**Learning objectives**

- Compute divergence of a vector field by thinking about mass flow into a box
- Understand the divergence theorem as giving 2 ways to calculate rate of change of mass inside a box
- By measuring the flow of mass into the box through all its bounding surface
- By measuring the rate of change of density of mass at each each point inside the box

- View divergence as a multiplier
- Flux of through the surface of a small box (div ) (Volume of box)

- Use the divergence theorem
- Derive the diffusion equation

**Before class**

- Read Hughes-Hallett sections 19.3 and 19.4
- Ponder Hughes-Hallett problems: 19.3: 4, 6, 7, 20; 19.4: 1,3.

**In class**

- Notes on the divergence theorem
- More information in section 1 of document on fluid mechanics
- Problems on divergence and divergence theorem