# Why does breathing out have more effect on surroundings?

Theres a candle in front of me while ive been working. It got me thinking; why does it take so little to blow the candle out, but if i inhale near the candle almost nothing happens. I guess more generally than this, why does suction have such a small impact on its surroundings compared to blowing?

Related Other Physics Topics News on Phys.org
Svein
I guess more generally than this, why does suction have such a small impact on its surroundings compared to blowing?
At most you can create a vacuum in your mouth when inhaling (and I doubt that very much). Thus: max inhale pressure differential: 105Pa. When blowing, you can create a much higher pressure differential.

It is the same with suction pumps and pressure pumps: You can only suck water up to ≈10m (vacuum in the pump, 105Pa outside), but there is only a mechanical and practical limit to the height a pressure pump can push water.

rcgldr
Homework Helper
When air is blown from a higher pressure source, the flow is directional (assuming that a dispersal type nozzle is not used). When air is drawn into a lower pressure zone, the flow is not directional, but instead is drawn inwardws from all directions, so the net flow in any specific direction is much less.

Last edited:
CWatters, davenn and marcus
I also think that, for biological reasons, breathing out is a stronger reaction than breathing in. From what I remember, expelling carbon dioxide is more critical than taking in oxygen.

marcus
Gold Member
Dearly Missed
When air is blown from a higher pressure source, the flow is directional. When air is drawn into a lower pressure zone, the flow is not directional, but instead is drawn inwardws from all directions, so the net flow in any specific direction is much less.
That's a good explanation! To repeat/paraphrase: Blowing out creates momentum in the volume of air expelled, the directionality and coherence increases its effect. Breathing in only creates coherent directionality in the air going down your windpipe. I'll check out some of the other posts by Rcgldr. Also it was a good question by Sharker---hadn't ever occurred to me to ask.

davenn
Gold Member
2019 Award
I also think that, for biological reasons, breathing out is a stronger reaction than breathing in. From what I remember, expelling carbon dioxide is more critical than taking in oxygen.
naaaa, disagree, am sitting here reading this thread and my inhaling and exhaling are very even in strength

D

have to agree with davenn, surely you breathe the same amount in as out, and you pretty much control the force. But the momentum explanation makes sense, thanks.

there is a simple test using a U tube manometer filled with water (standard school demo) Blow into one end of the manometer....note the height difference(pressure)
Suck on the manometer, note the height difference (pressure).....let me know which is greater !!!
(the manometer needs to be about 2m tall)

That's a good explanation! To repeat/paraphrase: Blowing out creates momentum in the volume of air expelled, the directionality and coherence increases its effect. Breathing in only creates coherent directionality in the air going down your windpipe. I'll check out some of the other posts by Rcgldr. Also it was a good question by Sharker---hadn't ever occurred to me to ask.
Breathing in creates just as much momentum, but outside of one's body cavities it is diffused over a larger volume so the velocity of the air is generally less.

the fundanental thing is that you can 'blow harder than you can suck'..... this is so easy to check with a pressure gauge...

Drakkith
Staff Emeritus
the fundanental thing is that you can 'blow harder than you can suck'..... this is so easy to check with a pressure gauge...
You might be able to blow out harder than you can suck in, but that's not why you can blow out a candle but you can't 'suck' out a candle. The reason is that the outgoing air has momentum, just as explain in posts 3, 5, and 9.

A.T.
the fundanental thing is that you can 'blow harder than you can suck'.....
As others said, even you blow at the same rate as you suck in, the blowing will still be more effective in putting out the flame.

Put it another way: When you suck in air through a barely opened mouth, you feel a strong stream of air hitting your tongue that could put out a candle. So the issue not our limited suction strength.

I think that it is agreed that you can blow harder than you can suck....in pressure terms

Drakkith
Staff Emeritus