Sucking

An example of a common alternative conception

Sucking is commonplace

We all tend to be familiar with sucking – it is a common enough phenomenon. We might suck a drink through a straw, or suck on an ice cube, or use a vaccuum cleaner ('hoover') to collect dust in the house.

image of baby drinking from milk bottle
"Good job mum remembered to leave some air in the bottle so I can create a pressure differential…" (Image by Dirk (Beeki®) Schumacher from Pixabay)
So what is sucking?
  • When you use a straw, you suck up the drink by pulling it through the straw – right?
  • Your vaccuum cleaner has a motor that pulls air in through the pipe, bringing dust and other debris with it – doesn't it?

As everyday descriptions, these accounts work well enough. But unfortunately they are strictly inconsistent with the accounts used in physics. Understanding sucking as being an action which pulls something along is an alternative conception (misconception). If one is being pedantic, this not what happens – and in science we need to be pedantic if we are going to develop effective theories of the world.

My vacuum cleaner really sucks!

Well, have you tried cleaning the filter?

[Poor attempt at a physics joke]

So, what is wrong with the sucking account?

Consider what happens when someone sucks in. By the time we are adults this has become such an automatic action that we do not think about what we are doing. (We think 'suck', and experience the flow of air coming into the mouth!) In order to suck, a person expands the volume of their lungs (we do this using the diaphragm beneath the lungs, and the intercostal muscles between the ribs). If we consider the amount of gas, air, in the lungs then this is not changed directly by our expanding our lungs – so, its density must drop – and, so, must its pressure.

This means that the pressure of air inside our lungs is now a little lower than the external, atmospheric pressure. There is at that moment a slightly higher density of fast moving air molecules just outside the mouth than inside – at least until there is a net flow of air in through the mouth to equilise the pressure. We may feel we are pulling air in – but acutally it is just the random movement of air molecules that leads to a shift from where there are more air molecues moving about, to where there are less.

Perhaps this seems just a more sophisticated account, that does not justify calling the everyday. description an alternative conception?

This straw really sucks!

Have you checked to see if it is blocked?

[Another poor attempt at a physics joke]

Sucking a drink through straw

People may feel they are pulling in a drink when they use a straw. The liquid In the cup or glass is in contact with the air above it. The liquid surface is being continuously bombarded by fast moving air molecules colliding with it – so pushing down on it. This air pressure does not usually have a noticeable effect as the drink is not (to any relevant extent) compressible, so it resists the pressure of the air.

However, if we place a drinking straw into the drink, then that part of the liquid surface inside the straw is in contact with the air in our bodies (lungs and mouth).

  • Now if we reduce pressure ('suck') there will be a pressure difference as there is higher pressure pushing down on the surface outside the straw than inside.
  • That means a net force is acting that pushes liquid up the straw.
  • We are not pulling the liquid, but applying a smaller push (from the pressure of the air in our lungs/mouth) than the atmosphere is.

Air comprises of myraid tiny molecules that are separated by empty space. So we canot pull with air in the way we can with, say, a rope where the molecules are bound together to transmit force along the 'chain'. Air can only push due to its pressure – it only seems to pull when it applies a smaller force than something pushing the other way.

The vaccuum cleaner motor does not actively pull air into the hose, but rather pushes air out the back. The motor moves air so there is higher pressure after the motor than before: the higher pressure region is greater than atmospheric pressure, so air is pushed out in that direction. The lower pressure region is below atmospheric pressure – so air from outside is pushed in (through the hose) by the higher pressure.

So, whereas the typical person may think in terms of things being sucked in – the scientist understands the same phenomena as creating the conditions to allow those things to be blown in! The distinction is not important in everyday life, but matters if we want to be scientifically accurate.

An analogy that might be useful in teaching:

Imagine you are lifting weights and hold 500N (c.50 kg) above your head. Perhaps after a short time your muscles will start to fatigue, and you weaken so that you can now only push up on the weight with 450N ! The weight would drop. It does not move down to the ground because you are pushing up on it with 450N force (that explanation sucks), but because the Earth is pulling down on it with a greater force!

Inappropriate sucking?

Writers of popular accounts of science (asssuming they do appreciate the science) have to make choices about when to be pedantic and when to use popular notions and expressions familair to their readers, even if these are technically dubious.

"The early stratospheric dust collection tests carried out in 1970 were not very sophisticated, using high-flying balloons called 'Vacuum Monsters' that sucked in air and filtered the particles out."

Natalie Starkey: Catching Stardust. Comets, asteroids and the birth of the solar system

"Ptolemy became the first space instrument in history to collect a mass spectrum on a comet – essentially a graphical pattern of the distribution of ions of chemical elements present in a sample. Ptolemy sucked in the neutral exhaust gases from the comet surface, ionised them (by removing electrons to give them an electrical charge), then analysed them to produce a spectrum to be beamed back to Earth."

Natalie Starkey: Catching Stardust. Comets, asteroids and the birth of the solar system 1

Sucking as a sources of comparisons

Although the use of the idea of sucking can be problematic in science,when it is implied that to suck is to directly apply a force, this does not prevent it being used metaphorically to explain other ideas.

Note:

1 I was a little mystified by this example. 'Sucking-in ' air in the Earth's atmosphere requires creating a region of pressure less than atmospheric – but 'sucking in' in the near vacuum of space would surely require creating a pressure even lower than that of space itself (even nearer a complete vacuum)? And the pressure difference created could only be miniscule, having a very small effect? This did not seem feasible, even for those clever space scientists.

I found a description on the website of the UK Science and Technology Facilities Council which seemed to suggest that the instrument did not actually sample gases being given off from the comet as suggested in 'Catching Stardust', but rather sampled material mined from beneath the surface, which was then vaporised in the instrument,

"Once on the comet's surface, Ptolemy will be provided with sub-surface samples which are then heated in miniature ovens. Once gaseous, the samples are fed into a sophisticated chemical analysis system and from here into an ion trap mass spectrometer. The gas is ionised by an electron-source and then a controlling high-voltage field is used to selectively eject ions of differing mass into a counter, enabling isotope ratios to be measured to very high precision."

https://www.ralspace.stfc.ac.uk/Pages/Ptolemy.aspx
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