When will flow rate stay constant and when will it change?

[September24]

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For fluids, you have the equation Q=AV. In most questions, flow remains constant so increasing area will decrease velocity.

However, I'll notice that in some questions, increasing area will make velocity remain constant but simply increase flow rate. For example:

You are pouring water into two "1 liter" bottles. One of the bottles has an opening that is 2x the size of the opening of the second bottle.

Since the area of the opening for the first bottle is larger, flow rate is larger. How come here, velocity remains constant. What words should make me think that flow is constant and what words should indicate a changing flow?

 

[DrknoSDN]

The driving force when pouring is gravity and the source is not limited.

in Q=AV if you increase area and keep velocity the same Q can increase, but it cannot be greater than a previous part of the system. Or else you would essentially create pockets of air spontaneously.
It wouldn't be reasonable to have a small pipe lead into a huge one and not slow down.

However, if you had an unlimited driving force such as two pipes hooked up to the bottom of a water tank, both pipes would have the same velocity but the larger area pipe would have a greater flow.

Your scenario is assuming gravity can put as much water through the bottle as is available. In reality a constant flow example would be equivalent to: "you are filling two 1 liter bottles from a sink faucet, one of the bottles has an opening that is 2x the size of the opening of the second bottle". In this case they would fill at the same rate because it can't fill any faster than water can come from the previous pipe (the faucet).

Just think about what the source of flow is and you can intuit whether flow is capable of increasing. Also remember "system" flow is constant everywhere (like electric current in series I1=I2=I3).