Teaching Guidance for 14-16
When a piece of metal is heated, electrons escape from its surface. These free electrons can be accelerated in a vacuum, producing a beam. The hot metal surface and the accelerating plates are sometimes called an ‘electron gun’.
In an electron gun, the metal plate is heated by a small filament wire connected to a low voltage. Some electrons (the conduction electrons) are free to move in the metal – they are not bound to ions in the lattice. As the lattice is heated, the electrons gain kinetic energy. Some of them gain enough kinetic energy to escape from the metal surface. We sometimes say that they are ‘boiled off’ the surface or ‘evaporate’ from it. Although they do not form a gas in the strictest sense, these are good descriptions.
If the hot metal plate is in a vacuum, then the evaporated electrons are free to move. The electrons can be pulled away from the hot surface of the plate by putting a positive electrode (anode) nearby. The anode is created by connecting an electrode to the positive terminal of a power supply, and the hot plate is connected to its negative terminal. The hot plate is then the cathode.
As soon as the electrons evaporate from the surface of the hot plate, they are pulled towards the anode. They accelerate and crash into the anode. However, if there is a small hole in the anode, some electrons will pass through, forming a beam of electrons that came from the cathode – or a cathode ray.
This cathode ray can be focused and deflected and can carry small currents. This is the basis of the important experiments carried out by J J Thomson and others.
It is also the basis of early electronic devices.
You could explain the operation of an electron gun thus:
- At one end of the tube there is a little rocket shaped
gun. In that gun a starting plate is heated by a tiny electric grill. The plate has a special surface that lets electrons loose rather easily. Electrons come off that plate. They are speeded up in the gun by a large potential difference between that starting plate (‘negative cathode’) and the gun muzzle (‘positive anode’)._
- Electrons come out at high speed through a tiny hole in the cone-shaped muzzle.
- The electrons continue at that constant speed through the vacuum because there is nothing for them to collide with - until they hit a fluorescent screen, where they make a bright spot.
- The glass globe of the tube has been pumped out to a very good vacuum, removing air which would soon slow down electrons by collisions. But then a very little helium (or hydrogen) gas is let in. Because the helium atoms give out a green glow when hit by electron, you can see the path of the electrons made visible as a thin line of glow. (Hydrogen glows blue.)
- Look at the thin glowing line carefully. You are seeing the path of electrons flying through thin helium (or hydrogen), almost a vacuum, all by themselves, with no wires there.
The fine beam tube is improved by adding a small conical electrode – often connected to the anode. This produces a converging electric field which focuses the electrons and produces a tighter beam and sharper spot on the fluorescent screen.