Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

CRO, Magnet & Left hand Rule

  1. Jun 8, 2012 #1
    When I placed the south end of a magnet on the right hand side of the green CRO spot (when timebase is off) facing towards the left hand side of the screen, I expected the spot to move downwards in accordance with the left hand rule, but instead it moves upwards. The spot also moves down when I hold the same magnet on the left hand side with south facing to the right, when it ought to go up.

    I have checked the magnet and the pole is definitely south, and I have also tested other magnets and get the same result.

    By the left hand rule in the first case above, the field is left to right across the screen into the south pole of the magnet. The current is into the screen, opposite the flow of electrons in the beam towards the screen, so the force ought to be down.

    Can anyone explain what might be happening?
     
  2. jcsd
  3. Jun 8, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    How did you check that the south pole "was definitely south"?

    The only other explanation is that the CRO is using positively charged particles.
     
  4. Jun 8, 2012 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This is the wrong way round. The electron flow is towards the screen so the conventional current is back towards the electron gun. There's nothing wrong with the LH Rule! :wink:
     
  5. Jun 8, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    That's the first thing I thought of ... however: OP notes current opposite direction of electron flow. It appears to be a difficult mistake to make to imagine the electrons originating on the screen and heading towards the electron gun. "Into the screen", as you normally look at it, would be towards the electron gun.

    Reality check:
    If I am facing the CRO screen, the electrons are being fired towards me, then the current is going away from me, and the magnetic field goes left-to-right (towards the south pole placed to my right) so, with my left hand, I point my second finger away from me, and my first finger to my right, which points my thumb downwards...
     
  6. Jun 8, 2012 #5

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    That's ridiculous.
     
  7. Jun 8, 2012 #6

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    Left hand rule, eh? I had to look that up, it's kind of weird that you are using a LHR in a world full of right hand rules.

    If I understand your description, the bar magnet is in front of the CRT screen with the South pole just right of center (and the North pole far to the right of center)? Look at a diagram of the field from a bar magnet. Well back from the screen where the e beam is traveling, the horizontal component of field is pointing from right to left and not the other way. This explains the observed deflection.
     
  8. Jun 8, 2012 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The Left Hand Motor rule. At School, we learned that Motors drive on the Left (UK School!!!) and the other rule is the GeneRIGHTer rule!. Can't go wrong with that.

    The electron flow thing causes so much trouble because of this obsession with explaining everything electrical in terms of electrons. I realise it's essential that kids should be told about electrons but so many teachers are, themselves, struggling with the direction thing that they often manage to confuse students. I'm sure this is because they give snap answers to a kid's question and either the question or the answer gets mis-interpreted - bad news. I know that 'charge' is a bit of an abstract concept but it certainly avoids a lot of pitfalls. - The Left Hend Rule being a prime victim. If, as soon as we consider an electron beam, we say, out loud, "and that means current in the oppositie direction!!!!" then we can avoid the confusion.
     
  9. Jun 8, 2012 #8

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    Certainly a left hand rule can be made to work. It's just hard to adjust for a physicist who's been right-handed his whole life!
     
  10. Jun 8, 2012 #9
    "This is the wrong way round. The electron flow is towards the screen so the conventional current is back towards the electron gun. There's nothing wrong with the LH Rule!"

    When I said "into the screen" I meant back towards the electron gun (bad choice of words), i.e opposite the flow of electrons.

    I don't doubt the left hand rule, I have demonstrated it many times. I just can't account for the seemingly anomalous CRO spot motion.

    I checked the magnetic pole with a plotting compass - needle points into the south. Again, I used more than one magnet with the same result.

    Re: particles are not electrons - some other positive particles. A possibility, but nothing I have ever heard of.

    I was suspecting that perhaps it had something to do with the phosphorus atoms that emit the light rather than the electrons. Afterall, it is not the electrons that are seen, but the light emitted by the phosphorus (or other) coating once it has absorbed the KE of the electrons.

    I'd be interested to see if anyone else with a CRO gets a similar result. Timebase off, magnet coming from the side ( ie. perpedicular to the beam).
     
  11. Jun 8, 2012 #10
    I never knew there was a left hand rule. I just use the right hand rule.
     
  12. Jun 8, 2012 #11

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    You options are misidentifying the magnetic pole (see below), misidentifying the charges (unlikely, see below), or failing to describe the setup completely.

    If you can - try making a video.

    Did you check the compass (does it point north?) Also check the manufacturers mark on the magnet and/or try with an electromagnet. But you are correct - the north-seeking pole of the magnet points to the magnetic south pole.

    The rays don't have to be cathode rays ... you can, in principle, set up a tube using positive ions instead. However - I imagine it would look somewhat different to a regular CRT (at the very least it would be written on the label) so it does not look like an easy mistake. Although... it is the sort of trick I am known to pull on students. People get so complacent about anything electric being about negative charges.

    Are you using an actual oscilloscope screen for this or a tube mounted for the purpose?

    The phosphor is affected by the magnet but not very much and not in a way that would shift the spot by several centimeters.

    This sort of experiment is routine in teaching laboratories. If a result like this had been found before, then it would be all over the internet.
     
  13. Jun 9, 2012 #12
    The compass is working fine, I have several plotting compasses and they all point the same direction. I'm based in Hong Kong and the compasses point to China, so I'm fairly confident its not due to a mistake in identifiing the pole. Besides, the same magnet produces the correct expected motion in a current through wire experiment, following the left hand rule.

    The motion of the CRO spot is not several centimetres but several mm - not quite one vertical division. So it may be the phosphorus atoms moving as suggested if there is a known effect on them (assuming the screen coating is phosphorus that is).

    It shouldn't be the beam charge that's the reason, as the beam responds as expected to an applied external voltage to the Y-plates. I can even get the negative external applied voltage to move the spot down and have the magnet move the spot up at the same time.

    I'll see if I can do a video. I'm asking the manufacturer, see if they can explain it. My latest guess, is that CRO uses magnetic fields instead of plates to deflect the beam, and the external field is interfering with the internal field(s).

    Here's the CRO model
    http://www.gwinstek.com/en/product/productdetail.aspx?pid=3&mid=6&id=69 [Broken]
     
    Last edited by a moderator: May 6, 2017
  14. Jun 9, 2012 #13

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Even several mm is too big for an effect on the phosphor... you can try with a stronger magnet.

    Since you cannot see inside the box, and you can be sure the magnet is well behaved, the best bet is "something else messing with the field". There can be all kinds of things inside that box.

    When this is done in a teaching lab, the CRT is specially mounted to make the workings obvious. This kind of transparency is important in experiments - you could try opening the case and examining the works?

    You say the beam responds as expected to an applied external voltage to the y-plates? So you have looked inside the box to make sure that the y-plates are as you'd expect?

    Asking the manufacturer is the sane way to go about this though.
    Let us know what you discover.

    [edit] without opening the case, the RHS is difficult to get to ... to deflect the beam you have to have the beam intercept the feild - applying to the outside of the screen won't do that. Try applying the N pole to the LHS with the case removed.
     
    Last edited: Jun 9, 2012
  15. Jun 9, 2012 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I think this is getting out of hand. A CRT has an electron gun in it. What other beam could you possibly use in a real world? Electrons are light and easy to accelerate etc. etc..
    If you are getting an apparently strange result then look at your interpretation of what you see. The conventional current is flowing From the screen and the motor rule WILL apply. Secondary emission from the screen is not actually impossible but the design will discourage it as it would wreck the spot size.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: CRO, Magnet & Left hand Rule
  1. Left hand rule? (Replies: 4)

  2. Left Hand Rule (Replies: 5)

Loading...