Module 2 : Electrostatics
Lecture 6 : Quantization Of Charge
  QUANTIZATION OF CHARGE
 

Building blocks of matter are atoms, which consist of nucleus and electrons. Nucleus has positively charged protons and neutrons which are charge neutral. Negatively charged electrons move round the nucleus. It has been observed that the electric charge of all particles are integral multiple of an elemental value of charge. Denoting the magnitude of the charge of an electron by $\mid e\mid$, the charge of all particles are

\begin{displaymath}\pm\mid e\mid, \pm 2\mid e\mid, \pm 3\mid e\mid,\ldots\end{displaymath}

This is known as charge quantization. Neutral particles, like neutron and photon have zero zharge.

   
Physicists have revised their earlier thinking that particles like neutrons and protons are fundamental particles.
 

They are now regarded as belonging to a group of particles called Hadrons , which are built up of funadental constituents called quarks , which have fractional charge of magnitude one third or two third that of an electronic charge. Electrons, on the other hand are considered elementary particles, belonging to a group called Leptons.
   
Millikan's Oil Drop Experiment
 

The quantization of charge was experimentally established by Robert Millikan in 1909. Millikan sprayed a fine mist of oildrops into an evacuated chamber using an atomiser . The chamber has two metal plates, which are charged with high voltage. Some of the oil drops find their way into the region between these two plates through a pin-hole on the top plate. In this region they move under the action of gravity and air resistance. If these drops are exposed to an ionising radiation from an X-ray source, some of electrons of the ionized air attach themselves to the oil drops, making these drops negatively charged. The droplets are illuminated by a light source at right angles to a viewing microscope. With careful switching of the voltage, the droplet can be observed for a long time. Initially, the plates are uncharged. A droplet acquires a terminal speed $v_0$as it falls between the plates under the action of gravity and air resistance.
                                     Air Resistance = $\displaystyle{ 6{\bf\pi\eta} av_0}$ (Stoke's Law)
                                                       $Mg = 6\pi\eta av_0$

1
2