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Rutherford Alfa experiment and observations about Atom

Physics is a branch of science where we study nature. It deals with different properties of matter. We know that the matter has three different states like solids state, liquid state and gaseous state. All the states the matter consists of molecules. It is understood that molecules can be divided into atoms. When they were named as atoms, it means they are not divisible. But later we are able to divide atoms further into nucleus and the electrons which are revolving around the nucleus.

To know the structure of atoms, we have different theories and the corresponding experimental results.

We are having a model called pulp pudding model. It is also called watermelon model. According to this theory electrons and protons are uniformly distributed in the nucleus like the seeds of a watermelon. As this theory is unable to explain all the properties of matter, a new theory called Rutherford Alfa experimental model is introduced.

According to Rutherford Alfa experiment model nucleus is consisting of positive charges and the size of the nuclei is much smaller than that of the atom. To verify this, an experiment is performed. In this experiment helium particles are allowed to strike a thin gold foil. Helium particles consist of positive charge. As the gold foil is very thin, we can assume that helium particles can directly go and interact with the atom itself.

It is experimentally observed that, out of 8000 alfa rays that were driven towards the nucleus, only one is deviated. From that it can be concluded that the nucleus is also having a positive charge. Then only there will be repulsion and the alpha ray and nucleus so that path is deviated. It can also be concluded that the size of the nucleus is very small when compared with the size of the atom. That is why out of so many positive rays only very few are reflected.

As there is repulsion between the two positive charges, alpha particles cannot go and strike the nucleus. After they reach a certain distance, the repulsion dominates them so that either they deviates or reflects back. That’s why the particles are able to reach up to only a certain distance from the nucleus and this distance is called closest distance of approach. At that point all the kinetic energy of the alpha particle is converted into electrostatic potential energy between the Alpha particle and the protons of the nucleus.

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Photo Electric Effect Problems and Solutions

Problems and Solutions on Einstein's Photo Electric Equation

Dual Nature of Radiation and Matter complete lesson

De Broglie Hypothesis and Wave Nature of Particle

Problems and solutions on de Broglie's Hypothesis

Photo Electric Effect Experimental Observations

When a light of suitable frequency incident on a metal surface, electrons are emitted from the metal surface and these electrons are called photo electrons. The corresponding current is called photo electric current and the phenomena is called photo electric effect.

Photo electric effect is possible with any of the metal surface when the light of suitable frequency is allowed to incident on the metal surface. The incident frequency shall have a minimum value for this photo electric effect to happen and the minimum frequency is called the threshold frequency. When the incident frequency is more than threshold frequency, photo electric effect can happen.

We can express the threshold value even in terms of wavelength. Being frequency is reciprocal to wave length; threshold wavelength is the maximum wavelength of the light that is allowed to incident on a metal surface therefore photo electrons can be emitted. It means when the incident light is having a wavelength less than the threshold wavelength, photo electric effect is possible.

To observe the properties of photo electric effect , experimental arrangement is made as shown below. The apparatus consists of a discharge due with the cathode and anode. Light is allowed to incident on the cathode. The anode is further connected to a rheostat and then further to input a voltage.

When the incident frequency is more than threshold frequency, from the cathode photo electrons are emitted and the emitted photo electric current is measured with the ammeter connected in the circuit.

It is noticed that the photo electric effect is instantaneous process. It means immediately after the striking of light, photo electrons are emitted. There is no time lag in between .

When the voltage is not applied, the photo electrons are not having enough energy to continue travelling in the circuit and to make a consistent current. The applied voltage is enabling the flow of the current through the circuit.

When no voltage is applied, the released electrons get struck between the cathode and anode and they are called stacked electrons. These electrons further oppose the flow of the current and to overcome it, we need to apply the voltage. With the applied voltage, we can notice a steady flow of current in the circuit.

It is experimentally observed that, with the increase of intensity of light, the corresponding photo electric current is also increasing. The graph drawn between intensity of the light in the photo electric current is a straight line passing through the origin.

When the positive plate of the battery is connected to the anode and the negative plate is connected to cathode, there is an increase in the photo electric current. If reverse voltage is applied to the cathode, that is connecting a positive plate to the cathode, it is practically noticed that with the increase of voltage, photo electric current starts decreasing.

At a particular reverse voltage, photo electric current becomes zero and this particular voltage is called stopping potential. At the stopping potential the kinetic energy of the electrons is compensated by the potential energy acquired by the electron due to the stopping potential. We can equate both the energies basing on the law conservation of energy.

It is also practically noticed that stopping potential is independent of intensity of light. With different intensity of light, there may be different photo electric currents. But for all the intensities, stopping potentially is same. It is represented on the negative x-axis of the graph. On this graph voltage is taken on x-axis and the photo electric current is taken and y-axis.

It is also practically noticed that, stopping potentially is a dependent of frequency of the incident light. It is noticeable that for different frequencies of incident light, the corresponding stopping potentially is different. It is also experimentally observed that change of the frequency of the incident light is not going to affect the saturation current that is generated.

It is experimentally observed that higher the incident frequency, more the stopping potential.

We can draw a graph taking the incident frequency on x-axis and the stopping potential on y-axis. The graph is as shown below. It is observed that the incident frequency shall be more than threshold frequency for the photo electric current to emit. Then only we can apply reverse voltage so that somewhere in the photo electric current stops. Once if the applied frequency is more than the threshold frequency, it is observed that with the increase of frequency, the stopping potential also increases.

Representation of Light as Wave

Interference of Light an Overview

Fringe Width and Angular Fringe Width in Interference

Interference Fringe shift and Problems

Polarization Concepts and Problems

Wave Optics Complete Lessons

Diffraction in Wave Optics Overview

Resolving Power of Optical Instruments

Determination of Specific Charge of Electron J J Thomson Experiment

Millikan Oil Drop Method to determine Charge of Electron

We can calculate the charge on the electron basing on Millikan’s oil drop experiment. Between the two circular identical plates oil drops are sprinkled through a device called atomizer. The purpose of this device is to produce very small oil drops.

On the oil drop there are multiple forces acting. Weight is the force that is always acting in the downward direction, up thrust is the force that always acts in the upward direction. When the drop starts moving, there is another force called viscous force starts acting against the motion.

Viscous force is similar to frictional force which opposes the relative motion. As the drop starts moving in the downward direction, viscous force also increases in the upward direction. Being the downward force is constant and the upward force is steadily increasing, at a particular stage these two forces are going to be balanced with each other. At that instant the drop acquires a constant velocity and the velocity is called as terminal velocity.

The oil drops during the motion acquires a positive charge due to friction. If a electric field is applied in the upward direction, the drops starts experiencing a new electric force in the upward direction.

Again at the equilibrium state, the drop will acquire a different terminal velocity and by combining both the cases we can derive the equation for the charge.