# Interaction of charged particles with a conducting or insulating surface

If I have a charged particle floating in a vacuum, and it strikes a conductor or insulator on its way, what would happen? Would the electron be taken into the conductor? or would it just bounce off the surface. What about the insulator and why? And I think the particles kinetic energy does play a role in this. Thanks.

EDIT: In this question, by floating, I meant that the charged particle is not bound by any nucleus and has no other forces acting on it except for the force due to the conductor or Insulator.

and what I personally thought was that when the charged particle hits the conductor, it will get absorbed because of the induced negative charge, but for the Insulator, I don't what will happen. I am thinking since charges are almost immobile in an insulator, It won't be able to accept the charged particle unless it is of the opposite charge of the particle itself. So, maybe in all other cases, it would result in some sort of bouncing off effect. I don't know how, though. But when the particle has high enough energy, particle physics might play a role. This is where I need help and at what energies does particle physics come into play?

• It seems you have thought about this. What do you think happens and why? Jan 24, 2017 at 6:59
• This is an open-ended question with the OP showing no effort as to how it might be answered. What does the word "floating" mean in this context? Jan 24, 2017 at 6:59
• @Farcher : The OP has narrowed the options down to a choice of 2, so I don't think the question is too broad. "Insufficient effort" might be justified though. I think that "floating" must mean the absence of other forces such as gravity. Jan 24, 2017 at 7:04
• @sammygerbil Thank you. Your definition of floating helps in that if the electron had a lot of energy it could produce an X-ray photon. Jan 24, 2017 at 7:08
• @Farcher : Well that's not what I had in mind! I think the OP is asking about the electrostatic interaction, the possibility of the electron being attracted into the bulk material or repelled by it, rather than any electromagnetic interaction or any interaction with individual atoms. Jan 24, 2017 at 7:14

The lower the energy (say less that a few Volts), the higher the likelihood the charged particle will stick to the material (conductor or insulator) and simply cannot escape. If you have an insulator, then this sticking process eventually stops because charge particles stuck in the material will repel future particles of the same charge.

At higher kinetic energies (hundreds of volts and above), there are a bunch more processes that can happen, including:

1. Reflection
2. Absorption
3. Ripping new charged particles from the material (secondary electrons)
4. Generation of x-rays and visible light
5. Sputtering of atoms

You can use any of these processes to make an image, which is what is done in scanning electron microscopes.

• Thanks for the nice answer. May I ask where did you get this picture from? If it is coming from a book can you please share its title? May 21, 2020 at 2:49
• You will find similar images in almost any textbook on scanning electron microscopy. For this specific one in the answer above, take a look here thermofisher.com/blog/microscopy/… May 21, 2020 at 3:10

If the conductor is not charged, the floating charge will induce opposite charges on the surface of the conductor. It will be attracted towards it and therefore will be absorbed by the conductor.

If the conductor already carries a charge of the opposite sign it will of course be attracted, but if the charge is the same sign then it is more difficult to say whether the floating charge is attracted or repelled. The attractive force between the floating and induced charges is proportional to $\frac{1}{r^3}$, whereas the repulsive force between like charges is proportional to $\frac{1}{r^2}$, so if the floating charge has no KE and is sufficiently far from the conductor it will be repelled. The kinetic energy might be sufficient to overcome the net repulsive force.

The floating charge will also polarise an insulator to a much lesser degree, so if the insulator is not charged with the same sign the floating charge will be attracted and absorbed.