# Direction of an ionized electron's travel

• I
Hello! If I have an atom traveling with a given velocity and I ionize it in flight, will the electron remain at the place of ionization, or it will travel at the same speed along with the resulted ion? Thank you!

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BvU
Homework Helper
2019 Award
and I ionize it in flight
Depends on how you peform this miraculous act !

Baluncore
2019 Award
It will depend on the degree of ionisation. If the electron is stripped completely from a travelling molecule, the electron will probably continue to travel near the molecule until it encounters another molecule. That distance will depend on the mean free path.

Consider the effect of UV from the Sun, on the upper atmosphere or ionosphere of the Earth.

Staff Emeritus
2019 Award
will the electron remain at the place of ionization, or it will travel at the same speed along with the resulted ion?
You do realize those are not your only two choices?

Klystron and berkeman
berkeman
Mentor
Hello! If I have an atom traveling with a given velocity and I ionize it in flight, will the electron remain at the place of ionization, or it will travel at the same speed along with the resulted ion? Thank you!
You do realize those are not your only two choices?
@kelly0303 -- Are you doing this on Earth? In a vacuum or in the atmosphere? If in a vacuum, say the two stay close together at first, what effect would the Earth's magnetic field have on those oppositely charged particles?

Klystron
sophiecentaur
Gold Member
Whatever makes the ionisation happen, it will have Momentum and Momentum is always conserved. If the ionisation is caused by a photon of EM radiation (high enough energy, of course) or a fast particle, then the momentum of the photon or particle is added to the system so you have a new momentum which is shared between the electron and the ion and the slowed-up other particle (assuming all the photon energy goes into the ionisation process). It's not too hard to calculate if you can do simple mechanical collision calculations.

There is one important consideration here. If the photon (or whatever hit the atom) has only a small amount of surplus energy then the electron and ion may still be attracted to one another and recombine eventually - releasing another photon. It's just like the situation with space launches. If the projectile's velocity (relates to Kinetic Energy( is less than the escape velocity (relates to the Potential energy needed to escape) then it will fall back. More and it will keep on going away.

PeroK
Homework Helper
Gold Member
Hello! If I have an atom traveling with a given velocity and I ionize it in flight, will the electron remain at the place of ionization, or it will travel at the same speed along with the resulted ion? Thank you!
An electron being at rest depends on your reference frame. What you ask is, therefore, physically meaningless.

An electron being at rest depends on your reference frame. What you ask is, therefore, physically meaningless.
"Rest" is a concept dependant on frame of reference. There is no such thing as an object with energy at rest, only rest relative to other objects.

So, by observing it at ANY frame of reference that would produce "restful" behavior, you have achieved a restful state to experiment on, even though it is not ACTUALLY at rest. It's why relativity physics is so important. Under the correct frame of reference, one can create a control frame and call it rest, just as we say today is the present, even though it would be the future to a previous time frame and past to a later time frame.

Or how we can take the position of ANY object, number its location as 0 on an x-y-z graph and make calculations based off of it, even though its position is CERTAINLY not actually zero, we just "create" a rest frame and assign it the initial position of zero.

So, the asked question, just because its meaning is not readily understood or sensible, is not physically meaningless. Incomplete, perhaps, but not meaningless. It is only meaningless to any that wish to view it as such and ignore what meaning may be there. Even "wrong" ideas have meaning, even if it doesn't verify an idea or it's not the meaning expected.

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berkeman and PeroK
hutchphd
An electron being at rest depends on your reference frame. What you ask is, therefore, physically meaningless.
I do not understand your comment. The question is quite carefully framed and the references to motion obviously refer to the lab frame. It seems unnecessarilly didactic to me.
Doe the electron "remains at the place of ionization" have an ambiguity I am not seeing?

PeroK
Homework Helper
Gold Member
I do not understand your comment. The question is quite carefully framed and the references to motion obviously refer to the lab frame. It seems unnecessarilly didactic to me.
Doe the electron "remains at the place of ionization" have an ambiguity I am not seeing?
If the original atom were at rest in the lab frame, then it would make some sense. But, you can't have a process that depends on the lab frame. The original velocity is arbitrary.

In lab A the atom has velocity ##v_A## and the electron has velocity ##0##.

In lab B the atom has velocity ##v_B## and the electron has velocity ##0##.

The two experiments are identical, only the lab frame varies and yet the electron must be at rest in both frames?

That makes no physical sense to me

hutchphd
I now see your point. Thank you much.
So as usual this would be easiest to consider in the center of momentum frame, and the requirements of energy conservation (including the ionization) and momentum conservation would select for a particular energy photon in that frame and the electron momentum will similarly be fixed (and almost certainly nonzero)

If the original atom were at rest in the lab frame, then it would make some sense. But, you can't have a process that depends on the lab frame. The original velocity is arbitrary.

In lab A the atom has velocity ##v_A## and the electron has velocity ##0##.

In lab B the atom has velocity ##v_B## and the electron has velocity ##0##.

The two experiments are identical, only the lab frame varies and yet the electron must be at rest in both frames?

That makes no physical sense to me
If the atom has a velocity of vA in one frame and the electron has velocity zero, then wouldn't the velocity of the electron also be vA if the atom were at rest (v = 0)? So the same would be true of lab B, and velocity vB - it would have a velocity of 0 while the electron would have the velocity vB (relative to the atom), then the electron's velocity would be the difference of vA and vB, correct? Or am I doing the math wrong? Or is it not possible to use the atom as the frame of reference instead of the electron in a workable manner?