1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electron Deposition and Object Deflection

  1. Jul 4, 2012 #1
    A Big Problem:

    (a) sub-problem 1:
    Assuming that a radio transmitter emits a radial pulse 'K' times a second, what would that frequency need to be in order to detect the doppler shift of an object traveling a 6000 m/s?

    (b) sub-problem 2:
    If one had a cathode ray source which was outputting an average of N electrons per second in a cone; how much charge over time could be deposited onto a 4 gram, round metal (assume copper, for now) surface which was traveling towards the cathode ray source at 920 m/s and is distance 'r' away from the cathode ray?

    (c) sub-problem 3 (this one is easy):
    Given that an electric field is generated from a source some distance below the cathode ray, knowing the charge that can be generated on the surface within a certain time period; how intense should it be in order to deflect the metal object at least some distance 'd' above the cathode ray source?

    (d) overarching problem (not so easy):
    Assuming a system was created to detect a metal object (using information from part a), spray the object with electrons to give it a charge, then given a small acceleration from a potential to alter the trajectory of the object away from or around the system; how quickly could part (a) and (c) be accomplished? (this is an engineering question)

    (e) power problem:
    How much power would a system that ran like this consume? Note that it's assumed that it's outputting a radio signal K times a second and looking for doppler shift above a certain range, when doppler shift is detected above that range it activates the cathode ray in the direction of the object and generates a potential which interacts with the electrons on the metal object.

    Note: Not a homework problem... a truly crazy idea I had which probably isn't even technologically feasible. (It is physically possible!... I think)

    Sub-problem (b) is a scattering problem and probably the main limiting feature to this idea. I'm not sure that enough charge could be deposited in a reasonable enough time frame for this idea to work. Presumably you might be able to control N, the number of electrons output through temperature and type of material, (I really have no clue, this is outside of my area of expertise).

    If you've read up to this point, thanks for humoring me. What do you think of the idea? :)
    Last edited: Jul 4, 2012
  2. jcsd
  3. Jul 5, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Depends on the radio-detector's ability to compare outgoing and in coming frequencies.

    Depends on how "sticky" the sphere is to electrons, the electron flux, and the distribution of the flux within the cone.

    Depends on the charge, mass and speed, as well as the time the field is to apply. Recall though that the same field deflects both cathode rays and the small sphere accumulating negative charge.

    Not enough information.

    Your idea is to detect something, say a projectile of some kind, heading towards you, give it a negative charge so that it will be deflected by an electric field to go around you. You have a lot of unknowns to deal with. Note: your first step has industry solutions - you want a doppler radar to detect position and speed.
  4. Jul 5, 2012 #3
    Thanks for responding to me!

    Sorry, I wasn't clear. The frequency I was referring to was not the frequency of the electromagnetic radiation, I was assuming that frequency would be fixed; something in the radio band. If one is releasing multiple waveforms in succession and counting the rate at which they come back, this information can be used to detect doppler shift.

    This was why I posted in the Solid State part of the forum originally. I assumed that they would be more likely to have the information about what factors influence the 'sticky-ness' of a material and a surface. Many of them deal with deposition of one material onto another kind of material in their experiments all the time.

    Although that problem might be difficult in this application because I don't really know if a 4 gram metal ball has a large enough cross section to be detectable when it's traveling at 920 m/s. You're interpretation of the idea is correct. :)
  5. Jul 5, 2012 #4


    User Avatar
    2017 Award

    Staff: Mentor

    Assuming your object does not fly towards the system in a vacuum, the electron energy has to be significant to reach the object at all. While this is "just" an engineering problem, it will limit the distance of the mechanism.

    There are some upper limits which might be interesting: If the charge gets too large, the electrons are not bound any more. In air, I would expect this to happen at something like 1kV/mm. Using the radius of the object (a sphere is the best case for your device), you can calculate the maximal charge of it.
    For a radius of 1cm, this would be something like 11nC. If the electrons reach the object in a really short timescale, it might be possible to "overcharge" it a bit. As upper estimate, I will use 100nC.

    Now, which force does the object feel? Again, the strongest electric field you can reasonably achieve is of the order of 1MV/m = 1MJ/(C*m). Multiplied with 100nC you get a force of 0.1N. Within 1m (~1ms), you can change its velocity by ~25m/s. This does not help, even if it would have 10 meters with this field.

