# Rotational energy from linear fields

1. May 15, 2007

### Chris FW

I am trying to understand how a charged object or an object with mass which is being accelerated by a linear field and therefore should accelerate in a purely linear fashion could possibly increase its rotational energy?
I am not looking for trivial answers perhaps a certain distribution of mass/ charge that is the shape of an object? But I would consider that any object independant of shape would accelerate in a straight line through the centre of mass/charge distribution.
Any suggestions would be gratefully appreciated.
Chris FW

2. May 17, 2007

### Xezlec

Hmm, "rotational energy"? Do you mean you want to know how to increase it's angular momentum about some axis? When an object is moving in a straight line, it already has angular momentum about every axis except an axis passing directly through the line of its path (someone correct me if I'm wrong). Linear and angular momentum are the same thing from two different points of view.

That being said, uniform magnetic fields tend to accelerate things (moving charges) in a circular way. Is this what you're looking for?

3. May 17, 2007

### da_willem

What do you mean with a linear field? As you know central forces cannot change angular momentum as the force is parallel to the position vector. There are however combinations of electric and magnetic fields that can change the angular momentum of a particle, notice that in this case the electromgnetic force has angular momentum of itself transferred to the particle! E.g. an electrically charged particle in a magnetic field is surrounded by both electric and magnetic fields that carries angular momentum.

Last edited: May 17, 2007
4. May 19, 2007

### Chris FW

For Example an electron accelerated by a uniform linear electric field in say a cathode ray tube will receive energy from the field and accelerate but its frequency will also increase (wavelength decrease) nothing to do with magnetic fields (F= q(VxB) but E=hf

5. May 19, 2007

### Xezlec

True. That's classical physics, and no rotation involved.

What do you mean by "its frequency"? Do you mean the frequency of the quantum mechanical wave function of the electron? If so, this is a correct statement, but then it's a quantum physics question. And yet, I still don't understand what that has to do with rotation or angular momentum. There is still no rotation involved.

6. May 19, 2007

### Creator

I think he means the 'relativstic' increase in energy (frequency) ....and thus what happens to the spin angular momentum in such a relativistic case.