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so i bought a petrol pump nozzle and cut
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it in half you might call it a gas pump
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nozzle the reason i cut it in half is
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because i want to answer one question
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how do these things know when to turn
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themselves off you hear that click sound
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that's the nozzle turning itself off
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like when you go to fill up your car
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with petrol you don't have to worry
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about it overflowing that's because
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these nozzles switch off automatically
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when the tank is full but how my first
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thought was that there must be some kind
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of electronic sensor in there but the
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way these really work is much smarter
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than that there are actually two very
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clever mechanisms in here the first one
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relates to this tube you can see that
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runs to the end of the nozzle and the
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second relates to all these connected
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levers the whole thing's quite compact
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so i built a few different things to
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illustrate each part of the mechanism to
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make it clearer let's start with this
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hole that's usually located here or here
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it's called the venturi sensor which
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sounds like it's electronic but actually
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this is entirely fluid mechanical it's
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called the venturi sensor because it
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works on the venturi effect the venturi
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effect happens in this part of the
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nozzle but to be able to see what's
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going on i got this made it's called a
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venturi tube because it demonstrates the
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venturi effect it's a wide tube that
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narrows in the middle and then you have
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this narrow u-bend-shaped tube coming
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off the restriction in the middle there
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are two other u-bend shaped tubes either
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side but we're ignoring those for now
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look what happens when i partially fill
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this u-bend with water and then blow
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through the tube you might expect air to
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be forced into the u-bend causing the
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level of water here to go down in fact
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the water level here goes up that means
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there must be a reduction in pressure
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here in the constriction and that's
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sucking the water up by the way the
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water in the u-bend is there just so
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that you can see the change in pressure
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there's no equivalent to that water in
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the nozzle itself but why does the
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pressure go down well it's bernoulli's
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principle but actually we can explain it
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quite easily from first principles the
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air travelling through the constricted
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part of the pipe must be traveling
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faster than the air in the wider part of
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the pipe that makes intuitive sense to
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get the same amount of mass through a
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narrower pipe the mass must have to
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travel more quickly but you can also
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think about it in terms of conservation
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of energy the gas here has kinetic
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energy but it also has potential energy
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stored as pressure a bit like how you
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can store energy in a spring by
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compressing it
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but that total energy kinetic energy
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plus potential energy stored as pressure
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needs to be conserved
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that means that when the kinetic energy
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goes up in the fast-moving fluid in the
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constricted part of the tube the
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potential energy must go down the
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pressure must go down by the way those
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two other u-bend shaped tubes either
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side of the middle one are there to
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illustrate the fact that the pressure
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goes down in the wider parts of the tube
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as well when there's air flowing but
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just not to the same degree as it does
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in the constricted part of the tube the
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two outer u-shaped pipes have nothing to
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do with the discussion we're having here
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about petrol nozzles i just thought i
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should explain what they were but
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thinking about that central u-shaped
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tube what does that tell us about how
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the petrol nozzle works well this pipe
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in the model is this pipe in the nozzle
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the constriction in the pipe that
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happens here in the model actually
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happens here in the nozzle this
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spring-loaded stopper here opens
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slightly under the pressure of the
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petrol to reveal a really narrow ring
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for the liquid to pass through and you
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can just about see inside that ring
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there's a tiny hole there actually
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there's a number of holes the one i just
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illustrated with the paperclip there's
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also this one and there's probably one
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on the other side as well but they all
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lead up to here and then into this hole
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which comes down through here
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which feeds into this long tube
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this tube that runs to the end of the
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petrol pump nozzle is equivalent to this
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tube in the model so this constriction
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here creates low pressure in the tube
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there is low pressure here at the end of
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the nozzle that means air is actually
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drawn in through this tube that air
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simply mixes with the petrol in this
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part of the nozzle so how is this used
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to detect when your petrol tank or your
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gas tank is full this bit's really
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clever the tube that comes away from the
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constriction in the flow of the main
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pipe is actually forked one tube goes
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off to the end of the nozzle as we've
