![]() ![]() With the index card in place, the water surface is kept flat and the pressure is evenly distributed over the entire mouth of the glass.įor much smaller openings, surface tension is enough to stabilize the surface, and we actually don't need the index card. Then another drop of water will fall out and another bubble of air will enter, and the process will accelerate until all the water is emptied out of the glass. If the glass is tilted ever so slightly to one side, or if there is a tiny ripple in the surface of the water, a drop of water will fall out of the glass on the low side, and a bubble of air will enter on the high side to make up the missing volume. In practice, it's impossible to achieve these conditions without the help of the card. In principle, if we could invert the glass of water so that the glass was perfectly level and the water was perfectly still, the forces would balance as before and the water would stay in the glass. Why doesn't the water stay in the glass when we don't use the index card? ![]() Cohesion adds the extra force necessary to overcome small instabilities in the water. This is why you can keep water in a straw just by putting your finger over the top, leaving the bottom open. In containers with a small opening, like a straw, cohesion plays a bigger relative effect. As a result, the pressure difference required to keep the water in the glass is less than would be needed if there were no cohesive force. (This cohesive force is the origin of surface tension.) In the upside-down glass, it helps prevent the first water drop from separating from the rest of the water volume. Water molecules have a strong attractive "cohesive" force between them due to the fact that each water molecule can make four hydrogen bonds with other water molecules. There is another separate effect that helps keep the water in the glass. For a typical sized glass about half full of air, an air volume increase of less than 1% generates a big enough pressure difference to support the weight of the water. Once the card sags enough so that these three forces balance, everything will stay put. Now the pressure inside the glass pushing down is not as great as the outside pressure pushing up, and this pressure difference is enough to counteract the gravitational force pulling down on the water. As a result, the air pressure goes down a tiny bit according to Boyle's Law. The air molecules spread out so that fewer of them hit the edges of the volume each second, and they slow down so that they don't collide with the container quite as forcefully. The air inside the glass was originally at one atmosphere of pressure when you put the card over it, but when you inverted the glass and removed your hand, the water moved downward a very slight amount (perhaps making the card sag ever so slightly), thereby increasing the volume allotted to the air.Īs the air expands to fill this increased volume, several things happen at once. Of course there is also pressure from the air inside the glass pushing down on the water from above. The card transfers the force of the air pressure upward to the water, so there is a pressure of (almost 1) one atmosphere pushing up on the water from below. The details of this delicate balance are more easily understood by looking at the forces on the water, rather than on the card (see Figure 2). Together, the three forces balance out to cancel each other. The red arrow indicates the force of gravity. The blue arrows indicate the forces due to air pressure above and below the water. ![]() 2: Diagram showing the relevant forces on the water. $$Force=Pressure\times Area$$ If you've done the trick correctly, the force from the air below exactly counteracts the force from the water above, and the card stays in place.įig. The force on the card is just the pressure times the area over which the pressure is applied that's the definition of pressure. This air pressure is pushing up on the card from below, while the water is pushing down on the card from above. ![]() At sea level, the mean air pressure is one "atmosphere" (=101,325 Pascals in standard metric units). Any object in air is subject to pressure from air molecules colliding with it. Why doesn't the water fall out of the glass with the index card? If you hold the glass steady and level, the water should remain in the glass (Fig. While your hand is on the index card over the mouth of the glass, invert the glass and slowly take your hand away. Put an index card over the mouth of the glass and press the palm of your hand on the index card, pressing the card against the rim of the glass and depressing it slightly into the glass in the center (this part is very important). the trickįill a glass part way with water. ![]()
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