Cathode rays
When a metal is heated strongly, some of its free electrons gain enough energy to escape from the surface. This is thermionic emission. In a vacuum tube a heated filament (cathode) emits electrons. A high voltage between the cathode and an anode accelerates them in a narrow beam called cathode rays. A vacuum is needed so that the electrons do not collide with air molecules.
Choose the accelerating voltage. Find the speed of the electrons and compare it with the speed of light (3 × 10⁸ m/s).
| Property of cathode rays | Evidence |
|---|---|
| They travel in straight lines | Sharp shadows of objects in their path |
| They carry negative charge | They are deflected towards a positive plate |
| They are deflected by magnetic fields | The beam bends in a magnet’s field |
| They carry energy and momentum | A paddle wheel in the beam turns |
| They make fluorescent materials glow | The screen of a tube glows |
The cathode ray tube (CRT)
In an oscilloscope or old television, electrons from the heated filament are accelerated by the anode and form a beam. Plates (or coils) deflect the beam up and down (Y plates) and sideways (X plates). The beam hits a fluorescent screen and makes a spot of light. The oscilloscope shows how a voltage varies with time: the time base sweeps the spot across the screen at a steady rate while the signal moves it up and down.
X-rays
When fast electrons hit a metal target such as tungsten, they are suddenly stopped and give out X-rays. X-rays are high-frequency electromagnetic waves with very short wavelengths. A higher accelerating voltage gives more energetic, more penetrating X-rays (hard X-rays); a greater filament current gives more electrons and so a more intense beam. Most of the electrons’ energy becomes heat, so the target is cooled.
| Use of X-rays | Reason |
|---|---|
| Medical imaging (bones, teeth) | Bone absorbs more X-rays than flesh, so it casts a shadow on film |
| Cancer treatment | High-energy beams kill cancer cells |
| Airport baggage scanners | Different materials absorb different amounts |
| Finding cracks in metals | X-ray images show hidden flaws |
| Studying crystals | X-ray diffraction shows how atoms are arranged |
Key terms
- Thermionic emission
- The release of electrons from a heated metal.
- Cathode rays
- A beam of electrons in a vacuum tube.
- X-rays
- Short-wavelength electromagnetic waves made when fast electrons hit a metal target.
- Oscilloscope
- An instrument that displays how a voltage changes with time.
Summary
- Heating a metal releases electrons by thermionic emission; a high voltage accelerates them as cathode rays.
- eV = ½mv²; the beam is deflected by electric and magnetic fields.
- A CRT or oscilloscope uses deflection plates and a fluorescent screen.
- Fast electrons stopped by a metal target produce X-rays; use them with care.