The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substruc

ture.

OBJECTIVE:1) To study the type of charges and the basic interaction.2)to know the development of experiments that lead to the latest value of mass and charge of electron

WHAT IS ELECTRON ?

According to the experiment conducted by Benjamin Franklin (1706-1790):

A hard rubber/plastic rod that has been rubbed with fur is suspended by a piece of string. When a glass rod that has been rubbed with silk is brought near the rubber rod, the rubber rod is attracted towards the glass rod. If two charged rubber rods ( or two charges glass rod) are brought near to each other, the force between them is repulsive.

the conclusion of the experiment, like charges repel and unlike charges attract to one and another.

the object become charged by gainning or losing electron only electron can be transfered because electron is lighter than

proton

` There are two types of electric charges: positive and negative. Positively charged substances are repelled from other positively charged substances, but attracted to negatively charged substances; negatively charged substances are repelled from negative and attracted to positive. An object is negatively charged if it has an excess of electrons, and is otherwise positively charged or uncharged. The SI derived unit of electric charge is the coulomb (C), although in electrical engineering it is also common to use the ampere-hour (Ah)

I. Pre-History The charge on electron was first measured by J.J. Thomson and two co-workers (J.S.E. Townsend and H.A. Wilson), starting in 1897. Each used a slightly different method. Townsend's work will be described as an example. Townsend's work depended on the fact that drops of water will grow around ions in humid air. Under the influence of gravity, the drop would fall, accelerating until it hit a constant speed. Several items were measured in this experiment. 1. the mass of a water droplet (actually the average mass of many) 2. the total electric charge carried on all the droplets (this was done by absorbing the water into an acid and measuring the charge picked up.) 3. the velocity of the droplet 4. the total mass of all water droplets (found by measuring the acid's increase in weight) He determined the e/m ratio of the droplets (2 divided by 4), then multiplied by the mass of one droplet to get the value for e. Thomson, Townsend, and Wilson each obtained roughly the same value for the charge on positive and negative ions. It was about 1 x 10-19 Coulombs. This work continued until about 1901 or 1902.

Electron was discovered by J. J. Thomson in 1897 when he was studying the properties of cathode ray

Thomson won Nobel Prize in 1906 for discovering the elementary particle electron. Interestingly, his son G. P. Thomson also won the Nobel Prize in 1937 for proving the wavelike properties of electron

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 is given in figure 2.

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 next.

Cathode Ray Tube.mp4

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.

e/m=−1.76×108 Coulomb-per-gram

m=e/−1.76×108

Putting e=−1.60217733×10−19 Coulomb,

m=9.1×10−28gram.

Robert Millikan discovered charge of electron and won Noble prize in physics in 1923Millikan Oil Drop Experiment.mp4

CONCLUSION As the conclusion, we now know the two types of charges which is the positive charges is known as proton while the negative charges is known as electron. Like charges will repel and unlike charges will attract. 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/gmThe charge of an electron was measured by R. Millikan in Oil drop experiment.Charge of an electron is −1.60×10−19 CMass of an electron is 9.1×10−28 gram.Electron is approximately 2000 times lighter than hydrogen.

REFFERENCES

Raymond A.Serway and Chris Vuille. 9th Edition,International Edition. College Physics,2012

http://www.padakshep.org/otp/subjects/chemistry/physical-chemistry/discovery-of-electrons-protons-and-neutrons/

https://www.youtube.com/watch?v=XMfYHag7Liw http://www.socratic.org/chemistry/a-closer-look-at-the-atom/

millikans-oil-drop-experiment