5
$\begingroup$

What is the difference between conservation of the Hamiltonian and conservation of energy?

$\endgroup$
1
8
$\begingroup$

Consider the time derivative of the Hamiltonian

$$\frac{dH(q,p,t)}{dt}=\frac{\partial H}{\partial q}\dot{q}+\frac{\partial H}{\partial p}\dot{p}+\frac{\partial H}{\partial t}=-\dot{p}\dot{q}+\dot{q}\dot{p}+\frac{\partial H}{\partial t}$$

From this you see that the Hamiltonian is conserved if it does not depend on time,$t$, explicitly. $H$ may or may not be the total energy, if it is, this means the energy is conserved. But even if it isn't, $H$ is still a constant of motion.

$\endgroup$
5
  • $\begingroup$ But then my reference sheet says that the potential should be independent of velocity. How does that work? $\endgroup$
    – Artemisia
    Nov 28 '13 at 14:52
  • 1
    $\begingroup$ The potential is velocity independent in order to have conservative forces. If $V=V(q,\dot{q})$, then the forces wouldn't be conservative and you couldn't state that $H=E$ $\endgroup$
    – vnb
    Nov 28 '13 at 15:06
  • $\begingroup$ So if a hamiltonian is not conserved, it is time dependent and its potential is velocity dependent? $\endgroup$
    – Artemisia
    Nov 28 '13 at 15:20
  • 2
    $\begingroup$ In general yes. But you can have cases where only the Hamiltonian is a function of time OR the potential is a function of velocity. It's not necessarily that both cases should be true at the same time. $\endgroup$
    – vnb
    Nov 29 '13 at 7:11
  • $\begingroup$ how would it look in quantum mechanics? $\endgroup$
    – MycrofD
    Mar 14 '18 at 1:55

This site is temporarily in read only mode and not accepting new answers.

Not the answer you're looking for? Browse other questions tagged .