# What's different between electrons and planet under a central force?

1. Mar 14, 2004

### Student08

Why electrons do a circumferential motion under the magnetic force and planets do a elliptical motion under the gravity? I don't understand this because the magnetic force and the gravity are all central force. Can anyone tell me what's the difference between them, in words, mathematics or in physics principle. I'm a high school student who want to know about this. Thanks.

2. Mar 14, 2004

Magnetic forces are central? That's news to me!

Orbits are elliptical (or hyperbolic, depending on energy) because gravity is an inverse square law. Conic sections are the solutions to the orbit equation for an inverse square law. Why is gravity an inverse square law? Good question!

Electrons move in circles in a magnetic field, on the other hand, for different reasons.

I'd elaborate a bit more, but it's 5 in the morning and I'd kinda like to sleep before the sun comes up. So instead I'll kindly refer you to scienceworld, which has all sorts of nice equations with little explanation. Depending on what level of high school student you are, it may or may not be very enlightening.

Electromagnetic Field:
http://scienceworld.wolfram.com/physics/ElectromagneticField.html

The Lorentz Force (Electromagnetic force, basically):
http://scienceworld.wolfram.com/physics/LorentzForce.html

The Gravitational Force:
http://scienceworld.wolfram.com/physics/GravitationalForce.html

The electromagnetic field page has a nice little derivation of the motion of a charge in a constant electromagnetic field.

And welcome to the forums.

3. Mar 14, 2004

### Student08

4. Mar 14, 2004