Much of this came from the results of several seminal experiments that revealed the details of the internal structure of atoms. In the two centuries since Dalton developed his ideas, scientists have made significant progress in furthering our understanding of atomic theory. Define isotopes and give examples for several elements.Describe the three subatomic particles that compose atoms.Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford.Outline milestones in the development of modern atomic theory.Then he obtained various values for q, and is found to be a multiple of 1.6 x 10 -19C.By the end of this section, you will be able to: of electrons attaching to the oil drop varied. of times, each time varying the strength of X-rays ionizing the air. Now the total force acting on drop is F e-F’ v-F G=0.į' V is the new viscous force under the action of electric field. Where l 2 is the distance travelled by the oil drop and t 2 the time taken. Now the electric force will act upwards,gravity and viscous forces acts downwards.Ĭorresponding terminal velocity v 2 is calculated as,

If the applied electric force F e is greater than the downward forces, some of the drops (the charged ones) will start to rise. Where V is the balancing potential and d is the distance between the plates. If the electric forces F e, balances the gravitational force F G, the drop suspends in the air. Now a field is produced in the bottom chamber with the supply voltage. A likely looking drop is selected and kept in the middle of the field of view by adjusting the voltage. The apparent weight (true weight minus up thrust) for a perfectly spherical body is given by,Īt terminal velocity the oil drop is not accelerating, so the total force acting on it must be zero The drag force acting upon the drop is calculated from stokes’s law and is given as Where 'l 1' is the distance travelled by the oil drop and ’t 1’ is the time taken. The terminal velocity v 1 in the absence of an electric field is calculated as Due to gravity they accelerate first, but gradually slowdown because of air resistance.

Initially the oil drops are allowed to fall between the plates in the absence of electric field. He proposed that this was the charge of a single electron. He repeated the experiment for many droplets and confirmed that the charges were all multiples of some fundamental value and calculated it to be 1.5924(17) ×10 −19 C, within one percent of the currently accepted value of 1.602176487(40) ×10 −19 C. After many repetitions he concluded that charge could assume only certain fixed values. Millikan observed one drop after another, varying the voltage and noting the effect. By adjusting the voltage applied, drop can be suspended in air.

Speed of its motion can be controlled by altering the voltage applied on the plates. With the help of dc supply a voltage is applied. As the droplets traverses through the air, electrons accumulate over the droplets and negative charge is acquired. Using X-rays the bottom chamber is illuminated causing the air to ionize. Some of the oil drops fall through the hole in the upper plate. There are two plates, one positive and the other negative above and below the bottom chamber.dc supply is attached to the plates. By means of a short focal distance telescope, the droplets can be viewed. It measures the size of charge on a single electron.Īpparatus consist of an atomizer, which helps to spray tiny droplets. Oil drop experiment was performed originally by the American physicist Robert A. Millikan’s oil drop apparatus,oil, Dc supply.

To find the terminal velocity of the drop.

To experimentally demonstrate the concept of Millikan’s oil drop experiment.

Millikan atomic theory