Physics Teachers Forum

This is my second year teaching Physics at a new school and I am slowly getting familiar with all the lab equipment. We have a number of Cathode Ray Tubes and have had on and off success in demonstrating the different properties of the Cathode Rays. One demonstration in particular is the deflection of the Cathode Rays by an Electric Field. I have tried different methods of connecting an electric field (I assume it needs a high voltage) but the induction coils that we have are a bit temperamental. Any ideas on how this demonstration can be improved?

Peter,
I think you need a DC high voltage power supply. Induction coils are okay but can be temperamental - even though traditionally people have used induction coils to look at cathode ray tubes. .

There are a couple of Australian companies that sell power supplies that go to 6000 V DC. Unfortunately the price is just over $400.

guys, I am teaching yr12 physics for the first time and have no other physics teAchers at my school... Am just wondering, I have purchased the Maltese cross, paddle wheel etc.. WhAt voltage do these need to be run off? I have a rhumkoff coil, but no other Induction coils.. Are Maltese cross etc run off DC or AC? I'd imagine it would be DC, so really the induction coil wouldn't work.. I have never tested this, but are typical school power supplies true DC or just half wave rectification? N other words.. Do I need a high voltage DC source? Cheers

C W,
All of the cathode ray tube experiments require high voltage DC. For example you only want electrons striking the paddle wheel in one direction in order to move it. There won't be any net momentum given to the wheel if you operate in AC. To see the shadow of the Maltese Cross on the glass, the electrons have to move toward the Cross - those that don't strike the Cross will then move onto the glass, thus forming the shadow. Again, this requires DC since you only want the electrons going in one direction. Same for all the other cathode ray tubes.

Even though induction coils pulsate they should output high voltage pulses of one sign (mostly) if you connected their input to a DC power supply. Historically, cathode ray tube experiments were done with induction coils. I personally would recommend schools invest in high voltage power supplies since in this day and age we don't really need induction coils. Plus high voltage power supplies can supply more current than an induction coil so you can get clearer results.

Are you connecting the input of the induction coil to a DC power supply?

Most school power supplies will either be half-wave or full-wave rectified power supplies, smoothed with capacitance. This is as simple as these can be made to keep the price down. They should be reasonably close to DC but keep in mind the more current you draw out of them, the greater the ripple in the voltage they will develop. But this should still be fine for most if not all applications for the syllabus since the output voltage is still one polarity (i.e. one sign).

Hi Joe (and others),

I am just wondering with an electron being deflected in a magnetic field, let's say that the electron could complete full circular revolutions, would it ever lose energy and spiral inwards?

Ifso, why? :)

I was asked this question by a student and I am actually unsure of how to put the answer simply without going into inverse compton scattering?!?!

I tried explaining via mass dilation amongst other things, dont think i got my point across though, as i was more confused by the end of my rant than the students i think... Help? :)

Jay,
yes an electron made to travel in a circle, say by letting it enter at right angles to a magnetic field, will lose energy and spiral down to a point - eventually. The reason is that all accelerating charges lose energy by emitting electromagnetic radiation. From conservation of energy, the loss in kinetic energy of the electron goes into the emitted radiation. We make use of this effect in many ways, such as oscillating charges in transmitting antennas, which could be that in your mobile phone or the transmitting antenna of a radio station. In these cases, the electrons are moving back and forth along the antenna, thus accelerating, and therefore emitting radiation.

Similarly, circular motion involves accelerations towards the centre of the circle. Even if the speed of the particle is made constant, the fact that the direction is changing means the velocity is changing and therefore there is acceleration. As a result, there will be emission of radiation from a circulating electron charge. This goes by many names, depending on the application, but is generally known as synchrotron radiation or cyclotron radiation. A web search on these two concepts should bring up plenty of examples. The end result is that if the electron is left alone and allowed to just circulate, then it will lose energy due to radiation and spiral to an ever smaller circle. Generally, people apply energy into circulating charges so that this does not happen, such as in a cyclotron. In fact, in this case, if you apply more energy to it than it is losing then it will spiral outwards with ever increasing kinetic energy and an ever increasing circle.