Discovery of Electron
Who discovered the electron? Electron was discovered by J. J. Thomson in 1897 when he was studying the properties of cathode ray.
What is cathode ray?
J. J. Thomson constructed a glass tube which was partially evacuated i.e. much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of particle (ray) was coming out from the negatively charged electrode (cathode) to positively charged electrode (anode). This ray is called cathode ray and the whole construction is called cathode ray tube. The schematic of a cathode ray tube
o Cathode rays consist of electrons.
o Electrons are the basic unit s of all atoms.
Properties of cathode ray particle
1. They travel in straight lines.
2. They are independent of the material composition of the cathode.
3. Applying electric field in the path of cathode ray deflects the ray towards positively charged plate. Hence cathode ray consists of negatively charged particles.
J. J. Thomson measured the charge-by-mass-ratio (e/m) of cathode ray particle using deflection in both electric and magnetic field.
e/m =−1.76×108 coulomb per gram
The cathode ray particle turned out to be 2000 times lighter than hydrogen.
Although we got e/m ratio for electron from J.J. Thomson’s Cathode Ray Tube experiment, we still don’t know the exact charge (e) for electron. American physicist Robert Millikan designed an experiment to measure the absolute value of the charge of electron which is discussed below.
Millikan Oil Drop Experiment
In 1909, American physicist R. Millikan measured the charge of an electron using negatively charged oil droplets. The measured charge (e) of an electron is −1.60×10−19 Coulombs.
Using the measured charge of electron, we can calculate the mass of electron from e/m ratio given by J. J. Thomson’s cathode ray experiment.
Putting e=−1.60×10−19 Coulomb,
What we have learned
Electron was discovered by J. J. Thomson in Cathode Ray Tube (CRT) experiment.
Electrons are negatively charged particles with charge-to-mass ratio −1.76×108 C/gm
The charge of an electron was measured by R. Millikan in Oil drop experiment.
Charge of an electron is −1.60×10−19 C
Mass of an electron is 9.1×10−28 gram.
Electron is approximately 2000 times lighter than hydrogen.
Discovery of Proton
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