# pressure

## Hydraulic Elevator

This is a hydraulic lift kit for kids that was purchased online for only S$2.10 from Shopee, with free shipping! I am not, in anyway, affiliated to this, but simply sharing about one of several fun and cheap educational sets that I bought to occupy my kids during this mid-term break. Other than the syringe, joints and tube, the parts are mainly laser-cut from a piece of wood with a thickness of two millimetres. The instructions come with pictures for each step so even though the words are in Chinese, there is no need to read them. This kit demonstrates Pascal’s principle which states that a pressure change in one part of a closed container is transmitted without loss to every part. Hence, the pressure change is transmitted from one syringe to another, allowing work to be done. Do not expect it to lift up very heavy weights, though as the syringes are not perfectly sealed. I shall share about other kits that I bought for this break soon, including a$6.62 Tesla coil that I am looking forward to testing.

## Hydraulic Press Simulation

This simulation can be used for O-level Physics, for the topic of Pressure. I created it as it was relevant to our school’s IP3 physics as well.

It demonstrates the working principle of a hydraulic press. By adjusting the cross-section areas (A) of the two cylinders, you only need a small amount of force at the narrow piston to exert a large amount of force at the wider piston. This is how, when driving, the force applied by one’s foot is enough to supply a large force to apply the brake pads on a car’s wheels.

The advantage of using GeoGebra is that one can create such simple simulations within a couple of hours and it can be readily embedded into SLS – a wonderful tool to have during this period of full home-based learning.

## Pressure Variation in Stationary Sound Waves

For sound waves, we learnt that the compressions (position of maximum pressure) and rarefactions (minimum pressure) occur at the equilibrium position of the displacement of particles. This suggests that the pressure would vary the most in a stationary wave at the nodes of displacement. Right in the middle between two adjacent displacement nodes is the displacement antinode and we should expect the pressure variation to be the minimum there.

A displacement node is a pressure antinode.
A displacement antinode is a pressure node.

The standing waves associated with resonance in air columns can, therefore, be visualized in terms of the pressure variations in the column. Daniel A. Russell from The Pennsylvania State University made a wonderful animation showing how the variation of pressure occurs along an air column. (Link here)

It is a common misconception, even among physics teachers, that if a microphone is moved along the air column, it will pick up the loudest sounds at the displacement antinodes. However, according to Young & Geller (2007), College Physics 8th Edition, Pearson Education Inc. (pg 385), microphones and similar devices usually sense pressure variations and not displacements. In other words, the position within a stationary sound wave at which the loudest sound is picked up is at the displacement nodes which are the pressure antinodes.

Check out my own animation for a progressive longitudinal wave.

## 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

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.

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.

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.