### The Lorentz Force from the Classical Hamiltonian

In this section, we wish to verify that the Hamiltonian

gives the correct Lorentz Force law in classical physics. We will then proceed to use this Hamiltonian in Quantum Mechanics.

Hamilton's equations are

where and the conjugate momentum is already identified correctly . Remember that these are applied assuming q and p are independent variables.

Beginning with , we have

Note that . The momentum conjugate to includes momentum in the field. We now time differentiate this equation and write it in terms of the components of a vector.

Similarly for the other Hamilton equation (in each vector component) , we have

We now have two equations for derived from the two Hamilton equations. We equate the two right hand sides yielding

The total time derivative of has one part from changing with time and another from the particle moving and changing in space.

so that

We notice the electric field term in this equation.

Let's work with the other two terms to see if they give us the rest of the Lorentz Force.

We need only prove that

To prove this, we will expand the expression using the totally antisymmetric tensor.

Q.E.D.

So we have

which is the Lorentz force law. So this is the right Hamiltonian for an electron in a electromagnetic field. We now need to quantize it.

Jim Branson 2013-04-22