water by pouring in water through the screen.
3. Cover the opening with a card.
4. Hold the card in place while turning the jar upside down.
5. Slowly, let go of the card. The card should stay stuck to the jar. If it does not stay, redo steps 2-4.
6. While keeping the jar steady, carefully remove the card. Surprisingly, the water will stay in the jar.
7. Now, put the card back on and flip the jar right side up.
8. Remove the card, pour out the water, and amaze your friends!
Scientific Principle:
This demonstration is explained by the following principles: air pressure and surface tension.
In the first part, the card is magically suspended in the air due to the principle of air pressure. At sea level, the atmosphere exerts 15 pounds of pressure per square inch. Not only does the air push down, it also pushes upwards and sideways. Just like air pushes on us, the air below the cup actually pushes up on the card. Because the air molecules pushing up on the card is greater than the weight of the water pushing down, the card does not fall. When the 'seal' between the card and water breaks, gravity will win and the water will spill out.
The second part, where you remove the card, can be explained by the principle of surface tension. Even if the card is removed, the water stays in the cup because the molecules inside are joined together. This is due to the fact that the cohesion of similar molecules causes the water to form a thin membrane between each hole in the screen. The equation for surface tension is:
γ =F/L
where
γ is surface tension
F is force exerted parallel to the surface of a liquid
L is length of the line over the which the force acts
This means that the upward force from the surface tension balances the water's weight/force. It also tells us
that a larger surface area will need more energy to maintain due to the molecular forces. This is why the openings in the screen are small. If I used a screen with large openings, the surface tension will not be strong enough to hold the water up.
Safety Regulations:
1. Practice over a sink or bowl before trying it over something valuable.
2. Follow all standard laboratory procedures.
Page updated
Google Sites
Report abuse