What is an electron?
An electron is a subatomic particle carrying a fixed negative charge of e = 1.6 × 10⁻¹⁹ C and a very small mass of 9.11 × 10⁻³¹ kg. In a metal, some electrons are free to move; when the metal is heated strongly they gain enough energy to escape from the surface. This release of electrons from a hot metal is called thermionic emission.
Producing and controlling electrons
In a cathode-ray tube, a heated filament (the cathode) emits electrons by thermionic emission. A high positive voltage on the anode accelerates them into a fast, narrow beam called a cathode ray. Because electrons are charged, the beam can be deflected by an electric field (towards the positive plate) or by a magnetic field. When the beam strikes a fluorescent screen it produces a bright spot of light.
Key formula / 关键公式
Work done by the accelerating voltage becomes kinetic energy:
eV = ½mv²
where e = charge, V = accelerating voltage, m = mass, v = speed.
The greater the accelerating voltage, the faster the electrons travel and the greater their kinetic energy.
Worked example / 例题
An electron is accelerated from rest through a potential difference of 500 V. Find its kinetic energy.
KE = eV = (1.6 × 10⁻¹⁹ C)(500 V) = 8.0 × 10⁻¹⁷ J.
Remember / 记住
- Thermionic emission needs a hot cathode.
- Electron charge is negative, so the beam bends towards the positive plate.
- Higher accelerating voltage means faster electrons and more kinetic energy.