Name: 加油槍如何知道何時該跳停?（How petrol pumps know when to turn themselves off）
Uploaded: 2022-10-29T05:11:34.000Z
Duration: 13 min 56 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

- So, I bought a petrol pump nozzle and cut it in half.

情態助動詞 B1

is because I want to answer one question.

How do these things know when to turn themselves off?

Like, when you go to fill up your car with petrol,

you don't have to worry about it overflowing.

That's because these nozzles switch off automatically

My first thought was that there must be some kind

but the way these really work is much smarter than that.

There are actually two very clever mechanisms in here.

you can see that runs to the end of the nozzle,

and the second relates to all these connected levers.

to illustrate each part of the mechanism,

but actually this is entirely fluid mechanical.

The Venturi effect happens in this part of the nozzle,

I got this made; it's called a Venturi tube,

because it demonstrates the Venturi effect.

It's a wide tube that narrows in the middle,

and then you have this narrow U-bend shaped tube

coming off the restriction in the middle.

There are two other U-bend shaped tubes either side,

Look what happens when I partially fill this U-bend

with water, and then blow through the tube.

You might expect air to be forced into the U-bend,

causing the level of water here to go down.

That means there must be a reduction in pressure here

in the constriction, and that's sucking the water up.

is there just so that you can see the change in pressure.

There's no equivalent to that water in the nozzle itself.

but actually, we can explain it quite easily

The air traveling through the constricted part of the pipe

than the air in the wider part of the pipe.

To get the same amount of mass through a narrower pipe,

the mass must have to travel more quickly,

but it also has potential energy stored as pressure,

But that total energy, kinetic energy plus potential energy

stored as pressure, needs to be conserved.

That means that when the kinetic energy goes up

in the fast moving fluid in the constricted part

of the tube, the potential energy must go down.

By the way, those two other U-bend shaped tubes

either side of the middle one are there to illustrate

when there's air flowing, but just not to the same degree

as it does in the constricted part of the tube.

The two outer U-shaped pipes have nothing to do

with the discussion we're having here about petrol nozzles;

I just thought I should explain what they were.

But thinking about that central U-shaped tube,

what does that tell us about how the petrol nozzle works?

Well, this pipe in the model is this pipe in the nozzle.

The constriction in the pipe that happens here in the model

This spring-loaded stopper here opens slightly

And you can just about see, inside that ring,

The one I just illustrated with the paperclip.

There's also this one, and there's probably one

on the other side as well, but they all lead up to here,

and then into this hole, which comes down through here,

This tube that runs to the end of the petrol pump nozzle

So, this constriction here creates low pressure in the tube.

There is low pressure here at the end of the nozzle.

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加油槍如何知道何時該跳停?（How petrol pumps know when to turn themselves off）

potential

equivalent

slightly

negative