# Effect of a magnetic field on cathode rays in a cathode ray tube

My question is regarding the direction in which cathode rays bend in a magnetic field.

My book states that :

When only electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A. Similarly when only magnetic field is applied , the electron strikes the cathode ray tube at point C.

I tried to apply the Fleming's Left hand rule used to find the direction of force on a current carrying wire where the direction of current is taken opposite to the direction of flow of electrons.

The Fleming's Left hand rule states that :

Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Applying this rule I found that the electrons should hit the cathode ray tube at A in a magnetic field . But in the book it is given that they strike it at C.

Edit : I am adding a picture which makes it clear how I applied the rule. Please explain me where I went wrong . Thank you.

• Cross posting on Chemistry SE – Alchimista Apr 15 at 8:11
• It is just to signal that answers can be in another section of SE. I have no idea if the same Q is welcome as a multiple one. Both approaches have pros and cons. – Alchimista Apr 15 at 8:21
• Yeah, you're not wrong, following the rule, the electrons should be also at A when magnetic field is on! – rnels12 Apr 16 at 11:36
• @user8718165 The link you have provied show the same problem. But there also no conclusion has been reached as to why do the electrons deviate in downward direction when all the rules we use give opposite deviation. – Ashok Sharma Apr 17 at 6:57
• see this courses.lumenlearning.com/austincc-physics2/chapter/… the image just after the discharge tube. Its a pretty reputable site and you can be sure that you are correct (in this situation). You may remove some of your previous comments as they are making this section messy. – user8718165 Apr 17 at 11:05

The rule applied is correct but because electron is negatively charged your actual answer will be exactly opposite of what you would get. The answer that you got is correct for a positively charged particle.

• Sir , I have edited my question and it is now very clear that the rule can be used for cathode rays and the direction of deflection of the rays is indeed opposite from what the rule predicts. – Ashok Sharma Apr 17 at 7:44
• This answer is wrong. The quoted textbook is incorrect. – Ben Crowell Oct 3 at 23:26

You're right, and your book is wrong. The magnetic field is coming out of the page. The velocity vector of the electrons is to the right. The magnetic force acting on the electrons is $$-e\textbf{v}\times\textbf{B}$$, which is up. The is contrary to the information stated in the quote in the book. The deflection should be up.

I believe the force by Fleming rule is correct for POSITIVE charges, while electrons are negative and suffer force in opposite direction.

So, direction of current opposite than electrons movement; and magnetic force on electron opposite than Fleming rule.!

otherwise we can use the left hand with opposite direction, like in photo ;)

• This answer is wrong. The quoted textbook is incorrect. – Ben Crowell Oct 3 at 23:26

Contrary to everything you have learned about the so called “cathode rays”, I assert something completely different about them. Please look at this drawing:

This is a kind of cathode ray tube (CRT), also called Braun tube, which can be found in every CRT TV, monitor or oscilloscope. On the left side of the tube is the negative electrode (the cathode) and a little to the right is the positive (the anode), which is in the form of a metal disk with a small hole in the middle. To the right of the anode there are two additional electrodes which, when connected to a high DC voltage, deflect the beam from its straight line upwards to the positive electrode. The beam itself is actually invisible, but is made visible by adding a small amount of some inert gas into the tube (neon, argon etc.).

The contemporary physics asserts that this is a beam of negative particles, called electrons, traveling from the cathode through the anode and then hitting the opposite wall of the tube. I assert that this so-called “beam” is actually an electromagnetic vortex (EM-tornado).

(a real image of a CRT taken from the Youtube video)

What the author of this answer regards as contradictory in the assertion of moving negative electricity from the cathode through the anode to the opposite wall are two things. The first is of principle nature: one of the basic principles of nature is that the movement is always from the positive to the negative and not contrariwise (please read Why is the charge naming convention wrong?). The second is a matter of fact. Let us examine the nature of electricity around the right part of the tube in the drawing above, in other words, let us examine it in front of the screen of a CRT television, monitor, or oscilloscope - we will always find that the detector shows intense positive electricity.

That it is impossible, negative electricity to travel towards the screen and on its other side to appear positive electricity, shows the following experiment: we electrify a vinyl (gramophone) plate by rubbing it (as we know it is negatively electrified) and place it behind a big glass window. Then we test the nature of the electricity on the other side of the window. The detector shows presence of negative electricity just as it would have indicated without the glass. Glass does not change the nature of electricity on the other side.

Before we present our explanation of this phenomenon, let us consider a few more experiments. We place a stiff copper wire on a table. Parts of its length don’t touch the table. Above a wire section that does not touch the table we hold a strong cylindrical magnet with its positive pole down, so that the wire lies exactly under the middle of the magnet. Then we connect a new battery to the ends of the wire so that the positive pole is closer to us and the negative pole further away from us. At the moment of connection we will notice that the wire makes a strong deflection to the left and up. As soon as we turn the magnet over and repeat the same, the wire will make a strong deflection to the right and up. If we hold the magnet again with the positive pole down, now not directly over the wire, but left over it, however still close to it, we will notice that the wire after connecting to the battery makes a jerky movement to the right and down. How is this explained? In the first variant, the permanent magnet “blows” down; the magnetic wind in and around the wire blows clockwise spirally from the plus to the minus pole of the battery; it blows down on the right of the wire, up on the left of it; on the right of the wire both magnetic winds coincide (the effect intensifies), and on the left of the wire they collide (the effect weakens); the wire moves to where the effect only intensifies, namely to the maximum, and that is to the left and up. In the third variant, in which both winds only collide, the wire deflects to where the adverse effect is maximally attenuated or quite ceased, namely to the right and down.

