Water in an Inverted Cup

This demonstration can be modified for use as a magic trick.

Materials:

  1. Glass of water
  2. Piece of cardboard that is larger than the mouth of the glass.

Procedure:

  1. Fill the glass up with water.
  2. Place the piece of cardboard over the mouth of the glass.
  3. Holding the cardboard against the mouth of the glass, invert the glass.
  4. Release the hand slowly.

Explanation

Water can remain in an inverted glass with the piece of cardboard underneath because atmospheric pressure is acting upward on the cardboard, holding it up together with the water. There is little air pressure within the g;ass, so the downward force acting on the cardboard is mainly the weight of the water, which is to the order of several newtons whereas atmospheric pressure exert an upward force of several thousand newtons.

Modification:

  1. Drill a small hole in a plastic cup, near the base.
  2. Seal the hole with your thumb and fill the cup with water.
  3. Place the cardboard over the mouth of the cup.
  4. Invert the cup together with the cardboard, while keeping your thumb over the hole.
  5. Using a magic word as the cue, shift your thumb slightly to allow a little air into the cup. This will cause the cardboard and water to fall. As the air pressure within the cup is equal to that of the atmosphere.

Crushing Can

We are usually unaware of the immense strength of the pressure due to the atmosphere around us, having taken it for granted. This demonstration will utilize atmospheric pressure to crush an aluminum can while introducing concepts such as the relationship between pressure and the amount of gas in a fixed volume.

Materials

  1. Empty aluminum drink can
  2. Pair of tongs
  3. Stove or bunsen burner
  4. Tank of water

Procedure

Heating the Can over a Flame
  1. Put about a teaspoon of water into the drink can and heat it upright over the stove or Bunsen burner.
  2. Prepare a tank of water and place it nearby.
  3. When steam is seen to escape from the drink can, use the pair of tongs to grab the drink can, inverting it and placing it just slightly submerged into the tank so that the mouth of the can is sealed by the water.
  4. You should observe the can being crushed instantaneously.
Crushing the Can

Physics Principles Explained

Two physics principles work in tandem to crush the can. The cooling of the air within the can will reduce the internal pressure of the can as the movement of the air particles will slow down with reduced temperature.

At the same time, the sudden cooling will cause the water vapour in the can that exists at just slightly above 100°C to revert to its liquid state, greatly reducing the amount of gases inside the can.

Water Vapour Condenses Rapidly

As air pressure depends on both the kinetic energies and amount of particles within the system, it is significantly reduced. Atmospheric pressure, being stronger than the internal pressure, will cause the can to implode.