# Force from an Electron

#### Tralfagar

Joined Jun 4, 2007
5
hello all, I was pondering, and I came upon something I couldn't quite find an answer for.

Newton's law says all the forces are equal to zero as long as you account for all of the forces in a closed system. And an electron passing through a magnetic field has a force exerted upon it, causing it to oscillate. My questions are these:

To satisfy Newton's law, where does the opposite Force Vector apply to(remember, a force is a kg*m/s^2? I was thinking it applied to whatever was making the magnetic field, but would this be correct?

And if that hypothesis were correct, would you be able to cause a force on the electron by just moving the electric field similar to Electromagnetic Induction(I think you should be able to, due to Galilean transformations)

Assuming that is also correct, would the a rotating trio of magnets be able to keep a charged particle(positive or negative, doesn't matter, as long as the mass approaches 0, so you can factor out gravity and the attraction between masses, but that shouldn't be a problem anyways) traveling in a straight line down the magnets(which are rotating around the particle)?

The magnets(which for all purposes would be bars with very little width and x length are separated 120 degrees. They form an Equilateral Triangle all the way throughout the arms. The triangle is perpendicular to the direction the arms are facing. The motion of the particle would be parallel the arms, perpendicular the direction they are rotating.

#### beenthere

Joined Apr 20, 2004
15,819

And if you think about it, magnetic fields are the means by which charged particles have their paths controlled while they are being accelerated. I suspect that the triangular field from your trio of magnets would not create a uniform field that would always deflect the particle back to the central axis, even if rotated.

#### Papabravo

Joined Feb 24, 2006
18,431
You need to also be aware that the behavior of particles on an atomic scale violate many of the rules of classical mechanics. It is why electrons do not travel in orbits like the planets orbiting the sun.

#### Tralfagar

Joined Jun 4, 2007
5

And if you think about it, magnetic fields are the means by which charged particles have their paths controlled while they are being accelerated. I suspect that the triangular field from your trio of magnets would not create a uniform field that would always deflect the particle back to the central axis, even if rotated.
Sorry, I didn't mean to make it sound like a uniform field, but yes, it was supposed to keep it along a path in the center(and hence a straight line, as the RHR would force the electron parallel to the axis of rotation. So you got it right on, trying to control the path of the electron while it's accelerating.

And atomic scale particles don't obey classical physics? Darn, oh well. I'm sure I'll be able to find more things to occupy myself with #### Papabravo

Joined Feb 24, 2006
18,431
In particular the Heisenberg Uncertainty Principle prevents you from determining BOTH the position and the momentum(velocity) with arbitrary precision. The more accurately you know the position the more uncertain is the velocity. Conversely the more accurately you know the velocity the more uncertain you become about the position.

http://en.wikipedia.org/wiki/Uncertainty_principle

#### recca02

Joined Apr 2, 2007
1,212
i think the two assumptions will hold.
but abt the three rotating magnets i m not even able to imagine the case properly.

#### Papabravo

Joined Feb 24, 2006
18,431
I don't think any of the assumptions are valid.
The sum of the forces being zero applies only to a system in equilibrium. Electrons are most definitely not in equilibrium since they are moving. Furthermore their motion is not uniform. They can perform quantum leaps. The uncertainty Principle places a lower bound on the ability to determine position and momentum. The rotating magnetic field cannot determine a path for the electron; it can only alter the probability of finding it in a given region of space.

#### recca02

Joined Apr 2, 2007
1,212
but wont a electron moving in magnetic field exert an equal force on the cause of magnetic field?
this has to be true else this cud easily be exploited to generate free power since there is no reaction to the action.

#### Papabravo

Joined Feb 24, 2006
18,431
but wont a electron moving in magnetic field exert an equal force on the cause of magnetic field?
this has to be true else this cud easily be exploited to generate free power since there is no reaction to the action.
This only applies when there is no motion, or when there is uniform motion. Uniform motion is what you get when there is no acceleration. As soon as there is acceleration or deceleration there is an imbalance, the forces no longer sum to zero and things either speedup or slow down.

Large electromagnets are often used to focus beams of charged paticles. When they do this you're not suggesting that the beam moves the magnet are you?

#### recca02

Joined Apr 2, 2007
1,212
of course they cant,
but that might be due to large mass of electromagnets hence its effect are negligible.

well i m a staunch supporter of classical theory and a bit skeptic abt it being invalid where the effect of quantum physics are more pronounced.
i have read abt Heisenberg Uncertainty Principle and the thought experiments but still i wud say that if a frame of reference is not an accelerated one the above rules should hold.

#### Tralfagar

Joined Jun 4, 2007
5
I am. Much in the same way when you jump on Earth, Earth should move towards you(an infinitesimal distance but still distance).

An example of this(in my thinking) using rotational motion:

A man has entered into the hammer throw for the Olympics. He starts rotating it. On the hammer there are two vectors(overall): The velocity and the centripetal force. Assuming the man stays in one spot(allowed to rotate though), he has no net movement(side-to-side), but the centripetal force must be out done by another force. I think it would probably be the frictional force between him and the ground.

Now to bring that example to here: When the particle beam moves across a magnetic field(I'll assume it's uniform, the particle will change direction and start orbiting a certain point as long as the entire span of the circle is over the magnet. Now, at any point, there are going to be two vectors; the velocity, and the centripetal acceleration(in this case, the force from the magnet onto the particle). Would it not have to be mitigated by a secondary net force?

#### Papabravo

Joined Feb 24, 2006
18,431
Well that would be the calssical viewpoint, and we know that it does not give the whole picture. There is more going on and it's useless to try and explain it if you cannot accept the quantum view. One thing that is required is that asking where is it, what is it's path, how fast is it going, and when will it get there, are all unanswerable questions. In point of fact nobody even asks those questions precisely because they are unanswerable.

Classical Newtonian Physics does not deal well with probabilities and statistical distributions.

ok, thank you 