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seen but there's actually a second tube
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that goes off up here because of the way
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this thing has been cut in half it's not
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that easy to see but there is a tube
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coming off here most of it's been cut
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away but it was there and that tube
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leads to this cavity here that cavity is
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sealed by a membrane you can see part of
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the membrane just there before it was
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cut in half in other words before i cut
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it in half this whole chamber was sealed
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off with a membrane so schematically it
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would look like this with two tubes
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forking off the restriction this now
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represents the tube that goes to the end
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of the nozzle and this represents the
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tube that goes to the sealed chamber so
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when i blow through this pipe it will
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reduce the pressure in this tube and
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this tube
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but look what happens when i put my
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finger over this tube
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it turns out that this tube was
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relieving some of the negative pressure
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and when i put my finger over it look
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you see a sudden jump in the water level
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in this tube the same thing happens with
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the petrol nozzle this opening is
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allowing some of that negative pressure
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to be relieved by allowing air to flow
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into the system but then what happens
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when the level of petrol in your tank
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reaches the end of the nozzle well it
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covers up that hole the tube is now
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sucking on petrol instead of air petrol
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is heavier than air so the tube can't
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suck as much it can't relieve as much of
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that suction force and just like when i
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put my finger over one pipe in the model
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the other pipe experiences an increase
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in suction force this is my attempt to
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put all that together so you've got the
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main tube it has a constriction here
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here's the tube coming off from the
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constriction here is where it forks one
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tube goes to the end of the nozzle the
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other goes to this chamber that's sealed
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off with a membrane here
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and look when the liquid in the tank
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reaches the end of the venturi tube you
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see that membrane gets sucked into the
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chamber the membrane moves up only
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slightly in my model that's because i
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don't know much about fluid dynamics i'm
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sure there's a lot of things i could
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tune in this model like how much is the
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pipe constricted how wide are the
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venturi tubes where does the fork happen
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all that sort of stuff but for me it was
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incredibly satisfying to see that
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membrane move at all so when the petrol
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in your tank reaches the end of the
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nozzle this membrane moves and you'll
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notice that this membrane is attached to
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something it's attached to this rod here
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and that's really important that's how
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the nozzle actually turns off it's quite
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hard to see what's going on here so i
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built another model so imagine this is a
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valve that lets petrol through i need to
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push this thing up to open the valve in
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the actual nozzle this valve is
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spring-loaded
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in this model i'm representing that fact
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with a mass
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the mass is pushing back down on the
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valve like the spring in the real thing
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and this here is the handle of the pump
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so hopefully if i pull this handle up it
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will open the valve but look it doesn't
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actually work because we've got a lever
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happening here pulling the handle up
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just makes this thing move down what i
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need to do is hold this thing in place
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so that this becomes the fulcrum of the
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lever
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now when i pull the handle up it opens
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the valve great now one way i can hold
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this thing in place is to put a couple
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of circles in here
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and then i put this piece in to hold
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these circles in place to stop them
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falling into the middle
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and there you go that works perfectly
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now here's the clever part
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this funny wedge thing is attached to
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the membrane and remember when petrol
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reaches that venturi tube it causes the
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membrane to pop up and when it pops up
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it pulls this part with it
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and when it does that those circles are
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now free to fall into the middle they're
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no longer jamming that shaft in place
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and this point stops being the fulcrum
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everything collapses and the valve shuts
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in three dimensions this is achieved
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with three ball bearings in these two
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positions and one round the back that
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you can't see so let's put all those
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mechanisms together inside the nozzle
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itself and recap so petrol comes in here
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under pressure and it meets this closed
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valve so you pull on this handle here
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you'll notice it doesn't open the valve
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instead this thing moves but look you
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get to this point here and these ball
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bearings they get jammed against the
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constriction here in the housing of
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course because i've cut this thing in
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half that doesn't work the constriction
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doesn't work and the thing can move when
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it shouldn't so i'm just going to use
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brute force to hold that in place to
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illustrate the point and look now