    What about a magnetic field? The CMS magnet at the LHC achieves 4 Tesla over several meters. Maybe we can get 10 Tesla... this gives a force of 6mN, even less than the electric field.

    If I did not make an error somewhere, this device will not work. The basic physical concept is correct, of course, but even with vast overestimates for the size and capacity of the mechanism it cannot change the velocity or direction of the projectile in a significant way.

    In space, things are a bit different - while the charge is still limited, you can increase the volume of the electric field as far as your science-fiction story requires. However, conventional armor should be cheaper, it can block several projectiles at the same time and it is passive.
  6. Jul 6, 2012 #5


    User Avatar
    Gold Member


    Your thought experiment (sub-problem number one) where “a radio transmitter emits a radial pulse” I assume that you mean the emitted pulse spreads radially since a pulse itself cannot be "radial". Its not clear why that makes any difference, anyway. I also assume that “'K' times a second” means transmitted pulse repetition frequency. Your question does not include the direction of the object’s relative velocity…only its magnitude. Since Doppler shift is a vector quantity, it has both magnitude and direction. So, one must specify if the target is moving towards or away from the transmitter/receiver.

    Your description does not mention the receiver, so I will assume there is one, and that its bandwidth is wide enough to receive the transmitted frequency PLUS the up-doppler (higher) frequency reflected from the moving (closing) target, or the original transmitted frequency MINUS the down-doppler from the moving (opening) target, and that the receiver can discriminate (measure) those differences in frequency.

    Assuming all this, then only one transmitted pulse and the receipt of its reflection from the target would be enough to detect both the target’s range and its closing or opening velocity.

    You have seemingly described a pulse radar, often used in fighter aircraft to detect the range and closing (or opening) velocity of another aircraft.

    What greater problem is this “sub-problem number one” a sub-part of?

    If you are interested in learning more about this subject, please check out this (and the references at the end): http://en.wikipedia.org/wiki/Pulse-Doppler_radar
    Last edited: Jul 6, 2012
  7. Jul 6, 2012 #6
    I was thinking that the separation between the pulses could be used to give a frequency. It's true that a single pulse could be used to detect doppler shift, but do you think that a projectile would be significantly blue shifted to easily detect? I wasn't sure, that's why I thought a pulsed radio signal would be more useful... that's a frequency you can control.

    Well, I was thinking of an active system that would react to whatever was sensed in it's environment. As such, in order for the electron gun (cathode ray source) to know which general direction to fire, the system would require a sense of direction. The issue is that I don't know enough about signal analysis and electrical engineering at the moment, but fortunately, that problem has already been solved. As mfb mentioned, with the numbers he/she gave, the idea probably isn't technologically feasible. Energy requirements to power the thing would be high.

    If one was trying to create a body suit that can deflect bullets, it would probably be bad for the user if such a suit required an electric field high enough to stimulate an electrical breakdown of the air. Heh, you might deflect the bullet, but you're no better off if the suit electrocutes you in the process.

    Admittedly, I was trying to think of a way that one could create a body energy shield that made physical sense, this was what I came up with. If the concept could be used on a large scale, it might be useful for deflecting particulates from spacecraft in the distant future; these days they have stiff energy limitations, so I'm not holding my breath to see this idea implemented in the near future.
  8. Jul 6, 2012 #7


    User Avatar
    2017 Award

    Staff: Mentor


    Well, to protect a human, you could assume lower projectile velocities. 6km/s is much more than any conventional firearm can achieve, and even the experimental railguns (which would be big weapons against armored tanks, not humans) do not reach this yet. But at the same time, the scale of the whole system would have to be smaller, so it does not help.
  9. Jul 6, 2012 #8
    Well, I thought I read that a typical M-16 round can travel at about 920 m/s; that was what I originally had in mind. I mentioned 6 km/s because if you can move that a significant amount, you can easily deal with the other.

    At the very least, the idea would be decent science fiction fodder. :)
  10. Jul 7, 2012 #9

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    You get to control the radio frequency - and combine approaches: eg. in chirp radar.

    Detection is the easy bit - eg. it gets used in some gun clubs to track shots on the range (I've heard).

    The hard part is deflecting the projectile ... you realize that as the projectile becomes charged it will tend to deflect incoming charges?
    Anyway you end up needing something like van-Allen belts around the target and even the Earth-sized ones let some projectiles through.
    Your best protection is still to not be there.
    Last edited: Jul 7, 2012
  11. Jul 7, 2012 #10


    User Avatar
    Gold Member

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook