*Intermediate Physics for Medicine and Biology*.

### Divergence

The divergence is defined in Section 4.1 in *IPMB*, when discussing the continuity equation. The divergence is one way to differentiate a vector field. I our case, the vector field is the current density (or some other type of flux density), **j**. Its divergence is defined as

When you take the divergence of a vector (a quantity that has both magnitude and direction), you get a scalar (a quantity that has magnitude but no direction). In electrostatics, the electrical charge is conserved, implying that the divergence of the electrical current density is zero.

### Curl

The curl is defined in Section 8.6, when analyzing electromagnetic induction. It is another way to differentiate a vector,

The symbols **x̂**, **ŷ**, and **ẑ** are unit vectors, and the vertical lines indicate that you follow the rules for determinants when expanding this expression. The curl appears often when analyzing the magnetic field. In our case, the curl of the electric field equations the negative of the time derivative of the magnetic field (Faraday’s law of induction).

### Gradient

The gradient is a way to differentiate a scalar field to get a vector.

You can think of the gradient, ∇, as representing the vector **x̂ **∂/∂x + **ŷ** ∂/∂y + **ẑ** ∂/∂z. The divergence is then found by taking the dot product of the gradient with a vector, and the curl is found by taking the cross product of the gradient with the vector. In electrostatics, *V* represents of the electric potential (a scalar) and **E** represents the electric field (a vector). The two are related by

### Laplacian

The Laplacian, ∇^{2}, is just the dot product of the gradient operator with itself. In other words

You can apply the Laplacian to a vector, but it is more commonly applied to a scalar (such as electrical potential, temperature, or concentration). The Europeans use ∆ to represent the Laplacian, but that’s just weird and we Americans know better than that.

### Other Coordinate Systems

We have written the divergence, curl, gradient, and Laplacian in Cartesian coordinates. These operators are more complicated in other coordinate systems. Appendix L of IPMB provides expressions for these operators in cylindrical coordinats of spherical coordinates.### The Divergence Theorem

The divergence theorem says that the volume integral of div J is equal to the surface integral of the normal component of J. We don’t dwell on this theorem in IPMB, but we do ask the reader to derive it in Homework Problem 4 of Chapter 4.### Stokes’ Theorem

We don’t discuss Stokes’ Theorem in IPMB, but I’ve pointed out how we might include a homework problem about it in a previous blog post. Stokes’ theorem says that the line integral of a vector around a closed loop is equal to the surface integral of the curl of that vector of an area bounded by the loop.div, grad, curl, and all that, by h. m. schey. |

*IPMB*. If, however, you want a little more detail, Russ and I recommend the wonderful book

*div, grad, curl, and all that*, by Harry Schey. I learned vector calculus from the first edition of that book as an undergraduate physics major at the University of Kansas. Schey died five years ago, but his book lives on.

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