# The direction of flux vectors in derivation of conservation of mass

• Mart1234
In summary, the conservation law of mass states that the net mass entering a control volume must be equal to any variation of mass within the control volume during a given time interval. This applies to control volumes of any size and can change during the time interval.

#### Mart1234

In the derivation of the conservation law of the conservation of mass, the flux on one side enters and the flux on the other side leaves the control volume. I presume this is due to the assumption that the volume is infinitesimally small and hence v(x,y,z,t) will not change directions dramatically within the control volume. Is this the correct way of thinking about this or am I missing something?

That sounds plausible, but without the context it's difficult to say.

No, what the law of conservation of mass says is that, during a given time interval, any variation of mass inside a control volume must be equal to the net mass that traverses the control surface during that same time interval. Not only the control volume can be of any size, but it can also increase or decrease during that time interval.