Electron Emission 🚀
1 Electrons — tiny charge carriers
J. J. Thomson’s cathode-ray work showed that every metal throws up the same particle: the electron. Experiments measured its charge-to-mass ratio as
\( \dfrac{e}{m}=1.76\times10^{11}\,\text{C kg}^{-1}\) and its speed as ≈ 0.1–0.2
Robert Millikan later pinned down the elementary charge \(e = 1.602\times10^{-19}\,\text{C}\) and, together with \(e/m\), calculated the electron’s mass 💡:contentReference[oaicite:1]{index=1}.
2 Why electrons don’t wander off
Inside a metal, free electrons zip around positive ions. The moment one tries to leave, the surface turns slightly positive and yanks it back. An electron must therefore gain extra energy to break free ✊:contentReference[oaicite:2]{index=2}.
3 Work function \(f_0\) 🔑
- The work function \(f_0\) is the minimum energy an electron needs to escape the surface.
- Physicists like the electron-volt: \(1\;\text{eV}=1.602\times10^{-19}\,\text{J}\) 🌟:contentReference[oaicite:3]{index=3}.
- \(f_0\) changes with the metal’s composition and how clean or rough the surface feels :contentReference[oaicite:4]{index=4}.
4 Thermionic emission 🔥
Heat the metal and thermal energy pumps up the electrons. Once an electron’s energy reaches \(f_0\), it shoots out into the surrounding space. That jump is called thermionic emission 😎:contentReference[oaicite:5]{index=5}.
High-yield ideas for NEET 🎯
- Definition of the work function \(f_0\) and its role in electron escape.
- The handy conversion \(1\;\text{eV}=1.602\times10^{-19}\,\text{J}\).
- Thermionic emission: heating helps electrons beat the \(f_0\) barrier.
- Charge-to-mass ratio \( \dfrac{e}{m}=1.76\times10^{11}\,\text{C kg}^{-1}\).
- Quantisation of charge (Millikan’s oil-drop insight).
