Science -- Your Future, Scotland's Future
PP4SS Exhibits
Electrons and Magnets
There are two principle physical properties which underpin all of the PP4SS exhibits, and which are introduced here, in this first exhibit:
  • Charged particles are accelerated by electric fields, and
  • their paths are bent by magnetic fields.

As seen in the picture above, a CCTV monitor (displaying images generated by two webcams) is used to display a monochrome image on the surface of the cathode ray tube. This is used to help describe the formation of a cloud of electrons within the heating element of the tube, their acceleration to high speed through an electric field towards the front of the monitor, and their deflection and distribution onto the phosphorescent display by means of magnetic fields.

To illustrate the point further, and to let pupils carry out some experimental work, magnets are supplied (the toroidal one is shown above), which let the pupils experience directly the way in which a magnetic field will deflect the path of charged particles. (The pupils are all told not to try this out on colour CRT televisions, as it would permanently damage the registration of the three colour beams. And in today's litigious society...)

Deflecting electrons with magnetic fields
More advanced pupils (and we expect that this work could be carried out by fifth and sixth years with assistance) could work through problems such as the following:
  • Given the left-hand rule (which could be supplied if necessary), and knowing that electrons are negatively charged, what would happen if a magnet was placed with its north pole facing inwards, on the left side of the monitor?
  • Knowing the above, what is the actual field distribution around the toroidal magnet? Which face is north (assuming that the poles are arranged on the flat faces)?
  • Given the toroid's field distribution, and knowing the left-hand rule, what would you predict should happen if the magnet is placed flat in the centre of the monitor screen?

A simple (and provisional) worksheet (in Word format), used to take pupils through these steps, can be downloaded by clicking here.

As an aside, it's interesting to think that there will be pupils who have never seen a television or computer monitor driven by a cathode ray tube, in which case this exhibit carries some historical significance!