# Is a proton's movement considered a subject of Quantum Mechanics?

1. Jan 4, 2009

### karkas

Is a proton's movement considered an object of Quantum Mechanics?

I had this thought a while ago when we were studying movements of protons in a force field, specifically inside the field of a charged capacitor.

The thing is that we used Newton's Second law to find the magnitude of the force exerted to the particle in a specific point. Then I thought "Hey,what the h*ll? Is this possible in a quantum level, isn't it sense-less?"

Am I wrong or right? I asked my sister and she told me this has nothing to do with quantum mechanics. My thought is that we have to use Schrodinger's Equation instead of Newton's. Enlighten me please!

Last edited: Jan 4, 2009
2. Jan 4, 2009

Staff Emeritus
Classical mechanics is the large distance limit of quantum mechanics. It's an approximation for protons and it's an approximation for baseballs. Whether it's a good enough approximation depends on the system at hand and your toleration for inaccuracy. It's entirely possible that it's good enough for the system at hand - particularly in elementary physics classes.

3. Jan 4, 2009

### Staff: Mentor

Consider the motion of an electron beam in a cathode-ray tube such as an old-fashioned TV set or oscilloscope. Classical physics is good enough for designing these devices.

4. Jan 4, 2009

Staff Emeritus
Or giant racetrack proton accelerators - you need SR, of course, but not QM.

5. Jan 5, 2009

### karkas

So I was right, what we were doing was a (good perhaps) approximation. Thank god, I thought I had misunderstood what QM is!

Now something optional. Can you show me perhaps a way to work with Quantum mechanics in such an exercise, per say? Thanks already, you are a very nice forum!! :)

6. Jan 5, 2009

### malawi_glenn

Well instead of force, you go to potential and do the shcrödinger equation.

For instance the Coulomb force between electron and proton, go to potential, solve for bound states -> Hydrogen atom.