Why does adding up certain partial derivatives have anything to do with outward fluid flow?
Warmup for the intuition
In the last article, I showed you the formula for divergence, as well as the physical concept it represents. However, you might still be wondering how these two are connected. Before we dive into the intuition, the following questions should help us warm up by thinking of partial derivatives in the context of a vector field.
Reflection question: A two-dimensional vector field is given by a function defined with two components and ,
A few vectors near a point are sketched below:
Intuition behind the divergence formula
Let's limit our view to a two-dimensional vector field,
Remember, the formula for divergence looks like this:
Why does this have anything to do with changes in the density of a fluid flowing according to ?
Let's look at each component separately.
For example, suppose , meaning the vector attached to has no horizontal component. And let's say happens to be positive. This means that near the point , the vector field might look something like this.
- The value of increases as grows.
- The value of decreases as gets smaller.
Therefore, vectors to the left of will point a little to the left, and vectors to the right of will point a little to the right (see the diagram above). This suggests an outward fluid flow, at least as far as the -component is concerned.
In contrast, here's how it looks if is negative:
- The vectors to the left of will point to the right.
- The vectors to the right of will point to the left.
This indicates an inward fluid flow, according to the -component.
The same intuition applies if is nonzero. For instance, if is positive and is also positive, this means all the vectors around point to the right, but they get bigger as we look from left to right. You can imagine the fluid flowing slowly towards from the left, but flowing fast away from it to the right. Since more is leaving than is coming in, the density at this point decreases.
Analyzing the value is similar. It indicates the change in the vertical component of vectors, , as one moves up and down in the vector field, changing .
For example, suppose , meaning the vector attached to has no vertical component. Also suppose is positive, meaning the vertical component of vectors increases as we move upward.
Here's how that might look:
- Vectors below will point slightly downward.
- Vectors above will point slightly upward
This indicates an outward fluid flow, as far as the -direction is concerned.
Likewise, if is negative, it indicates an inward fluid flow near as far as the -direction is concerned.
Divergence adds these two influences
Adding the two components and brings together the separate influences of the and directions in determining whether fluid-density near a given point increases or decreases.
Want to join the conversation?
- When will there be practice exercises for this content? I feel like I am learning content but will not be able to say, pass a test. Answers will be appreciated.(15 votes)
- The author of these lessons told in another article that he prefers to teach us the concepts and to give the formulas, but that it isn't very important to practice again and again on examples. You can find some by yourself, but I suppose there won't be any here on these articles :/(4 votes)
- but why they are linear add, not "Pythagorean Add": sqrt(a^2+b^2)(1 vote)