Granny's Parlour

New generations of children are born with computers. Most do not even question the use of such a device or even how it functions, not anymore than people of my generation questioned the science behind the rotary phone. If something is functional, if it serves its purpose, than we mold it into our lives without fuss, naturally. It does not matter how complex or simple it is. If it serves a purpose and serves it well, there is little need for questions, except if you are curious and hungry for knowledge.

Perhaps it is a good thing that we do not question everything, for then many of us might be found staring at the lawn mower, in awe, instead of accomplishing what we set out to accomplish; mow the lawn. Others yet might be found standing in the kitchen, mesmerized by the food dehydrator or grain mill. In truth, some children do this (guilty as charged). Perhaps it is sad that we outgrow this sense of awe. On the other hand, this “oversight” allows us to keep moving. All good.

Ha! But turn on the water sprinkler or the hose and you’ve got little ones’ attention. What fun! Such a simple, practical device. But it would be useless without a faucet.

Turn on the faucet. The familiar sound of approaching water begins and soon reaches far into the garden hose, round and round the hose reel. It is almost a surreal sound, like the tiny traveling pod in the classic Fantastic Voyage movie within the blood stream. It is at once propelled by its own energy and called forth by the opening valve that offers a new portal.

A simple experiment, one we have all conducted at one time or another, raises arguments that in fact explain the process. Ever placed your thumb at the end of the hose to show the kids how much more pressure you can get that way? In truth, it is not possible to increase the pressure in this manner.

A typical garden hose that is connected to a spigot, that is in turn connected to a system of pipes that transport water from the underground offers an average of 40 psi (pounds per square inch) of pressure. The diameter of the nozzle, how much it is open or shut or placing your thumb against its opening cannot influence the water pressure. However, these influence flow, thus giving the illusion of varying pressure.

Water disperses everywhere there is an opening that allows it to spread. Opening the valve on the spigot or the nozzle provides an escape for water that has already dispersed through a long network of pipes. The hose is just another, more flexible pipe. The only way to increase the pressure would be to provide mechanical assistance. This is why water pressure cleaners are equipped with a pump.

Indeed, placing the thumb on the end reduces the flow, but a pressure gauge attached to the nozzle would still read the same psi as prior to the obstruction. Varying the flow at the nozzle only affects the velocity of the water as it flows out.

This is explained by a mathematical equation called the continuity equation, which states that  the flow into the hose (or other conduit) must equal the flow out of it. Thus, as we change the area of the nozzle (how much it is open), velocity increases to maintain constant flow. The smaller the opening, the greater the velocity and vice versa. The nozzle affects the flow pattern, not the pressure. This is why, also, water will take the same amount of time to reach the nozzle whether the hose sits flat on the ground or it is coiled up on a hose reel.

Funny science, isn’t it?