Describe Rutherford’s gold foil experiment. How did the experiment prove that the plum-pudding model of the atom was wrong?
Solution : The above figure illustrates the Millikan’s oil-drop experiment, which was used to measure the char ge of the electron. Small drops of oil were allowed to fall between electrica lly charged plates. The drops picked up extra electrons as a r esult of irradiation(process by which an object is exposed to radiation) by X-rays and they became egatively charged. M illikan m easured the varying voltage between the plates that affected the rate of fall. From these data he calculated the negative charge on the drops. Because the charge on any drop was always some integral multiple of 1.602×10 C, Millikan -19 deduced this value to be the charge of a single electron. (a) What causes their rate of fall to vary from their rate in the absence of an electric field In the absence of electric field the two opposing forces which are gravitational force (applying downwards) and frictio nal force (applying upward) that is applied on oil drop vary the rate of fall of the oil droplet. The charged oil droplet falls downwar d due to the g ravitational force. But the rate of its fall is varying due to the rictional force that is applied on it. The f rictional force arises due to the friction between oil droplet and air particles.This force is in upward direction. Thus, gravitational force which ac ts downwards and f rictional force which acts upwards causes the oil droplet’s rate of fall to vary from their rate in the absence of an electric field. (b) Why do some drops move upward In the absence of electric field, the drop down of oil drops is depending on the gravitational force and frictional force that acts upwards. Whereas, In the presence of electric field, the drop down of oil drops is dependingon the g ravitational force and electrostatic force. When the electric field applied is high in magnitude than the e lectrostatic force applied in upward direction is reater than the force of gravity of the ea rth and hence the drop moves in u pward direction. Would the masses of the oil drops be changed significantly by any electrons that accumulate on them No, the elec trons are of negligible mass compared with an oil drop hence the masses of the oil drops would not change by any electrons that accumulate on them.