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i've lost a ball bang doesn't matter now
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when i put on this handle this point
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acts as a fulcrum and
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the lever mechanism opens the valve and
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petrol can flow through
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so let's imagine the valve is still open
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petrol flows through here it looks as if
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this housing is in the way of the petrol
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but actually there's a gap underneath
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and there's a gap on the top as well so
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petrol flows around this part of the
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mechanism and then the pressure of the
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petrol pushes against this spring-loaded
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thing here which creates a thin circular
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channel for the petrol to continue to
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flow through the nozzle
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now because it's a thin channel and due
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to the venturi effect the pressure is
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lower in that part and look there's a
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hole here and a hole here that links to
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that low pressure region and that hole
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leads over here down into this tube but
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it also goes up into
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this cavity here that is bounded by a
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membrane at the bottom here it's cut in
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half now
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because this hole is open to the air it
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sucks air in and that acts to relieve
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the negative pressure
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but when your petrol tank is full and it
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comes up to that hole
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it's harder to suck on petrol than it is
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to suck on air so this tube here is less
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good at relieving that negative pressure
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which means that this chamber here feels
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a stronger negative pressure which pulls
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up on the membrane the membrane is
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attached to this thing so this thing
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gets pulled up as well so let's put all
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that together at the very end and see
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what happens when the petrol nozzle in
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your hand makes that clicking sound
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so the ball bearings are holding this
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thing in place and look you've opened
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the valve but now like this thing i can
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try and reach over and this thing lifts
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up you see the ball bearings fall into
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the middle of that thing which means it
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can now slide down which causes this
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thing to move down which causes this
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thing to move down
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which closes the valve and of course
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that prevents petrol from spilling out
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of your full tank onto the full court
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which would be a pretty big fire hazard
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this video is sponsored by 80 000 hours
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a non-profit that helps people to find
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careers that solve the world's biggest
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00:11:37,600 --> 00:11:40,560
problems like i remember careers advice
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at school where they'd ask questions
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like do you like being outside do you
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like people and at the end they'd say
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something like you should be a flight
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attendant or whatever and you're left
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thinking
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did they even ask the right questions
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you know what i mean the thing is there
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is actually good advice out there for
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how to find a fulfilling career it's
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just that careers advice at school
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00:11:59,279 --> 00:12:00,880
doesn't tend to focus on that most
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00:12:00,880 --> 00:12:02,880
importantly it's hard to find good
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careers advice for people who want to
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make a difference the advice tends to be
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follow one of these well-known career
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paths be a doctor be a teacher be a
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charity worker but those aren't the only
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options and they might not be the best
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option for you either 80 000 hours aims
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to help you find a career that's
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fulfilling and makes a big difference
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the name comes from the average length
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00:12:23,360 --> 00:12:25,920
of a career 40 hours a week 50 weeks a
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year for 40 years their insights come
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from 10 years of research alongside
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academics at oxford university and it
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turns out the best way to make a
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positive difference might be pretty
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00:12:36,000 --> 00:12:38,320
different to what you'd expect and you
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00:12:38,320 --> 00:12:40,160
won't find any unsupported
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00:12:40,160 --> 00:12:42,720
generalizations at 80 000 hours either
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00:12:42,720 --> 00:12:44,880
like their recommendations are based on
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careful research but if they aren't sure
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about something they'll say so and to me
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that says a lot if you care about what
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the evidence says about having a
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fulfilling and impactful career and you
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want advice that goes beyond follow your
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dreams then 80 000 hours can help and by
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00:13:01,680 --> 00:13:03,440
the way everything they provide is free
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00:13:03,440 --> 00:13:05,360
forever because they're a non-profit
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00:13:05,360 --> 00:13:08,560
their only aim is to help solve global
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00:13:08,560 --> 00:13:11,120
problems by helping people like you find
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00:13:11,120 --> 00:13:13,360
the most impactful careers they can if
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00:13:13,360 --> 00:13:14,639
you sign up for their newsletter now
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00:13:14,639 --> 00:13:16,639
you'll get a free copy of their in-depth
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00:13:16,639 --> 00:13:19,360
career guide sent to your inbox you can
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00:13:19,360 --> 00:13:21,680
go to eighty thousand hours dot org
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00:13:21,680 --> 00:13:23,839
forward slash steve to sign up the link
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00:13:23,839 --> 00:13:25,680
is also in the description so check out
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80 000 hours today i hope you enjoyed
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00:13:28,079 --> 00:13:30,880
this video thanks to ulysses mendes for
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00:13:30,880 --> 00:13:33,279
the idea if you did enjoy it consider
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00:13:33,279 --> 00:13:35,040
subscribing and the algorithm thinks
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you'll enjoy this video next
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[Music]
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