Now, facing a CRT oscilloscope, we let its beam run slowly and uniformly from left to right (visible as a bright dot moving horizontally from left to right in the middle of the screen); then, exactly over the center of the screen, we place a magnet with its plus-pole down. We will notice that the dot moves no longer horizontally, but that it slopes downwards and passes through the center. When we turn the magnet upside down, the dot slopes upwards, passing through the center again. If we compare this observation with what we have just said about the experiment with the copper wire and the magnet, we find the same thing happening in both cases. We conclude that the rotational direction of the magnetic wind generated by the beam in the oscilloscope coincides with that of the wire, as long as the positive pole of the battery is closer to us. So it's also the oscilloscope's plus side closer to us when we stand in front of it.

[ The (+)pole of the magnet points downwards, the beam of the oscilloscope approaches it from the left. On the right side of the beam its magnetic wind blows down, i.e. both winds match; so, the beam is shifted upwards. When it goes to the right side of the screen, also on the right side of the magnet, then their winds collide, so the beam is shifted downwards.]

We explain this phenomenon as follows: the positive electricity radiating from the anode spreads to the right into the broader part of the Braun tube in the drawing above. Since the anode is a disc with a circular hole in the middle, this electricity, with the help of the suctioning minus cathode on the other side of the anode, forms a vortex which is directed to the opening of the anode and continues to the cathode. This electromagnetic tornado is actually the beam that is visible when a small amount of an inert gas is introduced into the tube.

So when we stand in front of an oscilloscope and the bright dot lies still in the center of the screen, then it flows in the tube around the bright dot invisible positive swirling electricity towards us and from the very dot begins a vortex in the opposite direction towards the hole of the anode and onward to the cathode. The bright dot is actually the top of this EM-tornado. (Even with toys that cause a vortex in a water-filled container by means of a small electric motor located at the bottom, it can be noticed that the movement of the water around the vortex is directed upwards, but in the vortex downwards).

The fact that the vortex is deflected to the positive of the two additional electrodes does not contradict this explanation, because I assert that this is not something that can be simply accommodated under the postulate “plus attracts minus”, but rather a positioning of a motion consistent with ambient influences whereby maximum effects are achieved (we could observe something similar in the previous experiment, where the wire was deflected to the left and up while the magnet with its plus pole was positioned over it). For the effect of the vortex to reach the maximum, it is deflected to the positive electrode when additional electrodes are inserted in the tube.

In the above-mentioned toys, the water vortex is fully upright when the electric motor is positioned right in the middle of the bottom of a cylindrical or slightly conical vessel. However, when the motor is displaced to one side of the vessel, the vortex is curved towards the opposite side. In this way, it strives to achieve the maximum effect, in this case to capture the largest possible amount of water and make it spin (YouTube video Discovery Kids Tornado Lab extreme weather toys.Real Tornado Sound Effects). In our case the electromagnetic vortex makes a curve to the positive electrode; so it seeks to capture and spin the largest possible amount of positive electricity.

[ When an air-tornado inclines to one side, then it does it to the side where the air-pressure is higher. Higher air-pressure means more air, so it strives to capture more air, make it spin and thus stay alive. If we imagine the positive electricity as a higher pressure, the negative electricity as a lower pressure, then the EM-tornado inclines of course to the higher pressure, i.e., to the positive of the additional electrodes. ]

It can also be assumed that a non-symmetrical conical glass tube would make the vortex curved even without the additional electrified electrodes (drawing below).

[please see also (in slow motion, let’s say 0.25 of the normal speed) how the vortex gradually position itself in the middle of the bottle in this YouTube video from 2:03 to 2:05].

https://youtu.be/5-Bco8KRpmU?t=2m3s

Another detail indicating that this is a kind of vortex is the shape that the bright dot takes when turning off the oscilloscope. It “dissolves” circularly. Something similar is also notic­eable on the water surface of the mentioned toy after switching off the electric motor.

[ With the air- or the water-tornado, the force of the gravity is pulling the vortex down. With the electricity the suctioning minus cathode takes the role of the gravity, that is, the role of pulling the EM-vortex “down”.]

I don’t know whether you understand what this answer means if it is true. It means that the whole particle physics is questioned, because the CRT is the founding stone of the whole modern particle physics.

I am ready to advocate its truthfulness in front of any audience.

Source: https://newtheories.info

• This presents an alternative theory about what cathode rays are. But it doesn't answer the question. – Thomas Fritsch Oct 3 at 20:58
• I think it answers the question in the part of the answer where I speak about the magnet above the oscilloscope screen. – Mitko Gorgiev Oct 3 at 21:05
• This is a site for questions and answers within mainstream physics, based on mainstream accepted physical models. Answering to promote your own non-mainstream theories is not appropriate on this site. Answering based on non-mainstream physics doesn't really help people here. – JMac Oct 4 at 0:13
• In my view, the only important thing is whether there is truth or not in an answer. I don't philosophize, but I speak with experimental facts which are very easily verifiable. The experiments are the only thing that matter, nothing else. Everyone can prove them and if the results are true, then it throws undoubtedly a new light on the phenomena. – Mitko Gorgiev Oct 4 at 6:44
• I see that you want to kick me out of here. Surely you can kick me out of StackExchange, but also surely you cannot kick out the truth. It will find its way, sooner or later. – Mitko Gorgiev Oct 4 at 10:07